Health benefits of plants and green space: Establishing the evidence base

Abstract

Among the many benefits provided by plants, “health” stands out as a benefit that is especially highly valued. Over the past decades, people-plant studies have increasingly focused on empirically demonstrating relationships between plants and health. However, there are as yet no consensual standards in the field as to which research findings qualify as evidence for health benefits of plants. In this paper, we first argue that only studies that directly relate exposure to plants and green spaces to health outcomes should be admitted into the evidence base. We then discuss several studies that meet this requirement. An important conclusion is that the experimental evidence base for health benefits of plants seems to be less strong than the evidence provided by observational population studies linking green spaces to public health. Consequently, observational studies deserve more attention from horticultural professionals and other audiences who share an interest in health benefits of plants. However, as living in green areas is often accompanied by certain conditions and lifestyles that promote health, there is still an urgent need for more rigorous studies that allow for a causal interpretation of health effects of plants and green spaces.

Full text

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Van den Berg, A. E., & van den Berg, M. M. H. E. (2015). Health benefits of plants and green space:
establishing the evidence base. Acta Horticulturae 1093,19-30.
Health Benefits of Plants and Green Space: Establishing the Evidence Base
Agnes E. van den Berg
Department of Spatial Sciences
University of Groningen
PO Box 800
9700 AV Groningen
The Netherlands
Email: a.e.van.den.berg@rug.nl
Magdalena M.H.E. van den Berg
Department of Occupational and Public Health and the
EMGO Institute for Health and Care Research
VU University Medical Centre
Amsterdam
The Netherlands
Keywords: Human issues in horticulture, landscape, people-plant interaction, pyramid of
evidence, response to nature, green space and health.
ABSTRACT
Among the many benefits provided by plants, “health” stands out as a benefit
that is especially highly valued. Over the past decades, people-plant studies have
increasingly focused on empirically demonstrating relationships between plants and
health. However, there are as yet no consensual standards in the field as to which
research findings qualify as evidence for health benefits of plants. In this paper, we first
argue that only studies that directly relate exposure to plants and green spaces to health
outcomes should be admitted into the evidence base. We then discuss several studies that
meet this requirement. An important conclusion is that the experimental evidence base
for health benefits of plants seems to be less strong than the evidence provided by
observational population studies linking green spaces to public health. Consequently,
observational studies deserve more attention from horticultural professionals and other
audiences who share an interest in health benefits of plants. However, as living in green
areas is often accompanied by certain conditions and lifestyles that promote health,
there is still an urgent need for more rigorous studies that allow for a causal
interpretation of health effects of plants and green spaces.
INTRODUCTION
Plants provide a myriad of benefits to people, from the provision of food, fibers and
building materials to more intangible benefits such aesthetic pleasure and improvement of
health (Lohr, 2011). These benefits may occur not only with individual plants, but also with
natural environments or “green spaces” such as gardens, parks, forests and other natural
settings. Among the benefits provided by plants, “health” stands out as a benefit that is
especially highly valued in modern society. Health is generally considered the most valuable
asset in life (Schwartz, 1992). Unfortunately, in many parts of the world, this asset is
threatened by developments related to aging populations and unhealthy lifestyles. To relieve
the healthcare burden that derives from these developments, governments and other health
authorities are actively seeking new ways to prevent disease and promote health. Plants have
long since been associated with healing powers, and there is a long tradition of the use of
plants for medical purposes. Plants therefore provide a potentially powerful means to relieve
the healthcare burden of an aging and increasingly unfit population in nations worldwide.

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There is a growing awareness and recognition by individuals from many different
professions of the value of scientific evidence for health benefits of plants and green spaces.
This evidence can support policy makers and other key community and health decision
makers in making well-informed decisions on the use of plants and green spaces that optimize
the benefits for the community. In addition, evidence can yield practical guidelines for
horticultural therapy and other interventions that make use of health benefits of plants and
green spaces. In this paper, we sketch a first outline of the main features of an evidence base
for health benefits of plants, including criteria for the inclusion of studies and grading of the
evidence. We also provide an overview of currently available evidence for direct health
benefits of plants from studies with different experimental and observational research designs.
In this overview, special attention is paid to the evidence provided by observational studies of
relationships between green space and health at the population level. Because publications of
these studies have been scattered across different medical and public health journals, this
literature has thus far been relatively inaccessible to horticultural professionals and other
audiences who share an interest in plants and nature. The paper concludes with a discussion of
the strengths and limitations of the current evidence base for health benefits of plants and
green spaces, along with suggestions for future research.
WHAT QUALIFIES AS EVIDENCE?
To date, there are no consensual standards in the field of people-plant studies as to which
research findings qualify as evidence for health benefits of plants. In general, the term
“evidence” is used in a very loose manner and the bar for what qualifies as evidence is set
very low.
Following standard guidelines for the evaluation of health research, we propose three basic
criteria for inclusion of research in the evidence base for the effectiveness of plants and green
spaces to improve health (National Institute for Health and Clinical Excellence, 2009).
1. Use of health outcome measures. As a basic rule, only studies that have measured
health outcomes qualify for inclusion into the evidence base for health benefits of plants. But
what constitutes a health measure? Unfortunately, there is not yet a shared understanding in
the field of people-plant studies of what constitutes a health outcome. The concept of health is
typically very broadly interpreted, including not only direct measures of physical and mental
health, but also health-related outcomes such as physical activity, stress or air quality.
In general, two types of health outcome measures can be distinguished (College of
Emergency Medicine UK., 2012; Van den Berg et al., 2012). First, clinical and patient
centered health outcome measures cover objective and subjective measures of patient
functioning, such as symptom severity, mortality, hospital days, medication use, discomfort
(pain, nausea), physiological responses, and patient satisfaction. Second, public health
measures give an indication of the health status of a population. These indicators include
measures based on birth and death statistics, such as mortality rates and life expectancy,
measures of the prevalence and incidence of disease and illness (also called morbidity rates),
measures of self-reported general, mental and physical health, and measures of health-related
quality of life.
2. Focus on exposure to plants and green spaces. A second criterion is that health
outcomes must be directly linked to the exposure variable of interest, i.e. plants and other
natural elements and settings. A major implication of this criterion is that evaluations of
nature-assisted therapies, such as horticultural or wilderness programs, are excluded from the
evidence base for health benefits of plants unless they include a control group that receives a
non-natural intervention that is identical to the nature-based intervention except for the
exposure to plants or green space. In a similar way, population studies that link green space to

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health do not count as evidence for health benefits of plants unless they use some form of
statistical control to disentangle the influences of green space from other variables that
typically co-occur with the presence of green space, such as the higher socio-economic status
of greener neighborhoods. In general, only studies that tap into the unique contribution of
plants and green spaces to health outcomes qualify as evidence for health benefits of plants.
A much-debated issue in the literature on people-plant relationships is whether
exposure to “plastic plants” and other imitations of nature (e.g., photos or videos of natural
landscapes) provides a valid way to study health benefits of plants and green spaces (De Kort
et al., 2006). As yet only few studies have compared people’s (psychophysiological)
responses to real and virtual nature, with somewhat mixed findings (Kahn et al., 2008;
Felsten, 2009; Kjellgren and Buhrkall, 2010). Therefore, we suggest to exclude studies using
photos and other simulations of plants and green space from the evidence base for health
benefits of plants and green spaces, unless more evidence for the validity of such studies
should become available.
3. Comparison of health outcomes over time and/or across locations. A third basic
inclusion criterion is that a study should compare health outcomes over time, e.g. by means of
pre- and post intervention measures, or across locations that differ in the presence of plants or
green space. Following this criterion, opinion surveys, post-occupancy evaluation studies,
focus groups or other non-comparative studies do not qualify as evidence for health benefits
of plants. This is not to imply that such studies cannot convey useful information on the use of
plants for health purposes, only that this information does not count as evidence for the
effectiveness of plants to promote health or prevent or cure disease.
The above three criteria only outline the minimum requirements for the admission of
studies into the evidence base for health benefits of plants. Additional criteria, such as
publication in a peer-reviewed journal may be set to increase the strength of the evidence
base. However, given the limited availability of studies, it seems premature to set stringent
inclusion criteria. Instead, the most pressing issue is to establish consensual guidelines for
separating relevant from non-relevant research findings.
GRADING THE EVIDENCE
Studies that merit inclusion into the evidence for health benefits of plants may vary
significantly in research method or design and the strength of the evidence provided. In
general, research studies can be broadly classified as experimental or observational. In
experimental studies, the investigator controls the assignment of participants to a (“green”)
intervention or exposure, which is often but not always performed randomly. In observational
studies, the investigator “observes” phenomena or outcomes in general or specific populations
without specifically assigning an intervention or treatment. Experimental study designs are
generally considered to provide stronger evidence than observational designs, but this applies
only when the experiment includes a control group that is not exposed to the intervention or
treatment of interest. Uncontrolled experiments or intervention studies provide weaker
evidence than most observational studies, except for case series that observe the medical
histories of single patient or exposure groups.
The hierarchy in the strength of evidence provided by different study designs can be
graphically represented as a pyramid, with the weakest (uncontrolled) designs residing at the
bottom and the strongest (controlled) designs residing at the top. Figure 1 gives an example of
a pyramid with six levels of evidence that apply to research on health benefits of nature. This
pyramid presents only a summary and simplification of all the study designs that are available
for studying health benefits of nature. For more detailed classifications, see Ho et al. (2008)
and The National Institute for Health and Clinical Excellence (2009).

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Figure 1. Pyramid of evidence. RCT = Randomized Controlled Trial; NRCT = Non-
Randomized Controlled Trial
The bottom level of the pyramid is formed by “before-and-after studies” that measure
health outcomes before and after a green intervention or exposure. For example, Raanaas,
Patil and Hartig (2010) investigated the effects of a 4-week indoor plant intervention on self-
report measures of health, well-being and emotion in coronary and pulmonary patients before,
during, and after the intervention. Due to the lack of a control group it is not possible to
determine whether any effects observed are due to the plants or to other causes such as
general distraction or spontaneous recovery of the patients. Before-and-after studies therefore
generally provide weak evidence of causality.
The next three steps up the pyramid consist of controlled observational studies. First,
in cross-sectional studies, both exposures and outcomes are measured at a single point in time,
and the prevalence of the outcome is compared among those with and without exposure to
plants and green spaces. For example, Maas Verheij, Groenewegen, De Vries, &
Spreeuwenberg (2006) showed that the amount of green space in residential neighborhoods is
related to self-reported health of residents of these neighborhoods. Because attractive, green
neighborhoods tend to attract wealthier and thus healthier people, cross-sectional studies
typically use some form of statistical control for the influence of socio-economic background
variables. However, causal influences of green space on health cannot be established with
cross-sectional methods because self-selection cannot be completely ruled out.
Cohort or longitudinal studies provide a more rigorous type of epidemiological studies
in which groups of people are followed (retrospectively or prospectively) over time to identify
potential risk or preventive factors for certain diseases or outcomes. Because events are
measured in a temporal sequence, causes can be more easily distinguished from effects. For
“Before-and-after study”
Retrospective Cohort
NRCT
Prospective Cohort

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example a retrospective cohort study by Bell and colleauges (2008) found that economically
disadvantaged children and youth in greener settings were less likely to increase their BMI
scores over two years compared to their counterparts in less green neighborhoods.
At the top levels of the pyramid we find the controlled experimental studies. In these
types of studies, the investigator assigns participants to (green) experimental and (non-green)
control conditions. If participants are assigned by chance, the experiment is classified as a
“randomized controlled trial”, the “gold standard” of medical research. Using chance to
assign people to conditions that differ on selected variables means that the groups will be
similar and that causal inferences can be drawn on the influence of the variables. For example,
Park and Mattson (2008, 2009) found that patients who were randomly assigned to recovery
in hospital rooms with ornamental plants displayed better medical and psychological
responses than patients in similar rooms without plants. Despite their significant strength,
controlled experimental studies also have important limitations. They are typically expensive,
time consuming, and designed to answer questions about the effectiveness of a specific
intervention for specific groups. Thus, controlled experiments are usually very narrow in
scope and may be inappropriate to address questions on the often long and complicated causal
chains in general populations (Victora et al., 2004).
In the next two paragraphs, we will summarize the currently available evidence for
direct health benefits of plants and green spaces from experimental and observational studies.
This summary is not a comprehensive review. It highlights some of the studies that meet the
criteria outlined above and it is mainly intended to give an impression of the size and strength
of the evidence base.
EVIDENCE FROM EXPERIMENTAL STUDIES
Experimental evidence for health benefits of plants and green space mainly comes
from two lines of research: (1) evaluations of the effects of horticultural and other nature-
assisted therapies (e.g., gardening and wilderness programs) in various patient groups (see for
reviews Elings, 2006; Annerstedt and Währborg, 2011), and (2) controlled laboratory or field
experiments in which healthy participants are exposed to natural or built settings (see for
reviews Van den Berg, 2005; Bowler et al., 2010). Both lines of research have typically
reported findings that support the relevance of nature as a resource for health and well-being.
However, following the criteria outlined above, these findings do not qualify as direct
evidence for health benefits of nature. Evaluations of nature-assisted therapies typically
include no control group, or only a passive control group that receives no therapy.
Consequently, the effects of the therapy cannot be disentangled from the effects of other
variables, such as individual counseling and group activities, and thus do not provide direct
evidence for a link between plants and health. Controlled laboratory and field experiments
provide more direct, causal evidence for positive effects of plants and green spaces on healthy
individuals. However, most of these studies have used health-related outcome variables such
as physical activity levels (Thompson Coon et al., 2011) or psycho-physiological responses
(Van den Berg et al., 2007; Bringslimark et al., 2009; Park et al., 2010) instead of direct
health outcome measures. Therefore, this line of research also provides only indirect evidence
for health benefits of plants.
Only a few random controlled trials in patient populations provide direct evidence for
health benefits of plants and green space. Among these studies is a series of clinical trials in
which patients were randomly assigned to rooms with and without potted plants after
thyroidectomy, appendectomy, or hemorrhoidectomy surgery (Park and Mattson, 2008,
2009). These trials show, among other things, that patients in rooms with plants have shorter
hospital stays and need fewer intakes of postoperative pain medication than patients in rooms

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without plants. A longitudinal study in a Norwegian rehabilitation center examined the health
benefits of bedroom views of nature among coronary and pulmonary patients (Raanaas et al.,
2012). Patients were quasi-randomly assigned to rooms with an unobstructed view onto
mountains and a valley, or to rooms with only a partial view to nature or a fully blocked view
that was dominated by buildings. The results show that an unobstructed natural view
positively affected the mental health of men and the physical health of women. In addition,
pulmonary patients with an unobstructed natural view showed greater improvement in mental
health than coronary patients with such a view.
Outside the health-care setting, a random cross-over trial among children diagnosed
with ADHD revealed that the children perform better on an attention test after a 20-minute
guided walk in a park than after a walk in a more urban downtown or residential
neighborhood (Faber Taylor and Kuo, 2009). Another random cross-over trial demonstrated
that individuals diagnosed with major depressive disorder display better cognitive and
emotional functioning after walking in a park than after walking in a built setting (Berman et
al., 2012). Furthermore, a longitudinal cross-over study among healthy office workers in
Norway revealed a 23 percent decrease in symptoms such as coughing or a horse throat
during periods when foliage plants were added to their offices (Fjeld et al., 1998).
Summary
In sum, although many experimental studies have demonstrated positive effects of
plants and green spaces on people, only few of these studies provide strong and direct
evidence for health benefits of plants. Consequently, observational studies could make an
important contribution to the evidence base for health benefits of plants.
EVIDENCE FROM OBSERVATIONAL STUDIES
In the early 2000’s, research groups in the Netherlands (De Vries et al., 2003) and
Japan (Takano et al., 2002) demonstrated for the first time direct relationships between green
space in the living environment and public health. In the Dutch cross-sectional study, data on
three health outcomes (perceived general health, mental health and number of health
complaints) from a random sample of more than 10,000 people were linked to land-use data
on the amount of green space in their living environment. Results show that, after controlling
for socioeconomic and demographic characteristics, residents with a high percentage of green
space in a 1 or 3 km radius around their home reported better general and mental health and
fewer health complaints than those with a low percentage of green space around their home.
Relationships between green space and health were stronger among lower socioeconomic
status groups, the elderly, and others who stay home during the day, suggesting that the
impact of green space on health is related to the amount of exposure to the local environment.
Takano et al. (2002) examined the five year survival rate of a cohort of more than
3000 seniors living in a highly urbanized area of Tokyo. They found a significant association
with the self-reported amount of walkable green streets and spaces near the residence: older
people who perceived their neighborhood as greener and easier to walk in lived longer (13 %
higher odds for survival) independent of age, sex, social economic status and other potential
confounders.
In the past ten years, the findings of these pioneering studies have been replicated and
extended in different countries with various objective and self-reported indicators of green
space and health. Thus far, however, this emerging literature has not received much attention
in the field of people-plant studies. Below, we will discuss some of the key studies and their
findings.

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The Vitamin G Research Program
The initial publication of the Dutch research group (De Vries et al., 2003) has been
followed up by a series of cross-sectional studies that were conducted as part of the large-
scale Vitamin G (where G stands for Green) program in the Netherlands (for overviews of this
program, see Groenewegen et al., 2006; Groenewegen et al., 2012). In one of these studies,
the Vitamin G team looked at people’s health as assessed by doctors (Maas et al., 2009b).
Among other things, they found that having more green space within a one-kilometer radius
of the home is linked to a reduced risk of depressive symptoms and anxiety. For another
study, the Vitamin G team visited 80 Dutch neighborhoods to collect primary data on
residents’ health and green space quantity and quality indicators such as absence of litter,
accessibility, and colorfulness (Van Dillen et al., 2011). Results show that people’s self-
reported general and mental health is positively related to both the quantity and the quality of
the green space around their homes. In addition to the Vitamin G research program, many
other observational studies have been carried out in different countries. These studies can be
roughly divided into three groups according to the principal health outcome: general health
outcomes, specific health outcomes, and mortality rates (Van den Berg and Maas, 2012).
Green space and general health
Using census data for the entire population, a cross-sectional study in the UK showed
that a higher proportion of green space in an area is associated with better population health,
especially in suburban lower income areas (Mitchell and Popham, 2007). A recent
longitudinal study in the UK demonstrated that moving to greener urban areas was associated
with sustained mental health improvements in all three postmove years (Alcock et al., 2013).
Other cross-sectional as well as longitudinal UK studies have found that individuals report
significantly better general health and mental health when living nearer the coast (Wheeler et
al., 2012; White et al., 2013). These latter findings indicate that health benefits are not
exclusive to “green spaces”, but may also occur with “blue spaces”.
A cross-sectional study in Australia showed that residents who perceive their
neighborhood as highly green have better odds of physical and mental health compared with
those who perceive it as not so green (Sugiyama et al., 2008). Another cross-sectional study in
Denmark revealed relationships between distance to green space and health and health-related
quality of life (Stigsdotter et al., 2010). A study in 75 neighborhoods in the city of
Amsterdam, the Netherlands, showed that dissatisfaction with neighborhood green space was
associated with a higher risk of fair to poor self-rated health (Agyemang et al., 2007). A
comparative study in a “green” and “grey neighborhood in Belgium with similar socio-
demographic, housing and spatial conditions found that residents of the green neighborhood
reported higher levels of general health, bodily functioning and happiness. However, only the
difference in happiness was significant (Van Herzele and De Vries, 2011). A Norwegian
study examined relationships between indoor plants and health using cross-sectional survey
data from 385 office workers (Bringslimark et al., 2007). After controlling for gender, age,
and other factors the number of indoor plants proximal to a worker’s desk was significantly
related to reduced sick leave.
Green space and specific health outcomes
A cross-sectional study in the U.S. showed lower asthma prevalence among children
living in areas with more street trees (Lovasi et al., 2008). Several studies in the U.S. and
Spain have found a reduced risk of adverse pregnancy outcomes such as low birth weight in
greener residential areas, especially among mothers with lower education (Donovan et al.,
2011; Dadvand et al., 2012a; Dadvand et al., 2012b). In Australia, access to green public open

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spaces was found to be associated with a more healthy cardiometabolic profile (Paquet et al.,
2013)
Green space and mortality
Mitchell and Popham (2008) obtained mortality records of all deaths registered in
England from the working-age population in a 5-year period. After adjustment for potential
confounders and area-level income-deprivation index, they found that mortality rates
decreased as the greenness of an area increased. Thus, residents of greener areas had a lower
risk of dying before their retirement age. The relationship between mortality and green space
was stronger for lower-income areas, suggesting that the provision of green space can reduce
income-related health inequalities. Another study by the same research group observed
significantly lower risk of all-cause mortality (excluding external causes) in small urban areas
with more than 60% green space (Mitchell et al., 2011).
A recent study in New Zealand found no association between green space and 10-year
mortality rates (Richardson et al., 2010). Another study by Richardson and colleagues,
conducted in the U.S., found that mortality from all causes was even higher in greener areas
(Richardson et. al, 2012). This study was performed at the geographical scale of whole cities
with a total population of 43 million people. The authors of both studies argue that their
results are country-specific: the variation in green space availability in New Zealand may be
too small to find a relation and in the U.S. greater car-dependency related to urban sprawl as
compared to West-European countries may obscure the potential health benefits of green
spaces.
Summary
In sum, observational - mostly cross-sectional studies - indicate that access to plants
and green spaces may promote health and protect people from the negative health
consequences of living in poverty. However, to gain more insight into the causality of the
relationships, there is a need for more rigorous, prospective study designs that follow large
numbers of people with different exposures to green space over longer periods of time. To
obtain a more balanced picture of both the positive and negative impacts of plants on health,
future studies should also allow for the detection of possible health risks of plants and green
space, such as the development of allergies due to sensitization to pollens and other natural
substances (Cariñanos and Casares-Porcel, 2011).
CONCLUSION AND SUGGESTIONS FOR ADVANCING THE EVIDENCE BASE
In this paper we have outlined the evidence base for health benefits of plants and green
spaces. We have argued that only studies that speak directly to a relationship between plants
and health should qualify as evidence for health benefits of plants, and we have proposed
some basic criteria for inclusion and grading of evidence. We have also highlighted some of
the currently available evidence for health benefits of plants from studies with experimental
and observational designs. An important conclusion is that the experimental evidence base for
health benefits of nature does not seem to be very strong. Although experimental research has
generally demonstrated positive effects of plants and nature-based activities on people, only
few of studies have employed proper designs and measures that enable the identification of a
direct link between plants and health. In the past ten years, an increasing number of
observational studies have provided important complementary evidence for health benefits of
plants at the population level. However, as living in green areas is often accompanied by
certain conditions and lifestyles that promote health, there is still an urgent need for more

9
rigorous studies that allow for a causal interpretation of health effects of plants and green
spaces. To advance the evidence base for health benefits of plants, we propose three potential
research directions. First, there currently exists a large body of experimental studies on
nature-assisted therapy and restorative effects of nature that only indirectly provides evidence
for health benefits of nature. By implementing some changes in the study design, future
studies in these domains would qualify more directly for inclusion in the evidence base. For
example, evaluations of nature-assisted therapies could include control groups that receive a
non-nature therapy, or experimental studies on restorative effects of viewing or visiting nature
could more often use patient samples instead of healthy volunteers. In general, when
designing an experiment, investigators should become more aware of the potential relevance
of their study to the evidence base for health benefits of plants and green spaces.
A second direction for future research pertains to the need for more rigorous
prospective designs to assess the long-term health effects of exposure to plants to support
well-informed decisions on the use of plants and green spaces in public health policy and
practice. Finally, there is still little evidence on the mechanisms underlying relationships
between plants and health. While many studies have demonstrated links between green space
and possible mechanisms, such as stress, physical activity or social cohesion, only few studies
have conducted mediational analyses to determine whether these mechanisms can explain
relationships between green space and health (see for example Maas et al., 2008; Maas et al.,
2009a; Van Herzele and De Vries, 2011). Conducting more of these studies can lead to a
better understanding of the pathways by which plants may lead to better health, which can be
used to optimize the use of plants and green spaces for the benefits of the community.
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