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The influence of plants on productivity: A critical assessment of research findings and test methods

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Abstract

Purpose This paper aims to review available research into the impact of plants on people and labour productivity in order to test a number of hypotheses and the reliability and validity of “evidence based” statements. Design/methodology/approach An extended literature review was conducted of research concerning the potential impacts of plants on people and labour productivity. In order to be able to compare the findings of different researchers, an analysis was made of similarities and dissimilarities with regard to the research context, starting‐points and test methods. Findings The paper identifies a lack of precise descriptions of the research design and poor comparability between different research with regard to the characteristics of the plant, test persons, test procedures, surrounding conditions and contents of the reports. Although it can be concluded that plants can have a positive impact on the productivity of human beings, it is remarkable that in research reports and research papers the properties of the plant itself are only mentioned by exception. The condition of the plant – whether it is healthy or not – is not described at all. Research limitations/implications Only 17 studies and underlying papers were investigated and no new research was conducted with the proposed improvements. Practical implications The findings can be used by managers to legitimate investments in plants and by researchers to improve (the comparability of) research into plants. Originality/value In addition to the review of the impact of plants on different types of productivity a vision is presented about the impact of the vitality of plants. Furthermore recommendations are given on how to cope with the methodological problem of poor comparability of research.
The influence of plants on
productivity
A critical assessment of research findings
and test methods
Iris Bakker and Theo van der Voordt
Department of Real Estate and Housing and Center for People and Buildings,
Faculty of Architecture, Delft University of Technology, Delft, The Netherlands
Abstract
Purpose This paper aims to review available research into the impact of plants on people and
labour productivity in order to test a number of hypotheses and the reliability and validity of “evidence
based” statements.
Design/methodology/approach An extended literature review was conducted of research
concerning the potential impacts of plants on people and labour productivity. In order to be able to
compare the findings of different researchers, an analysis was made of similarities and dissimilarities
with regard to the research context, starting-points and test methods.
Findings The paper identifies a lack of precise descriptions of the research design and poor
comparability between different research with regard to the characteristics of the plant, test persons,
test procedures, surrounding conditions and contents of the reports. Although it can be concluded that
plants can have a positive impact on the productivity of human beings, it is remarkable that in
research reports and research papers the properties of the plant itself are only mentioned by exception.
The condition of the plant whether it is healthy or not is not described at all.
Research limitations/implications Only 17 studies and underlying papers were investigated
and no new research was conducted with the proposed improvements.
Practical implications The findings can be used by managers to legitimate investments in plants
and by researchers to improve (the comparability of) research into plants.
Originality/value In addition to the review of the impact of plants on different types of
productivity a vision is presented about the impact of the vitality of plants. Furthermore
recommendations are given on how to cope with the methodological problem of poor comparability of
research.
Keywords Plants, Productivity rate, Research methods, Work psychology, Workplace
Paper type Literature review
Introduction
In order to be able to design the optimal working environment where people can
flourish in their work and organisations will be successful, it is important to know how
the physical environment affects people and productivity. One of the variables is the
The current issue and full text archive of this journal is available at
www.emeraldinsight.com/0263-2772.htm
The authors want to express special thanks to Peter Vink/ TU Delft, John Klein Hesselink/TNO
Bouw, and Bert van Duijn/Fytagoras BV, for their valuable suggestions and comments. This
paper is part of an ongoing PhD research on maximising productivity in office environments,
that is being supervised by prof Peter Vink of TNO/Quality of life, and dr. Theo van der Voordt
of the Faculty of Architecture at the Delft University of Technology.
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Received November 2009
Accepted February 2010
Facilities
Vol. 28 No. 9/10, 2010
pp. 416-439
q Emerald Group Publishing Limited
0263-2772
DOI 10.1108/02632771011057170
presence of plants. In search for evidence-based knowledge about the impact of plants
on labour productivity it turned out that the existing literature is not always clear on
what the impact exactly is. It is needed to define this impact more exactly. Second, we
observed a large variety of research methods and test conditions. As a consequence,
the comparability of different research projects and the conclusions that came out of
the research is limited. And third, the first scan of a number of studies and included
references showed that in particular information about the plants themselves is often
lacking. This is an omission, because probably nobody will be more productive by
seeing a faded or dead plant. Apart from the appearance, the type of the plant may be
an important issue too. It may be expected that people respond differently when seeing
a cactus or a rose plant. These observations have led to three main questions for a more
extensive literature review on the impact of plants on productivity:
(1) What is the influence of plants on productivity?
(2) Are different studies sufficiently comparable to draw sound conclusions?
(3) What is the impact of the appearance and vitality of the plant?
These questions have been rephrased into three hypotheses:
H1. Plants have a different impact on different types of productivity.
Productivity covers a diversity of activities such as routine work and creativity.
Creativity tasks and complex knowledge work need inspiration and deepening.
Through history many statements of famous philosophers, writers and artists such as
Nietzsche or Liszt refer to the inspiring and deepening effect of nature. Our hypothesis
is that in case of routine work plants might help to support wellbeing and as such keep
people going on, whereas in case of creativity work a positive effect is expected in
relation to inspiration and deepening:
H2. Research concerning the impact of plants on productivity is not well
comparable.
Research is rather complex. Even when the focus is just on one “dependant” variable,
plants, many “independent” variables can influence the results. It is expected that
research so far does not use standardised research methods.
H3. Both the appearance, type and vitality of the plant have an impact on the
productivity.
One of the wonders of nature is its infinite variation combined within certain patterns
and structures. Each variety has its own characteristics. As a consequence one might
expect different effects of different plants. In particular, the vitality of a plant is
expected to be important. Probably a healthy plant has a more positive impact on
people than a plant that is not vital. In addition it is important that a plant lives in an
environment with healthy conditions that support the plant and conditions people
need.
Research methods and conceptual model
Initially, 17 studies from renowned researchers and research institutes were collected
(see the Appendix). These documents have been scanned on possible effects of plants
The influence
of plants on
productivity
417
on people and labour productivity, relevant variables and references for further
reading (see list of references). Without any exception all studies make a significant
contribution to the field. Together an incredible amount of data has been collected on
many different effects. Second, in order to enlarge the knowledge that came out of the
documents both technical and psychological discussions with specialists of the
knowledge institutes TNO and Fytagoras/TNO have taken place as well. Third,
because of the many different phenomena that are being mentioned in the studies and
additional references, the need came up to develop a conceptual model that visualises
the different types of impact of plants on human beings (Figure 1). Two different
mechanisms were traced:
(1) Evolutio nary influence. Since our genesis we have been surrounded by green
plants and trees. From this point of view it is generally assumed that seeing
plants has, in general, a restful effect (Ulrich, 1984; Kaplan and Kaplan, 1989).
(2) Healthy indoor climate. Plants have an impact on the indoor climate; this indoor
climate in turn affects people and their productivity (Wolverton, 1989; Wood
et al., 2004).
The evolution of human beings and a healthy indoor climate affect people in three
ways: plants evoke a physical/physiological response, an affective response and/or a
cognitive response. In the literature six components of the indoor climate are being
mentioned in relation to the impact of plants: light, temperature, relative air humidity,
air quality, sound and static electricity. Another point of attention is the characteristics
Figure 1.
Conceptual model of the
impact of plants on people
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of plants themselves, including form properties and metabolics. The latter are hardly
mentioned in the literature.
This conceptual model has been used as a guiding principle to analyse and discuss
the collected data to examine the research findings and conclusions in the studies more
closely. In a cyclic process of reading, reflecting, discussing, further reading, etc. a list
of items has been traced with regard to the test conditions (Table I). This list includes
six main aspects:
(1) characteristics of the plant;
(2) the test surroundings;
(3) the test persons;
(4) the test process;
(5) test strategies; and
(6) methods and variables.
Table II shows the variables that have been investigated in each research.
Research findings
Effects of plants on human beings: physical/ physiological, affective and cognitive
response
The next responses are mentioned rather often:
.
Physical/physiological. Primary physical responses are effects on blood pressure
and heart beat and physiological decrease of complaints of headache; secondary
responses are physiological phenomena like faster recovery (all documents
excluding nos 9, 6, 10, 13, 14).
.
Affective. Positive affective response on mood and affective behaviour like
self-confidence, alertness or less aggression and positive feelings like pleasure
(all documents, excluding nos 9, 13, 14, 16 and 17).
.
Cognitive. Positive cognitive responses are better concentration capacity and
higher response speed (all documents excluding Nos 9, 13 and 14). Ulrich (1984)
and Lohr et al. (1996) showed significant statistical correlations between seeing
plants and physical/physiological, affective and cognitive responses. These
researchers use different methods like questionnaires, the Zipertest (Zuckerman
Inventory or Personal Reactions), interviews and observation of behaviour.
Unfortunately a clear explanation of the set-up of these methods is often missing.
In most research quantitative effects were also mentioned, be they quite underexposed.
The following quantitative data are interesting:
.
Wolf (2002) mentions in her research at shops an increase of sale concerning all
products of 12 per cent when plants are present;
.
Lohr et al. (1996) appoint an increase of the response speed of 12 per cent at
simple recognition tests;
.
Fjeld (1995) shows a decrease of symptomatic physical complaints of 23 per cent
at 51 office employees;
The influence
of plants on
productivity
419
Plants Test surroundings Test subjects Test process Test strategies Methods and variables
Spot Institute Test subjects Reduction Hawthorne-
effect
Observation Information
Position in space Outdoor area Men Attention Observation by test subject Observation
Height of view Laboratory Women Habituation process Observation researcher By test subject
Indoor/outdoor Education Children Attention Technical supporting
measurements
By researcher
View Office Patients Data semantic questionnaire Biophysical
Sort Shop Students Test surrounding Data standard interview Questionnaires
Hospital/care Employees Clear information at the
beginning
Data interview/ survey Standard
Space type Age Intensive accompaniment Data question conversation Score model
Variety One person space Number Acceptation management Questionnaire Quantitative
Intensity Two persons space Sort of work Test aspects Computer program Qualitative
Dimension/number per
square m
Multi persons space Concentration Placebo Biophysical observation Interview method
Number Various Creativity Heartbeat Guidance question
conversation
Size Space characteristics Routine System blood pressure No guidance
Cleanliness order Number of windows Commitment of test
persons
Muscle tension Computer program
Maintenance situation Size of windows Relevance Skin conductance ZIPER test
Pot ground/hydroponics Size of space Seriousness of
participation
Electrical brain activity Fee
Pot size Relation to
temperature
To participate is own
choice
Number measurements Credit
Form pot Light level Involvement in final
result
Task
Artificial plant Relation lighting Preference for plants Test duration Association task
Image plants Fluorescent broad
spectrum
Hours Key typing task VDT
Flower Neon light
Days Computer task
Micro-organism Daylight
Weeks Sorting task
Relative air humidity
Months Concentration task
Ventilation system
Years Technology
Natural ventilation
Objectifying Air/ventilation systems
(continued)
Table I.
Overview of items to
compare different
research on plants
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Plants Test surroundings Test subjects Test process Test strategies Methods and variables
Mechanical ventilation Knowledge structure
questionnaire
Light systems
Air treatment Measurement air quality
Airco Concept
Quantity ventilation Effects Position plants
Design ventilation
quantity
Affective feeling Number plants
Real ventilation
quantity
Affective mood
Sound Affective behaviour
Static electricity Physical primary
Colour space Physical secondary
Fragrance Physiological effects
Interior elements Cognitive
Smoke Cognitive concentration
Specification and
VOCs
Cognitive memory
Parts and value parts
of dust
Cognitive reaction time
CO2 and value CO2 Cognitive errors
CO and value CO Cognitive discipline
Moulds Adiposis
Pathological micro-org. Other mentioned effects
Time Productivity/performance
Link to seasons Sound
Link day/night Ecologically/reduction
energy
One cell organism Staff keeping and
recruitment
Weather On working environment
On plants
Start conditions
Single plant
Many plants
Table I.
The influence
of plants on
productivity
421
Studies (numbers according to the Appendix)
Component Aspect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sum
Plants £££££ £££8
Spot Position in space ££££ 4
Height of view £££££££££££££££££ 17
Indoor/outdoor ££ £ 3
View ££ £££ £ £££ ££ 11
Sort ££££ £ ££££9
Variety £££ £ ££ 6
Intensity Dimension/number per square m ££ £££ ££££ £ 10
Number £££ ££ £6
Size £££3
Cleanliness order ££ ££4
Maintenance situation ££ ££4
Pot ground/hydroponics £££2
Pot size £££4
Form pot £ 2
Artificial plant £ 2
Image plants £ 2
Flower 1
Micro-organism
Test surroundings ££££4
Type of environment Outdoor area £££3
Laboratory £ 1
Education £££ £4
Office £ 1
Shop ££ £ 3
Hospital/care £££ £4
(continued)
Table II.
Aspects that were
mentioned in 17 studies
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Studies (numbers according to the Appendix)
Component Aspect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sum
Space type One person space 0
Two persons space £ 1
Multi-persons space 0
Various £££££ 5
Space characteristics Number of windows £ 1
Size of windows ££££££££8
Size of space £££££5
Temperature Known £££ 3
Light level Known £ 1
Type of light Florescent broad spectrum ££ £ 3
Neon light ££2
Unknown £££ £ £ 5
Daylight Known £££££ £ £7
Relative air humidity Known ££2
Ventilation system Natural ventilation ££ £ 2
Mechanical ventilation £ 1
Air treatment ££2
Airco ££ £ 3
Quantity of ventilation Designed ventilation quantity £ 1
Real ventilation quantity ££ £ 3
Sound Known £ 1
Static electricity ££ 2
Colour space Known ££2
Fragrance Known ££ ££ 4
Interior elements ££ ££ 4
Smoke £££££5
Specification VOCs ££££4
Value VOCs ££ £ 3
Parts of dust £ 1
(continued)
Table II.
The influence
of plants on
productivity
423
Studies (numbers according to the Appendix)
Component Aspect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sum
CO2 £££3
Value CO2 ££ £ £3
CO ££3
Value CO ££ £2
Moulds ££ 2
Path micro-organism £ 2
Time Which season(s) £ 1
Day/night 0
One cell organism £ 1
Test persons
Test persons No distinction ££ £ £ 4
Men ££££ ££ £ 6
Women ££££ ££ 6
Children £ 1
Patients ££2
Students £££ ££ 5
Employees £££££ 6
Age ££ 2
Number ££ £ £ 4
Type of work Concentration £££ £ 4
Creativity £ 1
Routine ££ £ 3
Commitment of test persons Relevance £££ 3
Seriousness of participation 0
To participate is own choice £££ ££ £6
Involvement in final result 0
Preference for plants £ 1
(continued)
Table II.
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Studies (numbers according to the Appendix)
Component Aspect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sum
Test process
Reduction Hawthorne effect Attention £ 1
Habituation process Attention £££ £ £5
Clear information at the beginning £ 1
Test surrounding Intensive accompaniment ££2
Acceptance management 0
Test aspects Placebo 0
Test strategies
Observation Observation by test person ££ £ £4
Observation researcher ££ 2
Technical measurements 0
Data semantic questionnaire 0
Data standard interview 0
Data interview/survey ££ 2
Data question conservation £ 1
Questionnaire ££££ ££ 6
Computer programme £ 1
Biophysical observation Heartbeat £££ 3
System blood pressure £££ 3
Muscle tension £ 1
Skin conductance £ 1
Electrical brain activity £ 1
Number of measurements Known £££3
Test period Ours ££££ 4
Days £££3
Weeks 0
Months ££ ££3
Years ££ 2
(continued)
Table II.
The influence
of plants on
productivity
425
Studies (numbers according to the Appendix)
Component Aspect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Sum
Objectivity Knowledge structure questionnaire 0
Effects Affective feeling £££££ ££ £ 8
Affective mood £££££ £ £ 7
Affective behaviour ££ £ 3
Physiologically primary £££££ £ £ 7
Physiologically secondary £££££ £ 6
Cognitive ££ ££ £ £ 6
Other mentioned effects Productivity/performance £££3
Sound 0
Ecological/reduction of energy £ 1
Staff retraining and recruitment £ 1
Impact on working environment ££££4
Impact on plants £ 1
Other ££2
Start conditions Single plant 0
Many plants £ 1
Total mentioned aspects 19 38 23 13 54 24 41 1 4 18 14 32 20 13 18 11 28 371
Table II.
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.
research by Fjeld among 48 employees of an X-ray division showed a 25 per cent
decrease of health complaints by using plants; and
.
in 2001-2002 Fjeld revealed an average 24 per cent reduction of physical
complaints among different groups of 48 bank employees after the introduction
of plants and light with a broad spectrum.
When the results are analysed more closely, a uniform effect on physical/physiological,
affective and cognitive responses comes up. This confirms the statements of many
famous people that emphasise the positive effects of nature on human beings. Greek
philosophers used the so called “peri-pathetic method”: walking through the academy
garden to discuss their ideas (Csikszentmyhalyi, 1998). Based on studies such as those
presented above, it can be concluded that a relation exists between seeing and
experiencing plants and physical/physiological, affective and cognitive responses.
This relation however is merely qualitatively described and to a lesser extent
quantitatively defined. The exact effect of plants on human beings is still not clear. In
accordance with the model, three explanatory options are possible. The effect can be
evolutionary: during centuries of development of human beings, plants have always
been an important part of nature and a strong foundation in our existence. A second
effect is the improvement of the indoor climate. Many aspects of the indoor climate are
strongly connected to the presence of plants. Third, metabolics may have an influence
on people. Plants form metabolics, chemical compounds with amongst other things
fragrances and colour properties. These substances may be expected to influence
people, but this has not been proven by research so far. Little attention has been paid to
the impact of intermediary variables such as research conditions and test persons. So
although the positive effects of plants on human beings are widely accepted and
supported by research, we have to interpret the research findings carefully.
Effects of plants on the indoor climate
Plants and indoor climate affect one another. To be able to interpret research findings
on the impact of plants correctly, detailed information is needed about the indoor
climate in the test situation. But due to differences in descriptions and lack of essential
information concerning technical data that might affect the process and the impact of
plants it is rather difficult to draw clear conclusions. Nevertheless some interesting
results have been found with regard to the six components of indoor climate that are
included in the conceptual model: light, temperature, relative air humidity, air quality,
sound and static electricity.
Light. With regard to photosynthesis the blue and red part of the spectrum are
necessary for healthy plants. In many buildings light with a broad spectrum is absent,
so probably insufficient blue and red light will be available for the plant. This
obstructs the growth and also the processes of photosynthesis and metabolism. It is
striking that in the examined studies both light colours (spectrum) and light intensity
are usually not mentioned at all, in spite of its importance for the health of the plant. By
contrast the reflection of light on the leaves of the plants affects the variation on light
colours in the physical surrounding.
Temperature. Stec et al. (2005) revealed that an outside awning of plants is more
effective than a regular awning. Schempp (2002) mentions a difference of two up to
The influence
of plants on
productivity
427
three degrees with regard to outside temperature by application of an outside awning
with plants in combination with plants inside.
Relative air humidity. Research of Costa and James (1995) and Strickler (1994)
showed that the relative air humidity of a space without air treatment increases with
approximately 5 per cent when plants are used. It is necessary to use a quite large
number of plants. Lohr et al. (1996) mentions an increase from zero to 15 per cent if
space is not ventilated; in a ventilated room there is an increase of 3 to 5 per cent.
Applying plants means that you have to take care for them. When for instance the
value of relative air humidity is too low, the stomata at the base of leafs will close.
Air quality. In the air volatile organic compounds (VOCs) occur, such as small dust
particles, moulds, bacteria, metabolics, CO and CO2. Air quality is expressed by the
VOCs concentration which is quantified in parts per million (ppm) value. Based on the
experiments of Wolverton (1989) it is known that a synergetic process between plant
and micro organisms that attaches themselves to the rootstructure of the plant
contributes to the reduction of the VOCs’ value. van der Wal and Hoogeveen (1993)
proves that unrealistic amounts of plants are needed to reach a sufficient reduction of
the VOCs’ value. Quite often the indoor climate in buildings is not optimal for plants
and therefore also not optimal for the process of VOCs reduction. Plants also have a
positive influence on the reduction of dust accumulation. Research of Lohr et al. (1996)
showed that plants in optimal conditions can cause a dust reduction of 20 per cent.
Plants are selected in buildings in such a way that they will not grow too rapidly,
because rapid growth increases the exploitation costs too much. It may be concluded
that a positive effect of plants is not the right argument to use of plants as a means to
control or improve the indoor air quality. Ventilation is much more effective.
Sound. Research by Costa and James (1995) shows that the reverberation time of
sounds with a high frequency is shortened when plants are used, and as such the space
will be quieter. At low frequencies more inflection of the sound takes place. Dependent
of the exact location and the spreading, sound absorption takes place.
Static electricity. Employees working at least four hours at screens undergo less
inconvenience from static electricity when plants are in their workspace than other
employees without plants in their rooms.
Overall we may conclude that in real working environments the influence of plants
on the indoor climate is rather small. So this cannot be a convincing argument to apply
plants in working environment.
The effects of plants on productivity
According to the studies that have been analysed, the question of whether plants have
an impact on the functioning and productivity of people can be answered in a positive
way (Table III). Most studies mention the positive qualities of plants. However, it is
hardly possible to compare the studies in a systematic way because of the lack of clear
definitions of productivity and performance and a lack of clear information about
which activities were measured, what exactly has been measured, what the
characteristics were of the test persons and in which way the measured results were
achieved. Because of the large amount of variables it is impossible to establish clear
conclusions.
In spite of the methodological shortcomings we can discern a common thread:
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Research Conclusions Document number (Appendix)
Asami et al. (1995) Indoor plants reduce fatigue of the
eye when working with screens 10
Conklin (1974, 1978);
Isen (1990, 1993)
Plants in offices lead to higher
employee morale and higher
effectiveness 7, 11
Knez (1995); Isen (1990, 1993) If people are in a positive mood,
their creativity raises 6, 11
Isen and Shalker (1982) Positive phenomena stimulate the
brain for recalling more information
and they initiate more cognitive
manipulation that causes a higher
level of creativity 6
Larsen et al. (1998) A larger number of plants improves
the mood, but reduces
concentration; the perceived
productivity increases in connection
to the number of plants 1, 6
Lohr et al. (1996) Plants lead to 12 per cent increase in
response speed and reduce the
number of mistakes 5, 8
Mayer et al. (2006) Plants strengthen the capacity to
think about life problems 1
Mayer and Frantz (2004) Plants evoke a positive feeling of
alliance and increase problem
solving capacity 1
Marchant (1980);
Srivens (1982)
With plants increase of productivity
10-15 per cent 7
Ottoson and Grahn (2005) Staying one hour in a green space
improves concentration 1
Shibata and Suzuki (2002) Plants have a larger impact on
performance than on women; in
spaces with a plant men perform
better; conducting a sorting and
association task men performed on a
lower level than women in case of no
plants in the room, but when a plant
was placed in front of them, men
performed better than women. The
impact of plants was larger at the
association task, then at the sorting
task. Plants had a negative effect on
women in sorting tasks 1, 5, 11
Shibata and Suzuki (2002) The presence of plants increases the
performance score of women; in
general the presence of a plant
increases the mood and the
appreciation of the space 11
Shoemaker (1992) Plants have no impact on work
satisfaction 5
Stone (1998) Plants have a negative impact on
performance and task perception 11
Table III.
Effects of plants on
labour productivity (by
alphabetical order of the
author)
The influence
of plants on
productivity
429
.
Plants put people in a better mood and improve confidence and openness of the
mind to the surrounding world. Plants have also a positive social effect in
relation to alliance and morality.
.
If people are in a better mood, the perceived productivity increases, whereas the
measured (“real”) productivity score decreases.
.
The amount of plants plays a role.
.
The presence of a plant stimulates people in different ways.
.
The effect of plants can be different depending on the activities.
.
With regard to productivity of creative work, a clear positive relation is evident
on the basis of the research above.
Reflections on the attention paid to five test items
As has been said before, to improve the comparability of research on plants, a test
structure has been developed with five test characteristics that should be described
very clearly: the plant; the test surroundings; the test persons; the test process; and the
test itself. Furthermore standard items have been formulated per aspect. The collected
studies have been examined on the attention paid to these five aspects and the
components (Table II).
The plant itself
Looking at the plant itself, most reports and papers only pay attention to its type,
variety and number and sometimes the spot. Heights and sizes of pots are mentioned
as well. The characteristics of the plant itself are usually not described at all. Several
types of plants are used, with different varieties (Table IV). Particularly the Dracaena,
Spathiphyllum and the Epipremnum are often used Because of the different plants that
are involved in the investigations, the conclusions from the studies are not comparable.
Test surrounding
Most studies mentioned whether the tests have taken place inside or outside. In all
studies, the environment of the test is described, including offices, a laboratory, shops,
care sector and education buildings. Most attention is paid to the size of the space and
the relative air humidity. All other aspects of the test surroundings are mentioned only
very briefly and to an insufficient degree. Colour specification is extremely limited,
whereas this variable affects the light frequencies required for the photosynthesis of
the plant.
Test persons
The test persons vary from children to students (graduates and undergraduates),
clients and employees and include men and women in different sectors. Usually reports
and papers do not give any information about the psychological and social
psychological situation of test persons or personal characteristics (beside age and sex),
personal conditions or mood specifications. So, no valid statements can be made about
the impact of these issues. Sometimes attention is given to the willingness of people to
participate in the experiment.
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Plant species
Lohr
(7)
Strickler
(5)
Burchett Tarran
(5)
Klein Hesselink
(5)
Wood
(16)
Wolverton
(3)
Larsen et al.
(6)
Shabita and Suzuki
(10 þ 11)
van der Wal
(13 þ 14)
Aglaonema £ £
Chamaedora £
Dracaena ££ £ £ £ £
Epipremium ££ £ £
Homalomena £
Hoya £
Philodendron ££ £
Sansevieria £
Scindapsus £
Syngonium £
Dizygotheca £
Ficus benjamina ££ £
Hedera £
Howea ££
Spathiphyllum £££ £
Schefflera ££
Orchidee £
Bromelia achtigen £
Augusta
Phycorapis £
Strelizia £
Note: Numbers refer to the numbers of the documents in the Appendix
Table IV.
Names of plants
appointed in the research
documents
The influence
of plants on
productivity
431
Test proce ss
Processes are very complex; there are many factors playing a role and also
influencing one another. No single study paid attention to psychological effects like
the Hawthorne effect. In a number of cases attention was paid to habituation.
However, the way that habituation has been defined and being measured is described
insufficiently. It is possible that both the habituation of the test persons and the early
effects of VOCs reduction of plants have affected the test results, but in which way is
still not known.
Test methods and variables
Observations, measurements, impact and test duration are only comparable in a
limited way. The observations vary from individual perceptions of the test persons to
observations by research workers and standard questionnaires with scores and/or
scales. Biophysical observation has taken place to a limited extent.
It may be concluded that because of the huge variety in test characteristics the
comparability of the 17 analysed documents is limited. Testing phenomena like
effects of plants on productivity is related to many variables, so it is a very
complex process. As a consequence it is nearly impossible to draw sound and
transparent conclusions. Many studies do not pay sufficient attention to important
terms. Quite often terms have not been formulated consistently or accurately. At
this moment, there is no standard research framework that can be used as a
guideline to design research. A positive exception is study no. 5 of TNO (Klein
Hesselink et al., 2006). The appointment of 55 aspects is a relatively complete
description. The analysis of Fjeld and Bonnevie (2002) scores also high with an
appointment of 44 aspects. The more technical considerations of Wood et al. (2004)
and van der Wal (1991) have high scores as well. They focus on a pure technical
and well-defined input.
A closer look at the appearance and vitality of a plant
Table V shows an overview of relevant aspects with regard to appearance and vitality.
Based on this scheme, all remarks about the appearance and vitality of plants have
been collected and analysed. It is obvious that researchers do not pay sufficient
attention to the appearance of plants or their health condition. Research with
significant evidence of the impact of the appearance and health condition of a plant on
human behaviour has not been found yet. It has been noted that plants with flowers
give most entertainment. Costa and James (1995) discuss the size of the leaf and/or the
length of the little hairs in connection with admission of specks of dust and chemical
substances. Only the study of Van Dortmont and Bergs (1997) discusses plant
properties based on conversations with garden experts.
The comparative analysis shows that hardly any attention is being paid to the
properties of the plant itself, like the shape of the leaves, colours and structures of the
vascular bundle. One can imagine that a cactus has another effect on people than a rose
plant, and that an unhealthy or nearly dead plant makes people feel less pleasantly
than a strong and healthy plant. These considerations are missing in nowadays
research.
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432
Plant characteristics 17 studies (numbers refer to the Appendix)
No. Aspect Subaspect 1234567891011121314151617Total
1 Chemical activity Bioproces system
2 Admission chem. Compounds
3 Quantity of transpiration £ 1
4 Plants differentiation Plant type ££ £ ££ 5
5 Plant sort £ £££ £ £ ££ 8
6 Plant variety £££ £ £ 5
7 Sizes Plant height £££ ££ £6
8 Plant width £ 1
9 Proportions £ 1
10 Form Apple/pear form
11 Fullness/mass
12 Horizontally or straight
13 Structure Structure plant £ 1
14 Structure branches
15 Structure leaf
16 Mesophyll
17 Leaf Number/intensity ££ £ ££ £6
18 Size £££3
19 Form
20 Surface/tactility characteristic £ 1
21 Colour mix £ 1
22 Colour expression £ 1
23 Structure £ 1
24 Position
25 Expression
26 Brilliance
27 Difference colour front/back
28 Leaf edge
29 Fragrance plant Nature ££ 2
(continued)
Table V.
Characteristics of plants
and its application in 17
studies
The influence
of plants on
productivity
433
Plant characteristics 17 studies (numbers refer to the Appendix)
No. Aspect Subaspect 1234567891011121314151617Total
30 Flower Mentioned ££2
31 Expression
32 Form
33 Structure
34 Colour
35 Colour differentiation
36 Position
37 Fragrance
38 Number/intensity
39 Brilliance
40 Seeds and fructification Number
41 Size
42 Form
43 Colour differentiation
44 Colour expression
45 Structure
46 Position
47 Dynamics Dynamics
48 External factors Pot form ££ £ £5
49 Pot ground/hydroponics ££ ££4
50 Position ££ 2
51 Integration environment
52 Care and carefulness ££ £££5
53 Solitarily/group
54 Performance Fine or grove structure
55 Metabolics
56 Health Degree of vitality £ 1
Total 2 2 11 4 8 3 5 0 041423236 60
Table V.
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Discussion and conclusions
H1. Plants have a different impact on different types of productivity
Although a consistent positive influence of plants on creativity came out from the
studies mentioned, the influence of plants on overall productivity varies. In general
plants have a positive impact on the physical/physiological and affective response of
people. Through centuries people are aware of the impressive nature. Modern research
supports the so-called “Biophilia Hypothesis” that refers to the biological basis for
human values in nature (Kellert and Wilson, 1993). There is also a growing awareness
of the importance of nature to children’s development intellectually, emotionally,
socially, spiritually, and physically (Kellert, 2005; Moore and Cooper Marcus, 2008).
Plants support people in their feelings of safety, because all plants have a clear
structure. Concerning cognition, the effects of plants are different for various reasons.
Many factors play a role. Another issue is the infinite diversity of people, their way of
being, living, doing, feeling and thinking. All people are completely different
concerning their Intelligence and Emotional, Spiritual and Physical Quotient. Their
personal situations are also different. So one might question if it is really possible to
measure the effects of plants on people.
H2. Research concerning the impact of plants on productivity is not well comparable
Because of the lack of essential information and indistinct and incomplete data, the
comparability of the analysed studies is limited. Accuracy concerning the various
aspects playing a role in research is necessary to establish clear conclusions. Because
of the complexity of this type of research and the lack of accurate information about the
many aspects playing a role there is doubt about the validity of the posited conclusions
from present research.
H3. Both the appearance, type and vitality of the plant have an impact on the productivity
None of the analysed studies discussed the appearance of the plant on a scientific
basis.
Only study 3 refers to the vitality of the plant, whereas, hypothetically it is assumed
that the more healthy the plant, the more positive the impact on people. It is remarkable
that researchers were looking for a physical environment that is healthy for human
beings, without paying sincere attention to the plant itself. Plants are like ourselves
living beings and are permanently changing their form, colours and fragrances. It is
really important to treat plants with respect. Nowadays, they are cultivated in a world
with emphasis on low costs and less time. So, it is really the question if the cheap pots
and cheap potting soils are benefiting the plants themselves. Moreover, the spots where
plants in buildings will be placed are often too windy, too dark without daylight, or
lack the blue and red light of the spectrum. When plants are unhappy, they cannot
make people feel happy. When more attention is paid to the plant itself and when the
plant stays healthier, this stronger interaction between people and plant will generate
positive effects in a more socialising way. An interesting example is a home for older
people, where the older men and women were allowed tot take care of their own plants,
which they had selected themselves. These elderly people felt better and had fewer
complaints. Just by bringing user involvement in the organisation, both plants and
users of a building will be happier.
The influence
of plants on
productivity
435
Recommendations
It is highly recommended to make the approach of future research less unambiguous in
order to improve its comparability with other research and to support sound
conclusions. For that purpose a more elaborate standard research approach is needed.
The tables and schemes that came out of this paper may be helpful here, in particular
in recording of the properties of plants in a structured way. It is also important to use
unambiguous definitions without overlaps and to pay more attention to the appearance
and vitality of plants. This will help to create a more complete picture. However, people
have to be humble. Nature is so infinite in her expressions that it is impossible to
gather all variations of nature in a model made by human beings. Finally it is
recommended to pay more attention to the health of the plants themselves. It is
hypothesised that the happier the plant, the more positive effect the plant has on
human beings. It is interesting to study this hypothesis more closely.
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Appendix. List of the 17 examined research reports
(1) van den Berg and Winsum Westra (2006);
(2) Fjeld and Bonnevie (2002);
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(3) Dortmont and Bergs (1997);
(4) Klein Hesselink and Hopstaken (1995);
(5) Klein Hesselink et al. (2006);
(6) Larsen et al. (1998);
(7) Lohr et al. (1996);
(8) Loomans and Klein Hesselink (2005);
(9) Schempp (2002);
(10) Shibata and Suzuki (2001);
(11) Shibata and Suzuki (2002);
(12) Ulrich (2002);
(13) van der Wal (1991);
(14) van der Wal and Hoogeveen (1993);
(15) Wolf (2002);
(16) Wood et al. (2002); and
(17) Wood et al. (2004).
Corresponding author
Iris Bakker can be contacted at: iris.bakker@levenswerken.eu
The influence
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