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Citation: Ahrens, A.; Llewellyn, D.;
Zheng, Y. Longer Photoperiod
Substantially Increases Indoor-Grown
Cannabis’ Yield and Quality: A Study
of Two High-THC Cultivars Grown
under 12 h vs. 13 h Days. Plants 2024,
13, 433. https://doi.org/10.3390/
plants13030433
Academic Editor: Keith R. Davis
Received: 7 January 2024
Revised: 26 January 2024
Accepted: 30 January 2024
Published: 1 February 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
plants
Article
Longer Photoperiod Substantially Increases Indoor-Grown
Cannabis’ Yield and Quality: A Study of Two High-THC
Cultivars Grown under 12 h vs. 13 h Days
Ashleigh Ahrens , David Llewellyn and Youbin Zheng *
School of Environmental Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada;
aahrens@uoguelph.ca (A.A.); dllewell@uoguelph.ca (D.L.)
*Correspondence: yzheng@uoguelph.ca
Abstract: Indoor-grown Cannabis sativa is commonly transitioned to a 12 h daily photoperiod to
promote flowering. However, our previous research has shown that some indoor-grown cannabis
cultivars can initiate strong flowering responses under daily photoperiods longer than 12 h. Since
longer photoperiods inherently provide higher daily light integrals (DLIs), they may also increase
growth and yield. To test this hypothesis, two THC-dominant cannabis cultivars, ‘Incredible Milk’
(IM) and ‘Gorilla Glue’ (GG), were grown to commercial maturity at a canopy level PPFD of
540
µ
mol
·
m
−2·
s
−1
from white LEDS under 12 h or 13 h daily photoperiods, resulting in DLIs
of 23.8 and 25.7 mol
·
m
−2·
d
−1
, respectively. Both treatments were harvested when the plants in the
12 h treatment reached maturity according to established commercial protocols. There was no delay
in flowering initiation time in GG, but flowering initiation in IM was delayed by about 1.5 d under
13 h. Stigma browning and trichome ambering also occurred earlier and progressed faster in the 12 h
treatment in both cultivars. The vegetative growth of IM plants in the 13 h treatment was greater
and more robust. The inflorescence yields were strikingly higher in the 13 h vs. 12 h treatment,
i.e., 1.35 times and 1.50 times higher in IM and GG, respectively, which is 4 to 6 times higher than
the relative increase in DLIs. The inflorescence concentrations of major cannabinoids in the 13 h
treatment were either higher or not different from the 12 h treatment in both cultivars. These results
suggest that there may be substantial commercial benefits for using photoperiods longer than 12 h
for increasing inflorescence yields without decreasing cannabinoid concentrations in some cannabis
cultivars grown in indoor environments.
Keywords: daylength; medicinalcannabis; lighting; controlled environment agriculture; flower initiation
1. Introduction
Cannabis sativa (hereafter, cannabis) is a dioecious annual that has been domesticated
worldwide for medicinal, textile, recreational, and nutritional uses. Cannabis is well known
for the cannabinoids it produces such as tetrahydrocannabinol (THC) and cannabidiol
(CBD), which are produced in high abundance in stalked glandular trichomes associated
with unfertilized floral tissues of female plants (Small, 2016) [1].
Most cannabis cultivars that are grown commercially in indoor environments are drug-
type (i.e., THC content > 0.3%), due to the relatively high value of their mature female floral
tissues. Indoor-grown cannabis cultivars typically have short-day photoperiod responses,
whereby daylengths greater than 16 h (i.e., uninterrupted dark periods
≤
8 h) will keep
plants in the vegetative growth stage while shortening the photoperiod below some critical
level will transition plants toward producing generative tissues (i.e., flowering initiation).
The robustness of photoperiodic cannabis’ flowering response tends to increase as
the daylength continues to be reduced below the critical photoperiod. Many modern
drug-type cannabis cultivars have been developed by hybridizing cannabis cultivars grown
from a broad range of latitudes with different seasonal photoperiod dynamics (Small,
Plants 2024,13, 433. https://doi.org/10.3390/plants13030433 https://www.mdpi.com/journal/plants
Plants 2024,13, 433 2 of 15
2022) [2]. Therefore, modern cannabis cultivars may have considerable variability in their
optimum daylengths for promoting robust flowering responses and maximizing yield
and quality. The range of photoperiodic responses has been well documented in hemp
cultivars (e.g., Zhang et al., 2021) [
3
], but the studies by Peterswald et al. (2023) [
4
] and
Ahrens et al. (2023) [
5
] are the only recent studies on photoperiodic responses of modern
indoor-grown cannabis cultivars. Indoor-grown cannabis crops are almost universally
switched to a 12-h daily photoperiod to initiate the flowering stage of production (Potter,
2014) [
6
]. This optimizes operational simplicity in cultivation systems that grow many
different cultivars concurrently. However, recent studies have shown that some modern
indoor-grown cannabis cultivars have robust flowering responses to moderately longer
photoperiods (Peterswald et al., 2023; Ahrens et al., 2023; Potter, 2009) [
4
,
5
,
7
]. Longer
photoperiods have the advantages of either (1) increased daily light integrals (DLIs), which
may result in higher yields (Rodriguez-Morrison et al., 2020) [
8
], or (2) lower installed
lighting levels (e.g., reduced fixture density), which reduces upfront lighting infrastructure
and installation costs.
Our recent study demonstrated that about half of the ten investigated cultivars had
minimal delays in flowering initiation in photoperiods up to 13.5 h (Ahrens et al., 2023) [
5
].
However, this study ended well before the cultivars reached normal commercial maturity.
Therefore, the effects of longer photoperiods on mature inflorescence yield and quality are
still unknown. The objective of the present study was to compare the yield and quality of
two cultivars (selected from the study by Ahrens et al. (2023) [
5
]) grown to commercial
maturity under 12 h or 13 h photoperiods. The hypotheses were that inflorescence yield
would be proportional to the DLI associated with each photoperiod and that there would
be no photoperiod treatment effects on the concentrations of the major cannabinoids.
2. Materials and Methods
2.1. Experimental Design and Plant Cultivation
This trial was conducted in a walk-in growth chamber at the University of Guelph
with two rows of three compartments separated by opaque white curtains to prevent inter-
compartment light contamination, as described in the study by Ahrens et al. (2023) [
5
]. Two
full spectrum LED fixtures (Jungle—LED G4i 1200, Allstate Garden Supply, Ontario, CA,
USA) were hung in each compartment on height-adjustable pulley systems. These fixtures
have onboard dimmers with fixed settings of 25%, 50%, 75%, and 100% of maximum
intensity. The relative spectral photon flux distribution of the LED fixtures is provided in
Figure S1, which was the same as in the study by Ahrens et al. (2023) [5].
Two cannabis cultivars, ‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG), were sourced
from a single commercial cultivator in Southwestern Ontario. Seventy-eight stem tip cuttings
were taken from vegetative mother plants of each cultivar and inserted into
3.6 ×4.0 cm
round rockwool plugs (Macroplug; Grodan, Milton, ON, Canada) on 7 November 2022 at
the cultivator’s facility. The rooted cuttings were delivered to the University of Guelph on
29 November 2022 and transplanted on 1 December 2022 into presoaked 10
×
10
×
7.5 cm
rockwool blocks (Grodan GRO-BLOCK Improved GR7.5 Medium 4”; Grodan). Transplants
were grown vegetatively (18 h light/6 h dark) with the LEDs at 85 cm above the canopy. The
dimmers were set at 25% on 29 November 2022, which provided an average canopy-level
photosynthetic photon flux density (PPFD) of
≈135 µmol·m−2·s−1
. Dimmer settings were
incrementally increased every two days until reaching 100%
(≈540 µmol·m−2·s−1)
on 5
December 2022.
Eighteen uniformly sized plants of each cultivar were randomly assigned to one of
two photoperiod treatments on 5 December 2022: 12 h light/12 h dark (12 h) or 13 h
light/11 h dark (13 h). All LED fixtures were set to maximum intensity, resulting in
uniform PPFD of
≈
540
µ
mol
·
m
−2·
s
−1
at an 85 cm hang height. The daily light integrals
for the 12 h and 13 h photoperiods were 23.8 and 25.7 mol
·
m
−2·
d
−1
, respectively. The
photoperiod treatments were randomly assigned to individual compartments, resulting
in 3 concurrent replications of each treatment. Three plants of each cultivar were placed
Plants 2024,13, 433 3 of 15
onto presoaked
100 ×15 ×7.5 cm
rockwool slabs (Vital; Grodan), one slab per cultivar,
in each compartment. The locations of each cultivar’s slab were randomized in each
compartment (Figure S2). Each slab was centered in a subirrigation tray (53
×
109 cm
grow trays, Botanicare, Vancouver, WA, USA). The plants were 35 cm apart on the slab and
70 cm apart between slabs measured ‘on center’ (Figure S3). The plants were subirrigated
as needed for the first 9 d after the photoperiod treatments began and then drip-irrigated
henceforth. One irrigation dripper (Supertif PCND-MOP 1.1 L
·
h
−1
; Rivulis, Gvat, Israel)
was placed in each rockwool block, and two drippers were placed in each rockwool slab,
centered between adjacent plants (Figure S3). There were eight irrigation events per day,
with event lengths increased, as needed, to maintain a daily leachate percentage of at
least 15%. The first irrigation event occurred
≤
1 h after lights turned on, followed by 1 h
rest periods between the next five irrigations and 2 h rest periods between the last two
irrigations. The length of individual irrigations was the same throughout any given day
but increased from 120 s at the beginning of the trial to 420 s by the end, to maintain
≥
15%
daily leachate levels. The cannabis flowering nutrient recipe from Zheng (2022) [
9
] was
used, mixed in deionized water to an electrical conductivity of 2.0 mS
·
cm
−1
, and pH was
adjusted to 5.7 with a 1.0 M potassium bicarbonate (Master Plant-Prod Inc., Brampton, ON,
Canada).
The chamber temperature was set at a constant 25
◦
C, and relative humidity (RH) was
set at 70% light/60% dark, which was controlled with a fogging system connected to the
climate computer (Titan I/O Plus, Argus Control Systems Limited, Surrey, BC, Canada).
The LEDs were turned on daily at 09:00 and turned off at 21:00 and 22:00 for the 12-h and
13-h treatments, respectively. The photoperiod was controlled with the computer. The
curtains at the front of each compartment were opened daily at 09:00 and closed daily
at 21:00. The LED fixtures were raised weekly to maintain the 85 cm distance between
the canopy and the LEDs. There was no CO
2
supplementation in this trial, but ample air
exchanges ensured CO2concentrations were consistently >400 PPM during lit periods.
2.2. Data Collection
Temperature and relative humidity data were logged every 300 s using three datalog-
gers (MX2301A; HOBO; Onset Computer Corporation; Bourne, MA, USA), with each logger
assigned to one replicate compartment from each treatment. The loggers were switched
between their respective compartments daily. The day and night aerial temperature and
RH for each compartment are summarized in Table S1.
Elapsed days to flowering (EDTF), which is defined as the number of days between
invoking the photoperiod treatments and the appearance of
≥
3 pairs of stigmas (i.e., an
inflorescence) at the top of the primary shoot, was monitored daily starting at 1 d after the
photoperiod treatments began. The apical inflorescence was defined as the conglomeration
of inflorescence and foliar tissues (i.e., sugar leaves) with no visible gaps located at the top
of the primary shoot. The percentage of brown stigmas and amber trichomes on the apical
inflorescence were monitored daily to track inflorescence maturity, beginning 37 d after
photoperiod treatments began.
The harvesting of each cultivar began after >95% of the stigmas in either photoperiod
treatment had turned brown, and either 10% of the trichomes had turned to an amber color
(IM) or the fan leaves began senescing (GG), whichever occurred first. The harvesting
of IM and GG began on days 58 and 72, respectively, after initiating the photoperiod
treatments. One replicate of each treatment (i.e., six plants) was harvested per day, for three
successive days.
The harvest process followed the study by Ahrens et al. (2023) [
5
] with the addition
of separating the vegetative tissues into fan leaves, sugar leaves, and stems. To differ-
entiate between fan and sugar leaves, any leaf with
≥
1 cm of petiole visible when the
longitudinal axis of the plant was at the nadir with respect to the observer (i.e., looking
straight downwards from the top of the plant) was considered a fan leaf. Prior to cutting the
plant at the substrate surface (i.e., the top of the rockwool blocks), plant height and width
Plants 2024,13, 433 4 of 15
(at the widest point and perpendicular to this) were measured to determine the growth
index following the study by Ruter (1992) [
10
]. The apical inflorescences were separated
from the rest of the plants to determine their volume (following Ahrens et al., 2023 [
5
])
and fresh weight (FW). The apical inflorescence FW was divided by volume to determine
apical inflorescence density (g
·
cm
−3
). The apical inflorescence of the most representative
plant from each photoperiod by cultivar combination in each replicate was air-dried at
23
◦
C and 70% RH to approximately 10% moisture content. During the harvesting process,
photographs were taken of the whole plant, the whole plant with fan leaves removed,
and the excised apical inflorescence of the most representative plant in each photoperiod
by cultivar combination in each replicate. The FW of the fan leaves, sugar leaves, stems,
and the nonapical inflorescence tissues of each plant were recorded. The harvest index
was calculated as follows: total inflorescence FW/(total inflorescence FW + aboveground
vegetative FW). The separated tissues of each photographed plant were dried to constant
weight at 70
◦
C. The representative plants’ tissue dry weights (DWs) were then used to
calculate the moisture content of each tissue type, which were then used to estimate the
DWs of the remaining plants in each respective cultivar by photoperiod combination. All
tissue weights were measured in grams using a digital balance (BCE2202-1S; Sartorius Lab
Instruments, Göttingen, Germany).
2.3. Cannabinoid Analysis
The air-dried apical inflorescences of all of the representative plants were sent to a
third-party lab (A&L Canada Laboratories Inc., London, ON, Canada) for the analysis of
moisture content (i.e., loss on drying) and the concentrations of the following cannabinoids:
cannabigerolic acid (CBGA), cannabigerol (CBG), cannabidiolic acid (CBDA), cannabidiol
(CBD), cannabinolic acid (CBNA), cannabinol (CBN), cannabidivarinic acid (CBDVA),
cannabidivarin (CBDV),
∆9
-tetrahydrocannabinolic acid (THCA),
∆9
-tetrahydrocannabinol
(THC), tetrahydrocannabivarinic acid (THCVA), and tetrahydrocannabivarin (THCV). The
total equivalent THC (T-THC) is an estimation of the amount of THC available to the
consumer, once THCA has been decarboxylated into THC, such as from adding heat.
The equation used is T-THC = (THCA
×
0.877) + THC. The limit of quantitation (i.e.,
LOQ) for these analyses was reported as <0.05% for each cannabinoid. All cannabinoid
concentrations are reported as the percentage of dry weight, based on the loss-on-drying
determination of the sampled tissues’ moisture content. The total inflorescence dry weight
was calculated by adding nonapical inflorescence dry weight (oven-dried) and apical
inflorescence dry weight (loss on drying) for each plant.
2.4. Statistical Analysis
The experiment was a completely randomized design, with two treatments and three
replications. Statistical analysis was performed using RStudio (v2021.9.0.351; Posit Soft-
ware, PBC; Boston, MA, USA). The parameter means of each replicate in each cultivar by
treatment combination were analyzed for the EDTF, harvest, and postharvest measure-
ments (n= 3). Each sample was considered a replicate in the cannabinoid data (n= 3).
Unpaired, two-tailed Student’s t-tests were performed between the 12 h and 13 h treatments
for each parameter by cultivar combination. Statistical significance was determined at the
p≤0.05 level.
3. Results
3.1. Elapsed Days to Flowering (EDTF)
The initiation of flowering of IM was delayed in the 13 h treatment by approximately
1.5 d, but there were no photoperiod treatment effects on EDTF in GG (Figure 1). However,
the rate of early inflorescence development appeared to be slightly delayed in the 13 h
treatment in both cultivars (Figure 2). Stigma browning was substantially delayed in the
13 h treatment in both cultivars (Figure 3).
Plants 2024,13, 433 5 of 15
Figure 1. Elapsed days to flowering (EDTF) responses to 12 h (filled bars) and 13 h (empty bars)
photoperiod treatments of C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG). Data are
means
±
SE, n= 3. Error bars are presented for all data but may be obscured for small SE values.
Percentage values marked in the 13 h bars signify the increase in EDTF relative to the 12 h treatment,
and the p-values above each cultivar represent the significance level of the comparison of means
according to Student’s t-test.
Figure 2. Representative photos showing early apical inflorescence development of C. sativa cultivars
‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG) under 12 h and 13 h photoperiod treatments from 14 d
to 21 d after the start of photoperiod treatments.
Plants 2024,13, 433 6 of 15
Figure 3. Temporal dynamics of stigma browning on the apical inflorescence of the primary shoot
of C. sativa cultivars ‘Incredible Milk’ (IM, black) and ‘Gorilla Glue’ (GG, red) grown under 12 h
(triangles) and 13 h (circles) photoperiod treatments. Data are means
±
SE of three replicates (n= 3).
Error bars are presented for all data but may be obscured for small SE values.
3.2. Inflorescence Yield
There were no photoperiod treatment effects on apical inflorescence DW in GG, but
the apical inflorescence DW in IM more than doubled under the 13 h vs. 12 h treatments
(Figures 4and 5).
Figure 4. Dry weight responses of individual plants’ apical inflorescence to 12 h (filled bars) and 13 h
(empty bars) photoperiod treatments of C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’
(GG). Data are means
±
SE, n= 3. Error bars are presented for all data but may be obscured for small
SE values. Percentage values marked in the 13 h bars signify the increase in apical inflorescence dry
weight relative to the 12 h treatment, and the p-values above each cultivar represent the significance
level of the comparison of means according to Student’s t-test.
There were no photoperiod treatment effects on apical inflorescence density in GG,
but the apical inflorescence density of IM was 36% lower in the 13 h treatment (Figure 6).
The total inflorescence DW was 35% higher in IM and 50% higher in GG in the 13 h vs.
12 h treatments (Figure 7).
Plants 2024,13, 433 7 of 15
Figure 5. Images of the apical inflorescence of the primary shoot of representative plants at harvest of
C. sativa ‘Incredible Milk’ (IM) plants on day 58 and of ‘Gorilla Glue’ (GG) on day 72 after the start of
the photoperiod treatments. The white scale bars in each image are 5 cm.
Figure 6. Apical inflorescence density responses to 12 h (filled bars) and 13 h (empty bars) photoperiod
treatments of C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG). Data are
means ±SE
,
n= 3. Error bars are presented for all data but may be obscured for small SE values. Percentage
values marked in the 13 h bars signify the decrease in apical inflorescence density relative to the 12 h
treatment, and the p-values above each cultivar represent the significance level of the comparison of
means according to Student’s t-test.
Plants 2024,13, 433 8 of 15
Figure 7. Total inflorescence dry weight responses (g/plant) to 12 h (filled bars) and 13 h (empty bars)
photoperiod treatments of C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG). Total
inflorescence dry weight was calculated by adding nonapical inflorescence dry weight (oven-dried)
and apical inflorescence dry weight for each plant. Data are means
±
SE, n= 3. Error bars are
presented for all data but may be obscured for small SE values. Percentage values marked in the
13 h bars signify the increase in total inflorescence dry weight relative to the 12 h treatment, and the
p-values above each cultivar represent the significance level of the comparison of means according to
Student’s t-test.
3.3. Apical Inflorescence Cannabinoid Concentrations
Both cultivars in this study had relatively low CBD and high THC concentrations
(Table 1). The CBGA and CBDA concentrations were 53% and 19% higher, respectively,
in the 13 h treatment in IM. There were no photoperiod treatment effects on CBG concen-
tration in either cultivar. The THCA concentration was 10% higher in the 13 h treatment
in IM. There were no photoperiod treatment effects in
∆9
-THC concentration in either
cultivar. The T-THC concentration in IM was 9% higher in the 13 h treatment, but there
were no photoperiod treatment effects on T-THC content in GG (Figure 8). The THCVA
concentrations were 22% and 16% higher in the 13 h vs. 12 h photoperiod treatments in IM
and GG, respectively. All other measured cannabinoids were below the limit of quantitation
(i.e., <0.05%).
Figure 8. Total equivalent THC (T-THC) concentrations (% of dry weight) in the apical inflorescence
responses to 12 h (filled bars) and 13 h (empty bars) photoperiod treatments of C. sativa cultivars,
‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG). Data are based on the loss-on-drying determinations
of the water content of the sampled tissues. Data are means
±
SE, n= 3. Error bars are presented for
all data but may be obscured for small SE values. Percentage values marked in the 13 h bars signify
the increase in T-THC relative to the 12 h treatment, and the p-values above each cultivar represent
the significance level of the comparison of means according to Student’s t-test.
Plants 2024,13, 433 9 of 15
Table 1. Responses of the apical inflorescences’ cannabinoid concentrations (% of dry weight) to
12 h and 13 h photoperiod treatments for C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’
(GG). Data are based on the loss-on-drying determination of sampled tissues’ water content. Data
are means
±
SE, n= 3. Percent change values signify the change in cannabinoid content of the 13 h
treatment relative to the 12 h treatment. The p-values are according to Student’s t-test.
Cultivar Cannabinoid z12 h 13 h Percent
Change p-Value y
IM
CBGA 1.34 ±0.025 2.05 ±0.025 53% <0.0001
CBG 0.14 ±0.007 0.13 ±0.006 −1% 0.8870
CBDA 0.07 ±0.001 0.08 ±0.001 19% <0.0001
CBD <LOQ x<LOQ
CBNA <LOQ <LOQ
CBN <LOQ <LOQ
CBDVA <LOQ <LOQ
CBDV <LOQ <LOQ
THCA 28.7 ±0.67 31.5 ±0.16 10% 0.0152
THC 0.51 ±0.020 0.45 ±0.065 −13% 0.3886
THCVA 0.15 ±0.007 0.18 ±0.007 22% 0.0325
THCV <LOQ <LOQ
GG
CBGA 0.90 ±0.059 1.13 ±0.059 25% 0.0539
CBG 0.12 ±0.006 0.13 ±0.006 11% 0.1998
CBDA
0.05
±
0.001 *
0.06 ±0.004 15% 0.1790
CBD <LOQ <LOQ
CBNA <LOQ <LOQ
CBN <LOQ <LOQ
CBDVA <LOQ <LOQ
CBDV <LOQ <LOQ
THCA 25.1 ±0.77 27.3 ±0.51 9% 0.0735
THC 0.24 ±0.018 0.26 ±0.001 6% 0.4573
THCVA 0.11 ±0.004 0.12 ±0.001 16% 0.0101
THCV <LOQ <LOQ
z
CBGA, cannabigerolic acid; CBG, cannabigerol; CBDA, cannabidiolic acid; CBD, cannabidiol; CBNA,
cannabinolic acid; CBN, cannabinol; CBDVA, cannabidivarinic acid; CBDV, cannabidivarin; THCA,
∆9
-
tetrahydrocannabinolic acid; THC,
∆9
-tetrahydrocannabinol; THCVA, tetrahydrocannabivarinic acid; THCV,
tetrahydrocannabivarin;
y
p-values are according to Student’s t-test between photoperiod treatments;
x
these
cannabinoids were below the limit of quantification (<LOQ); * n= 2 for this cultivar by photoperiod combination.
3.4. Vegetative Growth, Weight of Aboveground Tissues, and Harvest Index
The plants of both cultivars appeared larger in the 13 h vs. 12 h photoperiod (Figure 9).
With the fan leaves removed, the plants of both cultivars in the 13 h treatment appeared to
have more robust branches with stronger support for the inflorescence tissues (Figure 10),
despite also bearing the weight of substantially more inflorescence biomass (Figure 6).
The dry weights (DWs) of fan leaves, stem tissues (including side branches), and the total
aboveground vegetative biomass were 22%, 65%, and 28% higher in the 13 h treatment in IM
(Figure 11). There were no photoperiod treatment effects on the DWs of any aboveground
vegetative tissues in GG. The growth index of IM was 81% higher in the 13 h treatment
(Table 2). The harvest index of GG was 6% higher in the 13 h treatment. The total dry
weights of aboveground tissues (i.e., including inflorescence tissues) were 32% higher in
IM and 41% in GG in the 13 h treatment.
Plants 2024,13, 433 10 of 15
Figure 9. Whole-plant images taken just before the harvest of representative plants of C. sativa
‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG) on days 58 and 72, respectively, after the start of the
photoperiod treatments. The white scale bars in each image are 15 cm.
Table 2. Growth parameter responses (mean
±
SE, n= 3) of C. sativa cultivars ‘Incredible Milk’
(IM) and ‘Gorilla Glue’ (GG) to 12 h and 13 h photoperiod treatments. Harvest index and total
aboveground tissue data are based on dry weight (DW).
Cultivar Parameter 12 h 13 h Increase (%) zp-Value y
IM
Growth index x807 ±88.7 1460 ±69.6 81 0.0045
Harvest index w0.57 ±0.001 0.59 ±0.006 2 0.0844
Total aboveground
DW (g/plant) 128 ±3.2 169 ±5.3 32 0.0027
GG
Growth index 626 ±121.2 972 ±42.9 55 0.0543
Harvest index 0.57 ±0.008 0.60 ±0.005 6 0.0279
Total aboveground
DW (g/plant) 117 ±16.9 165 ±2.5 41 0.0492
z
These values represent the percent increase in the respective parameter in the 13 h vs. 12 h photoperiod treatment;
y
p-values are according to Student’s t-test.
x
Growth index = [height
×
width
1×
width
2
]/300 (Ruter, 1992) [
10
].
wHarvest index = total inflorescence FW/(total inflorescence FW + aboveground vegetative FW).
Plants 2024,13, 433 11 of 15
Figure 10. Whole-plant images with the fan leaves removed of representative plants of C. sativa
‘Incredible Milk’ (IM) and ‘Gorilla Glue’ (GG) on days 58 and 72, respectively, after the start of the
photoperiod treatments. The white scale bars in each image are 15 cm.
Figure 11. Dry weight (mean, n= 3) of aboveground vegetative tissues from individual plants under
12 h and 13 h photoperiod treatments in C. sativa cultivars ‘Incredible Milk’ (IM) and ‘Gorilla Glue’
(GG). Tissues are separated into stems (diagonal pink lines), sugar leaves (horizontal blue lines),
and fan leaves (gray dotted pattern). In each cultivar and each tissue type, photoperiod treatments
bearing the same lowercase letter are not significantly different at p
≤
0.05 according to Student’s
t-test. The p-values above each cultivar represent the significance level of the comparison of the total
aboveground vegetative DW according to Student’s t-test.
Plants 2024,13, 433 12 of 15
4. Discussion
An almost universal practice in indoor cannabis cultivation is to reduce the daily
photoperiod to 12 h to transition vegetative plants to flowering. While this protocol works
for virtually all indoor-grown cannabis cultivars, some cultivars have robust flowering
responses to photoperiods that are modestly longer than 12 h (Peterswald et al., 2023;
Ahrens et al., 2023) [
4
,
5
]. Since higher light levels can increase inflorescence yield and
quality (Rodriguez-Morrison et al., 2021; Llewellyn et al., 2022) [
8
,
11
], longer flowering-
stage photoperiods (with corresponding increases in DLIs) may have positive influences on
cannabis inflorescence yield and quality. Also, in greenhouse production during summer
months in higher latitude regions (e.g., Canada), blackout curtains can remain open longer,
allowing for better utilization of natural light and reductions in some other climate-control-
related costs. A 12 h flowering-stage photoperiod may not be optimized for maximizing
the yield of all cultivars. Hence, cultivators who use a 12 h photoperiod for all cultivars
may be ‘leaving yield on the floor’. The
≥
35% increases in the total inflorescence yield in
the 13 h treatment observed in the present study were similar to the yield increases in the
14 h vs. 12 h photoperiod reported by Peterswald et al. (2023) [
4
], who attributed the higher
yields to increased DLIs. However, in both the present study and that of Peterswald et al.
(2023) [
4
], the observed yield increases under longer photoperiods were disproportionately
higher than the increases in DLIs, contrary to our hypothesis. Other developmental and
morphophysiological responses to longer photoperiods may be contributing to the yield
enhancements.
In outdoor environments, after the summer solstice (i.e., 21 June in the Northern Hemi-
sphere), daily photoperiods gradually reduce a few minutes each day, with daily changes
in photoperiod depending on latitude and time of year. For example, in Guelph, Ontario,
Canada (43.5
◦
N,
−
80.2
◦
W), there are
≈
90 d between the summer solstice (15.5 h) and
12 h days (Sept 26) (Hoffmann, 2024) [
12
]. For short-day plants, the seasonal reductions in
daylength indicate the onset of the end of the growing season; photoperiodic cannabis cul-
tivars respond accordingly by transitioning from vegetative to generative growth, ending
in floral maturation, senescence, and plant death. In contrast to the outdoor environment,
reducing from
≥
16 h directly to 12 h in indoor cannabis cultivation is a relatively power-
ful signal that the end of the growing season (e.g., the onset of killing frost) approaches.
Accordingly, regardless of their provenance, virtually all indoor-grown cannabis cultivars
rapidly initiate flowering under a 12 h photoperiod, normally producing visible inflores-
cence tissues (groupings of ≥3 stigmas) in less than two weeks and mature inflorescences
six to ten weeks later (Peterswald et al., 2023; Potter, 2014; Rodriguez-Morrison et al., 2021;
Llewellyn et al., 2022; Potter and Duncombe, 2012) [4,6,8,11,13].
While some indoor-grown cannabis cultivars can flower under photoperiods longer
than 12 h, increasing the photoperiod may moderate the speed and intensity of the tran-
sition to reproductive growth. This is supported in the present study by the delayed
time to visible inflorescences in IM and slower early floral development in both cultivars.
These patterns were also reported by Ahrens et al. (2023) [
5
] for some cultivars, and by
Zhang et al. (2021)
for essential oil cultivars [
3
]. However, as seen in the present study,
delayed or repressed flowering initiation in the 13 h photoperiod led to enhanced rather
than repressed inflorescence biomass when plants reached commercial maturity. Since
much of the vegetative growth after switching to the flowering photoperiod occurs during
the first few weeks after the transition to short days (Potter, 2014; Yep et al., 2020) [
6
,
14
],
enhancements in vegetative growth during this period increase foliar biomass, probably
enhancing light interception and thus growth potential. Enhanced vegetative growth
during the early phases of the flowering stage may also increase the number of potential
flowering sites and capacity for structurally supporting higher floral biomass, which has
been observed in vegetative-stage cannabis (Moher et al., 2022) [
15
] and in flowering hemp
grown in protected culture (Hall et al., 2014) [
16
]. The plants of both cultivars in the 13 h
treatment in the present study had higher growth indexes and appeared to be larger. How-
ever, increases in the DW of individual vegetative tissues in the 13 h treatment were only
Plants 2024,13, 433 13 of 15
observed in IM. Regardless, the higher inflorescence yields in the 13 h treatment may be a
consequence of generally larger plants, which are known to increase commercially mature
inflorescence yield potential (Bevan et al., 2021) [17].
Despite the early delays in inflorescence development, by the time the plants in the
12 h treatment reached commercial maturity, the total inflorescence yield and the size of the
apical inflorescences were markedly higher in the 13 h treatment in both cultivars. Given
that the increases in inflorescence yield were disproportionately higher than the increase
in DLI, flowering photoperiod management may be one of the most efficacious cultural
practices available to commercial indoor cannabis cultivators for increasing yield that is both
simple and cost-effective to utilize. Furthermore, despite the plants in the 13 h treatment
appearing to have delayed inflorescence maturation rates (e.g., slower stigma browning and
reduced trichome ambering), the cannabinoid composition in the apical inflorescences was
of comparable quality in both treatments in both cultivars. There were moderately higher
CBGA concentrations in the 13 h treatment in IM, which suggests that the inflorescence
tissues in this treatment may have been less mature (Aizpurua-Olaizola et al., 2016) [
18
].
However, there was no detectable CBN in either cultivar, and the neutral form of THC was a
low proportion of total THC, suggesting that neither treatment had been harvested beyond
its optimum maturity level (Aizpurua-Olaizola et al., 2016; Russo, 2007) [
18
,
19
]. The CBDA
levels in IM were also higher in the 13 h treatment. However, as the THCA-to-CBDA
ratios were
≥
400 in both cultivars, treatment effects in CBD content in the cultivars used
in this study are likely not commercially relevant, at least in most markets that favor such
highly THC-dominant cultivars. This is also the case with the observed treatment effects
on THC concentrations, which represented only a small fraction of the total THC (T-THC)
content. More relevant are the effects of photoperiod on the T-THC content, which were
either insignificant (GG) or higher (IM) in the 13 h treatment. Since the 13 h treatment
increased the inflorescence DW, the 13 h treatment also increased THC yield (g/plant)
accordingly, by
≥
38%. These results are in contrast with those obtained by Peterswald et al.
(2023) [
4
], who found approximately 40% reductions in the floral THC concentrations in
the two THC-dominant cultivars when grown in the 14 h vs. 12 h photoperiod treatment.
These results are also contrary to our hypothesis, which distinguishes the present study
from many contemporary cannabis lighting studies that often showed only minor effects
of light treatments on cannabinoid composition (e.g., Rodriguez-Morrison et al., 2021;
Llewellyn et al., 2022; Potter and Duncombe, 2012) [
8
,
11
,
13
]. Further study is needed to
determine the mechanisms for how photoperiod, along with other environmental inputs,
affect the composition of the metabolome in indoor-grown cannabis.
Another important quality aspect of cannabis is the density of the apical inflorescences,
where more dense inflorescences are normally regarded as having higher quality. Apical
inflorescence density has been shown to increase linearly with light intensity (Rodriguez-
Morrison et al., 2021) [
8
]. However, inflorescence density in IM was lower in the longer
photoperiod in the current study, suggesting that the developmental ramifications of longer
photoperiods on apical inflorescence tissues may override the benefits of higher DLIs.
Overall, aside from lower apical inflorescence density in IM, the 13 h treatment substantially
increased the apical inflorescence size, total inflorescence yield, and cannabinoid yield.
Despite having similar prescribed days to maturity in commercial production
(Ahrens et al., 2023) [
5
], GG required
≈
25% longer to reach commercial maturity in the
present study, regardless of the photoperiod treatment. Factoring in the relative lengths of
the flowering cycle of each cultivar, IM was
≈
25% more efficient (i.e., g
·
d
−1
) than GG at
producing floral biomass in both treatments. While further study is required to elucidate
the specific mechanisms of photoperiod responses of indoor-grown cannabis cultivars,
the observed cultivar differences in photoperiod responses generally illustrate the need
to investigate individual cultivars’ photoperiod responses under the cultivators’ specific
cultivation environment for full flowering cycles.
Due to the relatively small plot sizes in the present study, the massive per-plant
yield increases may be tempered by competition for space and light in commercial indoor
Plants 2024,13, 433 14 of 15
cannabis cultivation systems, depending on the planting density. Readers are therefore
cautioned against inferring per-area yield increases (e.g., over commercially relevant pro-
duction areas) from the per-plant yield increases reported in the present study. Future
research should explore the effects of moderately longer photoperiods than 12 h on the
inflorescence yield and quality of different indoor-grown cannabis cultivars. Particular
focus should be directed toward investigating narrower discreet time differences between
photoperiod treatments (e.g.,
≤
15 min) and the effects of planting density on the treatment
effects of photoperiod on yield and quality, both on per-plant and per-area bases.
5. Conclusions
The 13 h photoperiod treatment increased inflorescence yield disproportionately
higher than the increase in DLI in both cultivars. In addition, while the longer photoperiod
somewhat delayed inflorescence development, the major cannabinoid concentrations in
the apical inflorescence tissues at commercial maturity were either unchanged or enhanced.
Therefore, increasing the photoperiod during the flowering stage of indoor cannabis culti-
vation is an easily employed cultivation protocol for enhancing indoor cannabis production.
However, cannabis’ photoperiod responses are strongly cultivar-dependent; growers must in-
vestigate the effects of photoperiods with their own specific cultivars and cultivation systems.
Supplementary Materials: The following supporting information can be downloaded at:
https://www.mdpi.com/article/10.3390/plants13030433/s1, Figure S1: Relative spectral photon
flux distribution used in all experimental plots (Ahrens et al., 2023) [
5
]; Table S1: Temperature and
relative humidity (mean
±
SD) in each experimental plot for the duration of the trial at canopy-level
height; Figure S2: Diagram of the experimental layout within a single growth chamber. The six
individual compartments comprise three replications of each photoperiod treatment (12 h and 13 h)
with three plants of each cultivar (Gorilla Glue (GG) and Incredible Milk (IM)); Figure S3: Diagram of
one compartment bench setup showing placement of individual rockwool blocks with Incredible
Milk (blue square) and Gorilla Glue (green square) plants, rockwool slabs (yellow rectangle), and the
locations of individual drippers (red circle). Image is for illustrative purposes only; the individual
elements are not to scale.
Author Contributions: Conceptualization, Y.Z.; methodology and experimental design, A.A., D.L.
and Y.Z.; validation, A.A., D.L. and Y.Z.; formal analysis, A.A.; trial execution and data collection,
A.A. and D.L.; data curation, A.A. and Y.Z.; writing—original draft preparation, A.A., D.L. and
Y.Z.; writing—review and editing, D.L., A.A. and Y.Z.; supervision, Y.Z.; project administration,
Y.Z.; funding acquisition, Y.Z. All authors have read and agreed to the published version of the
manuscript.
Funding: This study was supported by internal funding from Youbin Zheng; no commercial or other
third-party funding was used to complete this trial.
Data Availability Statement: All data from the study are included in the manuscript. Further
inquiries can be directed to the corresponding author.
Acknowledgments: We thank Allstate Garden Supply for providing LED fixtures, Cannim ((Syd-
ney, NSW, Australia) formerly Medisun) for providing plant materials, and Sebastian Dam for
logistical support.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Small, E. Cannabis—A Complete Guide; Small, E., Ed.; CRC Press, Taylor & Francis: Boca Raton, FL, USA, 2016; ISBN 9781315367583.
2.
Small, E. Genetics and Plant Breeding of Cannabis Sativa for Controlled Environment Production. In Handbook of Cannabis
Production in Controlled Environments; Zheng, Y., Ed.; CRC Press, Taylor & Francis: Boca Raton, FL, USA, 2022; pp. 41–90,
ISBN 9780367712570.
3.
Zhang, M.; Anderson, S.L.; Brym, Z.T.; Pearson, B.J. Photoperiodic flowering response of essential oil, grain, and fiber hemp
(Cannabis sativa L.) cultivars. Front. Plant Sci. 2021,12, 694153. [CrossRef] [PubMed]
Plants 2024,13, 433 15 of 15
4.
Peterswald, T.J.; Mieog, J.C.; Halimi, R.A.; Magner, N.J.; Trebilco, A.; Kretzschmar, T.; Purdy, S.J. Moving away from 12:12; the
effect of different photoperiods on biomass yield and cannabinoids in medicinal cannabis. Plants 2023,12, 1061. [CrossRef]
[PubMed]
5.
Ahrens, A.; Llewellyn, D.; Zheng, Y. Is twelve-hour really the optimum photoperiod for promoting flowering in indoor-grown
cultivars of Cannabis sativa?Plants 2023,12, 2605. [CrossRef] [PubMed]
6.
Potter, D.J. A review of the cultivation and processing of cannabis (Cannabis sativa L.) for production of prescription medicines in
the UK. Drug Test. Anal. 2014,6, 31–38. [CrossRef] [PubMed]
7.
Potter, D.J. The Propagation, Characterisation and Optimisation of Cannabis sativa L. as a Phytopharmaceutical. Ph.D. Thesis,
King’s College London, London, UK, 2009.
8.
Rodriguez-Morrison, V.; Llewellyn, D.; Zheng, Y. Cannabis yield, potency, and leaf photosynthesis respond differently to
increasing light levels in an indoor environment. Front. Plant Sci. 2021,12, 646020. [CrossRef] [PubMed]
9.
Zheng, Y. Rootzone management in cannabis production. In Handbook of Cannabis Production in Controlled Environments; Zheng, Y.,
Ed.; CRC Press, Taylor & Francis: Boca Raton, FL, USA, 2022; pp. 123–162, ISBN 9780367712570.
10.
Ruter, J.M. Influence of source, rate, and method of applicating controlled release fertilizer on nutrient release and growth of
‘Savannah’ holly. Fertil. Res. 1992,32, 101–106. [CrossRef]
11.
Llewellyn, D.; Golem, S.; Foley, E.; Dinka, S.; Jones, A.M.P.; Zheng, Y. Indoor grown cannabis yield increased proportionally with
light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Front. Plant Sci. 2022,13, 974018. [CrossRef]
[PubMed]
12. Hoffmann, T. Suncalc.Org. Available online: https://www.suncalc.org (accessed on 3 January 2024).
13.
Potter, D.J.; Duncombe, P. The effect of electrical lighting power and irradiance on indoor-grown cannabis potency and yield.
J. Forensic. Sci. 2012,57, 618–622. [CrossRef] [PubMed]
14.
Yep, B.; Gale, N.V.; Zheng, Y. Comparing hydroponic and aquaponic rootzones on the growth of two drug-type Cannabis sativa L.
cultivars during the flowering stage. Ind. Crops Prod. 2020,157, 112881. [CrossRef]
15.
Moher, M.; Llewellyn, D.; Jones, M.; Zheng, Y. Light intensity can be used to modify the growth and morphological characteristics
of cannabis during the vegetative stage of indoor production. Ind. Crops Prod. 2022,183, 114909. [CrossRef]
16.
Hall, J.; Bhattarai, S.P.; Midmore, D.J. The effects of photoperiod on phenological development and yields of industrial hemp.
J. Nat. Fibers 2014,11, 87–106. [CrossRef]
17.
Bevan, L.; Jones, M.; Zheng, Y. Optimisation of nitrogen, phosphorus, and potassium for soilless production of Cannabis sativa in
the flowering stage using response surface analysis. Front. Plant Sci. 2021,12, 764103. [CrossRef] [PubMed]
18.
Aizpurua-Olaizola, O.; Soydaner, U.; Öztürk, E.; Schibano, D.; Simsir, Y.; Navarro, P.; Etxebarria, N.; Usobiaga, A. Evolution of
the cannabinoid and terpene content during the growth of Cannabis sativa plants from different chemotypes. J. Nat. Prod. 2016,79,
324–331. [CrossRef] [PubMed]
19.
Russo, E.B. History of cannabis and its preparations in saga, science, and sobriquet. Chem. Biodivers. 2007,4, 1614–1648. [CrossRef]
[PubMed]
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