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The majority of experiments in plant biology use plants grown in some kind of container or pot. We conducted a meta-analysis on 65 studies that analysed the effect of pot size on growth and underlying variables. On average, a doubling of the pot size increased biomass production by 43%. Further analysis of pot size effects on the underlying components of growth suggests that reduced growth in smaller pots is caused mainly by a reduction in photosynthesis per unit leaf area, rather than by changes in leaf morphology or biomass allocation. The appropriate pot size will logically depend on the size of the plants growing in them. Based on various lines of evidence we suggest that an appropriate pot size is one in which the plant biomass does not exceed 1 g L-1. In current research practice similar to 65% of the experiments exceed that threshold. We suggest that researchers need to carefully consider the pot size in their experiments, as small pots may change experimental results and defy the purpose of the experiment.
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Pot size matters: a meta-analysis of the effects of rooting
volume on plant growth
Hendrik Poorter
A,C
, Jonas Bühler
A
, Dagmar van Dusschoten
A
, José Climent
B
and Johannes A. Postma
A
A
IBG-2 Plant Sciences, Forschungszentrum Jülich, D-52425, Germany.
B
INIA, Forest Research Centre, Department of Forest Ecology and Genetics, Avda A Coruña Km 7.5.,
28040 Madrid, Spain.
C
Corresponding author. Email: h.poorter@fz-juelich.de
Abstract. The majority of experiments in plant biology use plants grown in some kind of container or pot. We conducted a
meta-analysis on 65 studies that analysed the effect of pot size on growth and underlying variables. On average, a doubling of
the pot size increased biomass production by 43%. Further analysis of pot size effects on the underlying components of
growth suggests that reduced growth in smaller pots is caused mainly by a reduction in photosynthesis per unit leaf area,
rather than by changes in leaf morphology or biomass allocation. The appropriate pot size will logically depend on the size of
the plants growing in them. Based on various lines of evidence we suggest that an appropriate pot size is one in which the plant
biomass does not exceed 1 g L
1
. In current research practice ~65% of the experiments exceed that threshold. We suggest
that researchers need to carefully consider the pot size in their experiments, as small pots may change experimental results and
defy the purpose of the experiment.
Additional keywords: container volume, experimental setup, meta-analysis, pot size, plant growth, rooting volume.
Received 16 February 2012, accepted 11 May 2012, published online 15 June 2012
Introduction
A large number of studies in plant biology focus on gene
expression, physiology or biomass production of individually-
grown plants. To this end, experiments are often conducted on
plants grown in some kind of container, from here on referred to as
pots. Pot size has received special attention in forestry (Carlson
and Endean 1976) and horticulture (Kharkina et al.1999), where
commercial companies can prot from choosing the smallest pot
volume that still delivers a product with an appropriate quality.
Occasionally, the issue of pot size has received attention in other
elds of plant biology. Arp (1991), for example, emphasised
that pot size might be an important issue in experiments that
considered the effect of elevated CO
2
on plants; and recently, a
discussion in the eld of ecophysiology has emerged, where
studies on the recognition of roots of neighbours are thought to
be confounded by pot size (Hess and De Kroon 2007).
Apart from the elds mentioned above, pot size seems to
have received little consideration in the scientic literature and
is regulary not reported in the materials and methods section of
publications. Nonetheless, it is an important issue. In most
laboratories there is large demand for growth chamber facilities
and the use of small pots generally implies more experiments or
increased replication. Space is probably less of an issue in most
glasshouses. However, the plant biology community currently
makes a great effort to develop automated systems for plant
phenotyping (Granier et al.2006; Nagel et al.2012). These
high-throughput systems allow for the handling of many plants,
which automatically implies that spatial demands on growth
facilities will increase. The use of small pots has the additional
advantage that it does not exceed the capabilities of the transport
robots to move the weight of pot plus plant.
The use of small pots for research purposes may also have
disadvantages, which are more related to biological constraints.
A small container implies a small quantity of soil or other
substrate and thereby, almost invariably, a reduction in the
availability of water and nutrients to the plant. In addition to
reduced resource availability, pots generally impede root growth.
Many species easily produce roots of more than 1 m in length
(Jackson et al.1996), even at a relative young age (Drew 1975;
Fusseder 1987), thereby exceeding the dimensions of most
containers used. Large plants in small pots may have a large
fraction of roots pot-bound, with all kind of secondary
consequences (Herold and McNeil 1979). In this paper we
explore the consequences of the choice of pot size for plants
studied for experimental purposes. We rst analysed in a
quantitative way to what extent pot size affects plant growth in
the studies that explicitly considered this factor. We did so by a
meta-analysis of 65 studies reported in the literature. Second,
we reported on some of the morphological and physiological
components that might explain the observed growth patterns.
CSIRO PUBLISHING
Functional Plant Biology,2012, 39, 839850 Review
http://dx.doi.org/10.1071/FP12049
Journal compilation CSIRO 2012 www.publish.csiro.au/journals/fpb
Third, we analysed whether there is a threshold in the plant
size : pot size relationship above which growth is affected and
compared the data for the current pot-size experiments with a
more general database on plant research and to plants grown in
elds. Based on these comparisons we suggest what might be an
appropriate pot size for a given experiment.
1. Effect of pot size on plant biomass
To analyse the effect of rooting volume on plant growth, we
screened the literature of the last 100 years and arrived at a total
of 65 publications where this factor was studied in pots and
~10 where rooting volume was constrained in hydroponically-
grown plants (see Appendix 1). For this analysis we will focus
mainly on the results for the pot-grown plants. A description of
the methodological approach is given in Appendix 2. The effect
of pot size has been studied in a wide range of pot volumes, with
values ranging from 5 mL for a herbaceous greenhouse crop
(Bar-Tal and Pressman 1996), to 1700 L for trees growing over a
period of several years (Hsu et al.1996). The range of pot sizes
used within the various experiments was also large, varying by a
factor 2 at least and a factor 35 at most. A clear example in our
compilation is the experiment by Endean and Carlson
(1975), who followed the growth of Pinus contorta Douglas
over time. In the very beginning of the experiment, plants grew
presumably well in all pot sizes, but after 4 weeks of growth
biomass was already reduced in the smallest pot volume
(Fig. 1a), and by the last harvest, none of the pots seemed
large enough to ensure unrestricted growth.
In the full compiled data set the picture is similar: in almost
all of the experiments considered plants increase in weight with
every larger pot that is used (Fig. 1b). Most experiments
fall between the dotted lines, which indicate a xed ratio of
biomass to pot volume of 2 and 100 g L
1
, respectively, for the
lower and higher line. Only a few experiments with trees in large
containers exceed the 100 g L
1
. As all experiments start with
small seedlings, plants move over time from the bottom part of
the graph upwards (cf. Fig. 1a). Clearly, the experiments at the
upper right end of Fig. 1bare also the ones that were planned
to last for the longest time, 2 years in case of the experiment with
the largest pots (Hsu et al.1996), 5 years in case of the experiment
with the highest ratio (Bar-Yosef et al. 1988).
Theoretically, doseresponse curves should level off at
higher pot sizes, as in Fig. 1a. However, this is not the case
for many of the studies plotted in Fig. 1b, which mostly show
linear responses. We discuss this further in section 4. To scale all
data to an equal change in pot size, we calculated for each
species in each of the experiments the average slope of the
log-transformed mass versus pot size relationship and derived
from those values an easy to understand expression that indicates
the percentage increase in biomass for a 100% increase in pot
size (Appendix 3). On average, plants increased 43% in mass for
every doubling in pot size (Fig. 2,P<0.001), with no signicant
differences in response between herbaceous and woody species.
These response values are substantial and imply that plants
are likely to be over 3 times larger in 2 L pots than in pots of
0.2 L. Given these differences, it is clear that pot size should
be given careful consideration during the planning phase of
an experiment.
2. Effect on components of the carbon-budget
What is the cause of the growth retardation in smaller pot sizes?
As clearly shown by Endean and Carlson (1975), the effect of pot
size on biomass gradually increases throughout (the later part of)
the experimental period. The rate by which biomass of individual
plants is accumulated is proportional to the size of the plant and
conveniently described by the relative growth rate (RGR, the
rate of increase in biomass per unit biomass present; Evans
1972). The differences in RGR of plants growing in different
pot sizes are always smaller than the differences in biomass at the
end of the experiment. This implies that the physiological and
morphological factors that underlie the variation in biomass will
also be affected to a smaller extent than biomass itself (Poorter
et al.2012a). We analysed growth in terms of the plants carbon
economy, using a top-down approach where the RGR is
factorised into three components:
RGR ¼SLA LMF ULR;ð1Þ
0.01 0.10 1.00
0.001
0.01
0.1
1
10
0.01 0.10 1.00 10.00 100.00 1000.00
0.01
0.1
1
10
100
1000
10 000
100 000
Total dry mass (g)
4
8
14
20
0
Pot size (L)
Herb.
Woody
(a)
(b)
Fig. 1. (a) Doseresponse curves of total plant dry mass of Pinus contorta
as dependent on pot size. Plants were grown in pots with six different sizes
and harvested at several times during a 20-week period. Numbers indicate
the time of harvest after sowing, in weeks. Data from Endean and Carlson
(1975), except for seed mass (week 0), which was taken from McGinley et al.
(1990). (b) Summary of total biomass data of plants grown in pots of
various sizes, as reported in the literature (65 experiments on 69 species,
see Appendix 1). Each line connects the observations of one species or
genotype in one experiment. Values in red indicate woody species, in blue
herbaceous species. Dotted lines indicate a total plant biomass per unit pot
volume of 2 (lower line) and 100 (upper one) g L
1
. Additional, unpublished
data were obtained for Barrett and Gifford (1995) and Liu and Latimer
(1995), as well as from L. Mommer and H. de Kroon, and R. Pierik.
840 Functional Plant Biology H. Poorter et al.
according to Evans (1972). Here, SLA denotes specic leaf area
(leaf area per unit leaf dry mass; m
2
kg
1
), LMF the relative
allocation of biomass to leaves (leaf mass fraction, g leaf g
1
plant)
and ULR is a parameter that indicates the growth rate per unit
leaf area (unit leaf rate, g m
2
day
1
). ULR is basically the net
result of carbon gain through photosynthesis, corrected for the
rate of respiration in the whole plant and the C-content of the
newly added biomass. ULR and photosynthesis per unit leaf
area are often strongly positively correlated (Poorter 2002).
A characteristic of RGR is that an absolute difference in RGR
between treatments causes a relative difference in biomass over
time (Poorter and Navas 2003). However, in the case where we
want to understand the reason for the difference in growth and
only have fragmented information, it is more amenable to analyse
the proportional differences in RGR relative to that of the three
growth parameters that underlie RGR (Eqn 1). Only a few
experiments report data on these underlying components, but
the information we could gather points into the following
direction (see Fig. 2and Fig. S1, available as Supplementary
Material to this paper): a doubling in pot size increases RGR by
~5%. Consequently, each of the growth parameters at the right-
hand side of Eqn 1 may change by a very small proportion, or
one of them by a somewhat larger proportion in the range of
5%. At the nal harvest, SLA increased somewhat in some
experiments and decreased in others. We could quantitatively
compare the various experiments by calculating the percentage
increase in SLA with a 100% increase in pot size and found that
taken over all experiments, this variable did not deviate
signicantly from zero (Fig. 2). Most, but not all experiments
with root restriction in hydroponics conrm this response (e.g.
Carmi et al.1983; Kharkina et al.1999; but see Tschaplinski and
Blake 1985).
Allocation patterns are more frequently reported, generally
as the biomass ratio between shoot and root. We prefer a
classication in at least three plant organs (Poorter et al.
2012b) and therefore use LMF, SMF and RMF, which are the
fractions of total vegetative biomass invested to leaves, stems
and roots respectively. The relatively scarce information indicates
that LMF is not affected (Fig. 2a), whereas SMF increased
slightly but non-signicantly (0.05 <P<0.10) with pot size.
More information is present on RMF (or the root : shoot ratio).
Reviews are not equivocal in their evaluation. NeSmith and
Duval (1998) conclude that RMF often does not change with
pot size. In contrast, Hess and De Kroon (2007) expect RMF
to increase in larger pots. We found that over ~80 species
experiment combinations, RMF decreased on average by a small
but signicant extent (4% with a doubling in pot size, P<0.05)
We nd no clear differences between woody and herbaceous
plants in this respect (data not shown).
With two of the variables of Eqn 1 unaffected, we might
expect that the observed differences in growth rates between
plants grown in different pot sizes are caused largely by a
change in net photosynthesis and hence we would expect a
stimulation of ~4% in this variable. Some experiments report
a more than 30% higher rate of photosynthesis with a 100%
increase in pot size (Robbins and Pharr 1988; Ronchi et al.
2006; Fig. S1g). This is far more than the expected 4%. As pot
size stress builds up gradually over time, results may depend on
the actual timing of the measurements during the experiment.
Moreover, obtained rates will also depend on the specic leaf
measured and the time of the day that the measurement was
taken. The growth parameter ULR is generally closely linked to
the rate of photosynthesis and provides an estimate that is
integrated over the whole plant and the whole growth period
under consideration (Poorter 2002). A 2-fold difference is not
reported in the literature (Fig. S1f), but the pot-size effects on
ULR are signicantly greater than zero and larger than for SLA
and LMF (Fig. 2). The median increase in photosynthesis and
ULR with a doubling in pot size is quite comparable to the
median increase in RGR. Hence, we conclude that with the
–20
0
20
40
60
80
100
–4*
1ns
LNC
0ns
RGR ULR PSSLA RMFLMFTDM
1+
0ns
5**
9*
13**
% Change with 100%
increase in pot size
43***
SMF
(a)
(b)
0.5
1
3
10
1.37
0.91
2.0
RMF
h
y
droponics
RMF
pots
TDM
h
y
droponics
TDM (g) or RMF (g g–1)
in large/in small volume
TDM
pots
*
ns
2.6
Fig. 2. (a) Distribution of the percentage increase in a range of growth-
related traits when pot size doubles. The data are from a range of
experiments described in literature (Appendix 1). The distribution is
characterised by box and whisker plots, where the boxes show the 25th
and 75th percentile and the whiskers the 10th and the 90th percentile. The
median is represented by the line in the box and given as a number above the
box plot. The total (rounded) number of species experiment combinations
on which the boxplots are based are TDM (total dry mass): 90; RGR (relative
growth rate): 15; SLA (specic leaf area): 20; LMF (leaf mass fraction):
35; SMF (stem mass fraction): 30; RMF (root mass fraction): 80; ULR
(unit leaf rate): 5; PSa (rate of photosynthesis per unit leaf area): 15; LNC
(leaf nitrogen concentration): 15. The signicance level of a test whether
the observed distribution deviates signicantly from 0 is given above the
respective box plots: ns, not signicant;
+
, 0.05 <P<0.10; *, P<0.05;
**, P<0.01; ***, P<0.001. (b) Distribution of ratios in TDM and
RMF, taken from pot size studies (values for the largest pot relative to
those for the smallest pot within each experiment, n= 65) and studies
where rooting volume was constrained for hydroponically-grown plants
(ratio unconstrained/constrained, n= 11). The results of t-tests between the
two groups are shown between the boxplots.
Pot size matters Functional Plant Biology 841
limited and fragmentary evidence yet available, net
photosynthesis is likely to be the process that is strongest
affected by pot size and may explain best the observed pot
size effect on biomass (Fig. 1b). Additional support comes
from experiments where photosynthesis recovered quickly
after plants were repotted in larger rooting volumes (Herold
and McNeil 1979).
3. What mechanism could explain a reduced
photosynthesis in smaller pots?
Several factors could explain the reduced rate of photosynthesis
and thereby growth in smaller pots. A rst possible explanation
is that containers of smaller dimensions can be placed at a higher
density, with less light available for each shoot and hence, a
lower rate of photosynthesis. Although this could be the case in
some of the compiled experiments, strong pot size effects on
biomass and photosynthesis are generally also observed when
density is specically controlled for (e.g. Endean and Carlson
1975; Robbins and Pharr 1988; NeSmith et al.1992; Climent
et al.2011).
In the type of experiments that we included in our meta-
analysis, a smaller pot size will inadvertently decrease the total
nutrient content in the pot. Low nitrogen and phosphorus
availability are known to decrease photosynthesis (e.g.,
Sinclair and Horie 1989; Lynch et al.1991) and growth and
increase the root mass fraction (Poorter et al.2012b). Thus,
lower resource supply could form a plausible explanation. We
calculated the response of leaf nitrogen concentrations to changes
in pot size, expecting to see an increase if nutrient availability
would explain the pot size effect. On average, there was a slight,
but non-signicant increase in leaf nitrogen concentrations with
pot size (Fig. 2), suggesting that this factor cannot completely
explain the observed differences in photosynthesis or growth.
Similar results were found for phosphorus (Krizek et al.1985).
This conclusion is to a certain extent supported by observations
on hydroponically-grown plants, which have decreased
photosynthesis and growth (Fig. 2b) when the root volume
was restricted, despite a continuous high supply of nutrients.
However, unlike plants grown in pots, hydroponically-grown
plants do not show an increased RMF when restricted (Fig. 2b).
As increased RMF is a good indicator for nutrient stress, we
presume that nutrient limitation in small pots is still a factor,
although we cannot exclude possible allometric effects which
could explain a larger RMF in smaller plants as well (Poorter et al.
2012b).
Water is the other commodity that may be in short supply.
Small pots could negatively impact the water status of plants
as they have a reduced total water holding capacity and
will, therefore, dry out more quickly (Tschaplinski and Blake
1985) and at severe stress levels increase RMF (Poorter et al.
2012b). Ray and Sinclair (1998) demonstrated with their
drought experiment that soil in small pots dries out faster and
thereby caused more severe drought stress in plants. However,
pot size does not necessarily affect stomatal conductance or
leaf water potential (Ronchi et al.2006) and as for
nutrients with plants growing in hydroponics there is still a
clear effect of root connement, even though water availability is
not restricted.
Besides resource availability, the temperature of the
rooting volume could be affected by pot size (de Vries 1980).
Pots can intercept a substantial amount of solar radiation
especially in experimental gardens and glasshouses, which
may increase the soil temperature at the edge and eventually
in the middle of the pot if no precautions are made (Martini
et al.1991;Xuet al.2001). Small pots have greater surface
areas relative to their volume and thereby heat up more
quickly. Townend and Dickinson (1995) measured 5C
higher day temperatures in 0.19 L pots compared with 1.9 L
pots. Keever et al.(1986) suggest that the greater temperature
uctuations in small pots may explain the reduced growth of
the plants. High temperatures in the pot may have several direct
(respiration, root growth) and indirect (through increased
microbial activity) effects on plant growth. Pot temperatures
are rarely reported so it is difcult to evaluate how often
temperature differences between pots contribute to reduced
growth. However, given that growth reductions also have been
observed in hydroponically-grown plants suggests that
temperature differences alone cannot explain the observed pot
effects either.
If neither nutrient or water availability nor higher temperatures
can (fully) explain the pot size effects on photosynthesis and
growth, it could be that root connement per se may cause growth
retardation, with reduced photosynthesis as a consequence.
Root growth is known to respond directly to impedance.
Impeded roots stay shorter whereas the initiation and growth
of side branches increases (Bengough and Mullins 1991; Falik
et al.2005). Furthermore, Young et al.(1997) showed that within
10 min of increasing the impedance to root growth, leaf expansion
rate is reduced. This suggests that some kind of signal may
regulate shoot growth when a large proportion of the roots are
impeded. The actual signal for such a response remains as yet
unknown. Possibly a reduced sink strength of the root system
could cause a direct negative feedback on photosynthesis (Paul
and Pellny 2003). Alternatively, a specic rootshoot signal is
involved (Jackson 1993).
A crucial point in the evaluation of the lastly discussed
option is knowledge on the actual distribution of roots within
the pots. Although the vertical root distribution is relatively
easily measured (Price et al.2002; Suriyagoda et al.2010),
analysis of the horizontal distributions is more complicated.
Using non-destructive magnetic resonance imaging (MRI), we
followed the root development of Hordeum vulgare L. and
Beta vulgaris L. plants over time in three dimensional space.
Representative nuclear magnetic resonance (NMR) images of
root systems at the end of the experiment are shown in Fig. 3.
We calculated the percentage of roots that was located in
the inner half of the soil volume, furthest away from wall and
bottom and the percentage of roots present in the outer 4 mm of
the pot. Only 2025% of the root biomass was in the inner part
of the pot (Table 1), whereas ~50% was found in the outer 4 mm
(20% of the total volume). The proportional distribution
remained remarkably constant over time. Hence, if these
observations have wider validity, we conclude that a relatively
large fraction of roots is close to the edge of the pot, where
unfavourable environmental conditions, for example, large
temperature uctuations and impedance of the pot wall may
negatively impact growth.
842 Functional Plant Biology H. Poorter et al.
4. When does pot size limitation starts?
In sections 1 and 2 we considered for each experiment the
overall effect on plant growth, morphology and physiology
when pot size was doubled. However, it is to be expected that
a plant of a given size will be constrained more in a small than in a
large pot. That is, young plants are initially not affected by pot
size, but as plants grow older, the pot size effect becomes more
pronounced, even in medium-sized pot volumes (Fig. 1a). When
experiments last for sufciently long time, even the largest pot
size might not be large enough for unrestricted growth, i.e. the
saturating part of the curve extends beyond the largest pots used.
For the experimental data this implies that the relationship
becomes close to linear again. In fact, many of the curves in
Fig. 1bshow a linear relationship. For experiments where only
two pot sizes were used, it is impossible to deduce whether the
response of the plants is indeed linear or not. But even in many of
the other experiments in Fig. 1bno clear saturation is shown.
What is the reason for that?
One objective way to relate plant and pot size across
experiments is to calculate the plant biomass that is present at
a given volume of rooting space. This variable, for which we use
BVR as an acronym (total plant biomass : rooting volume ratio;
gL
1
), has, to our knowledge, been used only by Kerstiens and
Hawes (1994). BVR values vary widely and ranged in our
database from as low as 0.01 in work by Climent et al.(2008)
to over 300 g L
1
in work reported by Biran and Eliassaf (1980).
The median value in the pot size experiments is around 9.5 for
experiments both with herbaceous and woody species (Fig. 4b).
We tested whether the BVR could explain the form of the
doseresponse curves in the 65 experiments shown in Fig. 1b,
by calculating for each point what the slope of the doseresponse
curves was, as well as the BVR. In order to be consistent with
Fig. 2, we derived the percentage increase in biomass with pot
size doubling for these data as well. We found that very few
experiments had BVR values lower than 2 g L
1
(Fig. 4b). Hence,
for this part of the analysis we included not only data of the last
harvest, but also data of earlier harvests where available. This may
imply that not all data points in the analysis are formally
independent, but it increases the power of our analysis in this
crucial range. We binned all values in ve BVR ranges and show
the resulting distribution in Fig. 5a. Estimation of slopes is
always more challenging than determining the absolute values
per point and this may be one of the reasons that there is
considerable variation within each category. In the category
with a BVR between 1 and 2 g L
1
the effect of pot size is
clearly noticeable. Pot size effects are saturating when BVR
values exceed the 2 g L
1
(P<0.01).
Another way to obtain a greater insight into the relationship
between plant biomass and pot size is to express the biomass of
(a)(b)
Fig. 3. (a) NMR image of a Hordeum vulgare plant grown in a pot with a
volume of 1.3 L for 44 days. Roots in the inner 50% of the soil volume
(furthest away from wall and bottom) are colour-coded yellow, roots in the
outer 50% blue. The stem part that was masked from the analysis is shown in
red. (b) Idem for a Beta vulgaris plant 48 days after sowing. The developing
storage root is colour-coded red and was not included in this case.
Table 1. The proportion of the root mass that is present in the inner 50% of the pot volume (more than 12.5 mm from
the wall or bottom) and in the space less than 4 mm from wall or bottom of the pot, for Hordeum spontaneum and
Beta vulgaris plants growing in 1.3 L pots
Values are based on six plants followed over time. Time is days after sowing. Standard error of the mean was on average 1.6
and 2.9 percentage points for the inner half and outer 4 mm respectively
Hordeum vulgare Beta vulgaris
Time
(days)
Root mass in
inner half (%)
Root mass in
outer 4 mm (%)
Time
(days)
Root mass in
inner half (%)
Root mass in
outer 4 mm (%)
26 32.0 43.2 31 20.3 53.5
28 28.9 50.0 33 20.8 53.1
31 17.7 55.3 34 21.5 54.1
33 21.3 52.7 39 22.9 52.6
34 23.1 51.7 41 23.0 52.3
39 25.2 48.7 48 22.3 52.0
41 28.2 48.0 –– –
44 32.4 44.7 –– –
Pot size matters Functional Plant Biology 843
plants grown at various pot sizes relative to the biomass at
the largest pot size and plot these values against the BVR
(Fig. 5b). Experiments where pot sizes are limiting growth
throughout the full range of pot sizes are characterised by lines
that decline linearly with BVR. However, as long as biomass is
not affected, the line will remain around 1 and only drop at greater
BVR values when pot size starts to reduce growth. For the few
experiments where this was the case, we could show that this
inection point occurred somewhere between 0.2 and 2 g L
1
(Fig. 5b). Thus, from both the full sets of experiments compiled,
as for the more detailed analyses over time, we derive that pot
size effects are particularly strong when BVR values are greater
than 2 g L
1
.
5. How do these BVR values relate to other
experiments and the eld?
As mentioned in section 1, the range of pot sizes used in this
compilation is large. The median value is around 0.9 L (Fig. 4a).
How does that compare to common practice in ecophysiological
experiments? This will partly depend on the species studied.
Arabidopsis, for example, is generally grown in much smaller
pots (with an interquartile range of 0.080.21 L) than Zea
mays L. (1.85.0 L). An overall impression of used pot sizes
can be obtained from the metaphenomics database described
by Poorter et al.(2010), where the response of ~900 different
species to 12 different environmental factors is compiled for a
total of ~800 experiments from the literature. The median pot
size used in that compilation of experiments turns out to be
~23 times larger than those in the pot size studies (Fig. 4a),
whereas the median BVR value is ~4-fold lower (Fig. 4b). Hence,
we conclude that most studies on pot sizes have focussed on
relatively small pots and have grown plants to larger sizes
than is common in ecophysiological experiments. In contrast,
experiments in plant biology generally use relatively larger pots
and harvest plants at younger stages, when the BVR value is still
below 8 g L
1
.
0.01
0.1
1
10
100
(a)
(b)
9.3
0.5
1.6
2.5
3.0
2.0
10.5
Pot size (L)
1.2
0.01
0.1
1
10
100
Herb.
Woody
WoodyWoody
Metaphenomics
database
BVR (g L
–1
)
Pot size
experiments
Herb. Herb.
Fig. 4. (a) Distribution of pot volumes as represented in the current meta-
analysis of pot size studies and in a compilation of ~800 studies on the effect
of 12 environmental factors on growth and related ecophysiological traits
(meta-phenomics database, Poorter et al.2010). (b) Total plant biomass : pot
volume ratios (BVR) in the current meta-analysis and the meta-phenomics
database. The distribution is characterised by boxplots (see legend Fig. 2).
Blue boxes indicate the values for herbaceous species, red ones for woody
species. Numbers above/below each box show the median values.
0.1 1.0 10.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Scaled total dry mass (g g–1)
BVR (
g
L–1)
–20
0
20
40
60
80
100
120
(a)
(b)
>201–2 2–5 5–20<1
% Change in biomass with 100%
increase in pot size
48
53
38
21
7
Fig. 5. (a) Distribution of the percentage change in biomass with a
doubling in pot size for a total of ~80 experiment species combinations.
Distributions are characterised by boxplots and given separately here for
ve classes of BVR (total plant biomass : pot volume ratio). The ANOVA was
highly signicant (P<0.001), with an r
2
of 0.22. The (rounded) number
of observations in the ve classes are 30, 30, 90, 200 and 80 respectively.
(b) The total dry mass of plants in various pot sizes, scaled to the biomass
of the plants in the largest pot, plotted against BVR. Different lines indicate
data from different experiments or different harvests within an experiment.
844 Functional Plant Biology H. Poorter et al.
Most experiments with pots are conducted to eventually
understand how (agro-)ecosystems function. It may therefore
be relevant to consider what normal BVRvalues are in the eld.
Maximum dry matter yield of major crops varies between
10003000 g m
2
(Unkovich et al.2010; assuming 20%
biomass in roots). If we assume a rooting depth of 1 m, this
would correspond to BVR values in the range of 13gL
1
. Given
that at least half of the DM production takes place during
grain lling, we can expect that during the vegetative state, the
BVR value will not exceed 1.5 g L
1
. Similar calculations for
natural ecosystems are more difcult as large variation exists
in root depth and the standing biomass. Given a rooting depth
of 0.35 m (Nagel et al.2012) and a density of 500 plants per m
2
,
aeld of Arabidopsis thaliana Heynh. plants of 0.1 g dry mass,
would have a BVR of 0.15 g L
1
. Although we realise that plants
in the eld experience conditions that are very different from
those where plants are grown singly in pots in controlled
conditions, we conclude from these rough estimates that BVR
values around 1 are of the same order of magnitude as those of
vegetative plants in the eld.
6. Does pot size affect experimental conclusions?
Up to now we have considered the effect of pot size per se. Most
researchers are also interested in whether the outcome of their
experiments is affected by the choice of the pot size. Arp (1991)
was one of the rst to draw attention to the fact that pot size
might restrict the response to elevated CO
2
. This would limit the
possibilities to draw conclusions from experiments that have been
conducted in this eld.
The analysis by Arp (1991) was a compilation of different
studies that worked with different pot sizes. Kerstiens and Hawes
(1994), however, published a meta-analysis of the results of a
range of studies with trees in which they show that the biomass
responses to elevated CO
2
did not correlate with pot size, or even
decreased with a BVR over 18 g L
1
. This suggests that small
pot sizes do not reduce responses to elevated CO
2
universally.
However, in both meta-analyses the evidence could only be
circumstantial, as pot size was not an experimental factor in
the compilations. Here we analysed experiments where pot size
was specically included in the experimental design, not only
for interaction with CO
2
but also for nutrients, water and
irradiance (Fig. 6). Given that nutrient and water availability
already increase when pot size is increased, we would expect
less additional effect on plant growth if more nutrients or water
were supplied. However, we would expect increasingly stronger
growth responses with larger pot size when light or CO
2
would
be increased, simply because of the higher demand for nutrients
and water in larger plants. Although some of the experiments do
indeed follow the expected trend, results are not equivocal. As
results depend on the variability in at least four different harvests,
the number of experiments is likely too small to draw any strong
conclusion.
Besides possible interactions with abiotic factors, interactions
with biotic factors have been shown. For example, vesicular
arbuscular mycorhizae (VAM) infection rates, which would
normally increase with reduced nutrient availability, are
reduced in small pots (Bååth and Hayman 1984; Koide 1991).
As a consequence, VAM colonisation is less benecial for
nutrient uptake and results in smaller growth increases when
small pots are used (Kucey and Janzen 1987; Koide 1991).
Similarly, Baldwin (1988) showed that pot-bound Nicotiana
plants do not respond to leaf damage, whereas repotted plants
do. Thus, although we cannot draw a rm conclusion here we
suggest that the use of small pots with crowded roots carries a
substantial risk of inuencing the experimental results.
7. Other considerations
This analysis focussed on the effect of pot volume on plant
growth. However, choosing a pot for an experiment not only
includes choosing the right volume, but also the right shape.
Although shape is less important than volume (McConnaughay
et al.1993), shallow and deep-rooting species may respond
differently to the actual diameter and height of the pots, at
equal pot volume (von Felten and Schmid 2008). Pot height is
also an important factor in determining the free-draining water
content of pots and thereby the water potential as well as the
oxygen availability in the pots (Passioura 2006).
An alternative to standard plastic pots are containers that have
ribbed inner sides and small air holes. Such containers promote
self-pruning of roots close to the holes, which avoids root
spiralling and promotes development of lateral roots (Rune
2003). Not only shape, but also the material (Bunt and
Kulwiec 1970) and the colour of the pot (Markham et al.
2011) may affect plant growth, mainly through their effect on
soil and root temperature. For a broader discussion on the use of
pots for growing plants in the context of experimental setup see
Poorter et al.(2012a).
Conclusions
A meta-analysis of the effects of pot size on growth shows
that on average a doubling of the pot size results in 43% more
biomass. In most cases reduced growth in small pots will be
caused by a reduction in net photosynthesis. It is the plant
0.1 1 10 100
0.7
1
2
3
4
5
Ratio biomass high vs. low (g g–1)
Pot size (L)
Irradiance
CO2
Nutrients
Water
Fig. 6. Interaction of pot size with various abiotic factors. Results are
given for total plant biomass and are expressed as the ratio between the
biomass at a high level of that factor and the lower level. Data are from
Bilderback (1985), Thomas and Strain (1991), McConnaughay et al.(1993),
NeSmith (1993), Ismail et al.(1994), Nobel et al.(1994), Barrett and Gifford
(1995), Will and Teskey (1997), Houle and Babeux (1998), Centritto (2000)
and R. Pierik (unpublished data).
Pot size matters Functional Plant Biology 845
mass per unit rooting volume that is relevant rather than pot size
per se. Large plant mass per pot volume not only reduces growth
of plants but also carries the risk of inuencing the
relative differences between treatments. We conclude that it is
important for researchers to minimise such effects by choosing
pots that are large enough for their plants, even at later stages of
growth. Our advice is to avoid plant biomass to pot volume
ratios larger than 2 gL
1
and preferably work with plant and pot
sizes where this ratio is <1.
Acknowledgements
Damian Barrett, Phil Grime, Joyce Latimer, Liesje Mommer and Ronald
Pierik kindly provided (additional) unpublished data for this analysis.
Christian Heinemann enlightened us with the mathematical aspects and
comments of Liesje Mommer, Thijs Pons, Marc Faget as well as two
anonymous reviewers on a previous version of the ms are greatly
appreciated as well.
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7, 364368; Climent et al. (2011) Eur. J. Forest Res. 130, 841850; Nord et al. (2011) Funct. Plant Biol. 38, 941952; Mommer and
De Kroon (pers. comm.); Pierik (pers. comm.).
B. Hydroponically grown plants
Richards and Rowe (1977) Ann. Bot. 41, 729740; Carmi et al. (1983) Photosynthetica 17, 240245; Tschaplinski and Blake (1985)
Physiol. Plant. 64, 167176; Hameed et al. (1987) Ann. Bot. 59, 685692; Peterson et al. (1991) J. Exp. Bot. 42, 12331240; Thomas
(1993) Plant Growth Reg. 13,95101; Ternesi et al. (1994) Plant Soil 166,3136; Bar-Tal et al. (1995) Sci. Hortic. 63, 195208; Bar
and Pressman (1996) J. Amer. Soc. Hort. Sci. 121, 649655; Kharkina et al. (1999) Physiol. Plant. 105, 434441; Xu et al. (2001)
J. Plant Nutr. 24, 479501.
848 Functional Plant Biology H. Poorter et al.
Appendix 2
For this meta-analysis we screened the literature of the last 100 years. A total of 63 publications plus two additional unpublished
experiments dealt with plants grown in pots of various sizes, 11 with plants grown in hydroponics with different levels of root
connement. The references are listed in Appendix 1. As we were interested in not only the physical aspect of container volume, but
also in the resources that come with it, we compared pot size treatments based on size, including the possible additional benets of
increased nutrient and water availability. The experiments with hydroponically-grown plants are not included in the main analysis.
Differences in the shape of the pots were not independently analysed either. In several publications only pot diameter was reported.
For a sample of 30 different pots ranging in diameter (Ø) from 7 to 40 cm we derived an estimate for pot volume (V) based on the
empirical equation V=p(Ø/2)
2
(0.46 + 0.8397 Ø0.002307 Ø
2
). If additional data were missing, we assumed pots to be lled
with substrate up to the rim.
For each species or genotype in a given experiment, we determined the biomass at the last harvest and separated this variable in
biomass of leaves, stems, roots and reproductive mass as far as data were provided. To capture the importance of pot size for growth we
calculated for each experiment the proportional increase in total plant dry mass relative to the proportional increase in pot size, by
calculating the slope of the lines that were tted through the observed log-transformed plant masses and pot volumes, separately for
each species in each experiment. Because experiments differed in the range of pot size used, we scaled these slopes in such a way that
the number reects the percent change in biomass (or another variable) given a doubling in pot size (see Appendix 3 for more details).
For a more detailed analysis, log-transformed data from experiments which included three or more pot sizes were also tted with a
saturating equation, as described in Appendix 3 and calculated with the nls procedure in R (R Development Core Team 2011).
Appendix 3
Let V
1
and V
2
be pot volume 1 and 2 and B
1
and B
2
the total plant dry biomass that is observed at the respective pot volumes:
because proportional differences are the focus of interest, we calculate the slope of the line that connects these points as:
S¼logðB2ÞlogðB1Þ
logðV2ÞlogðV1Þ;ð1Þ
Suppose we want to know the fraction fby which plant mass increases when pot size doubles. Then
B2¼fB1;ð2Þ
and
V2¼2V1:ð3Þ
Assuming a common slope sover the whole trajectory of pot masses considered, this results then in
S¼logðfB
1ÞlogðB1Þ
logð2V1ÞlogðV1Þ¼logðfB
1
B1Þ
logð2V1
V1Þ¼logðfÞ
logð2Þ;ð4Þ
and, after rearrangement
f¼10slogð2Þ¼ðloglogð2ÞÞS¼2S
:ð5Þ
The same procedure applies if slope s is determined by linear regression through more than two points.
To relate the slope of the line to the observed values of total plant mass per unit pot volume (BVR), we took the following approach:
In cases where an experiment consisted of two pot sizes, a linear regression as above was calculated and the resulting slope attributed
to both points. In case an experiment consisted of more than two pot sizes and a second order polynomial showed no signicant
saturating trend, we tted a straight line through all points. Otherwise, a saturating curve was tted through the points, of the form
B¼aPþb
Pþc;ð6Þ
with the constraint that a,band cshould be positive. The slope of the line at each pot size is then given by the derivative:
S¼ac b
Vþc:ð7Þ
After the slope and fwere calculated for each pot volume in each experiment, data were binned in ve categories of BVR.
Pot size matters Functional Plant Biology 849
Appendix 4.
We measured the root distribution of Hordeum vulgare (barley) and Beta vulgaris (sugar beet), grown in a glasshouse in cylindrical
containers with a volume of 1.3 L (length 26cm, inner diameter 8 cm). Measurements were done by non-destructive imaging of roots
using nuclear magnetic resonance imaging (MRI) as described extensively by Jahnke et al.(2009). This method is able to detect roots
with a diameter down to 300 mM, which implies that ne roots go unnoticed. Image segmentation of the root system was done by
thresholding the image and removing the stem and the sugar beet, with the RegionGrowingMacro module in MeVisLab (ver. 2.2.1;
MeVisLab, Bremen, Germany). After segmentation, the pixel values were integrated for the inner and outer regions of the soil core and
divided by the integral over the whole pot.
850 Functional Plant Biology H. Poorter et al.
www.publish.csiro.au/journals/fpb
... Symbiotic associations between plants and fungi can vary from parasitic to mutualistic depending on environmental conditions (Allen et al., 1993;Johnson and Graham, 2013;Konvalinková and Jansa, 2016;Mandyam and Jumpponen, 2015). For fungi such as DSE and AMF that can improve plant-nutrient uptake, the benefit of the association may not occur if plants are growing in small pots that restrict the fungal ability to search for nutrients (Allen et al., 2003;Jumpponen, 2001;Poorter et al., 2012). ...
... A meta-analysis by Poorter et al. (2012) indicated that a plant-biomass to pot volume ratio higher than 1 g L -1 leads to growth limitations due to the small size of the pot. For the cone-tainer experiment, the proportion of A. tridentata biomass to pot volume at the time of harvest was approximately 1 g L -1 , suggesting that the pot size was adequate for the analysis of growth responses. ...
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