Green Thumbs and Blue Moods: The Effect of Indoor Plants on Mental Health

Background

It is well established that mental wellbeing plays a crucial role in overall health. This includes not only diagnosable mental illness, but also everyday stress and mood that everyone experiences. The absence of positive mental health – not just the presence of a specific mental illness or symptoms – has been shown to increase the probability of mortality among men and women of all ages (Keyes & Simoes 2012). Clearly mental wellbeing has profound effects on all people, but pinpointing exactly what factors contribute strongly to mental wellbeing is less clear.

One factor that many can relate to is spending time in nature. Most people have gone for a walk, sat in a park, or otherwise spent some time outdoors in nature and experienced the calming yet uplifting effect that the tranquility of nature has on them.

But because of the complex modern society we have built for ourselves, for many people spending time outdoors is not as easy as simply stepping out of their front door. According to the UN, as of 2018, 4.1 billion people live in urban areas, or 55% of the Earth’s total population; this number increases to a whopping 82% of those living in the US (United Nations). Though not all areas that are defined as “urban” are massive concrete jungles, it’s fair to say that the majority of the Earth’s population and the overwhelming majority of Americans do not live surrounded by nature, and for many, access to natural landscapes of any kind is limited. This can have detrimental effects on the lives and mental health of those urban residents. Higher proportions of green space surrounding one’s living area and a smaller distance to useable green space has been associated with lower levels of treatment for anxiety and mood disorders (Nutsford et al. 2013), while those living further away from green space have been shown to be more likely to experience stress (Stigsdotter & Grahn 2011). Even for those who are fortunate enough to have access to nature, many take it for granted. On average, Americans spend 92% of their time indoors (Klepeis et al. 2001). While this figure is now two decades old, it is the most recent reliable estimate that we have. It represents a shocking separation between modern humans and the natural world, even in a time when working from home was far less common, and long before the threat of a pandemic forced hundreds of millions of Americans to shun themselves in their homes for months.

It is evident that modern society’s indoor-based lifestyle has deleterious effects on the mental wellbeing of people. One proposed solution to this problem is indoor plants and greenery. The question that I hope to answer in this study is whether bringing a little bit of nature into the home or workplace can help to offset some of the negative effects that time spent indoors has on mental health. There is currently some evidence to suggest that tending to plants can have a positive impact on mental wellbeing. Having houseplants or a garden has been associated with markers of positive mental health; it is hypothesized that plants in the home can give the impression of being away while at home (Dzhambov & Dimitrova 2021). This was particularly useful during the COVID-19 pandemic. During a time of unprecedented stress for all, 73% of respondents in one study reported that owning and tending to indoor plants had a positive impact on their mental health during pandemic lockdowns, while only 3.5% of respondents disagreed with that sentiment (Pérez-Urrrestarazu et al. 2021). In another study conducted on young adults, a simple 15-minute plant transplanting task demonstrated a decrease in blood pressure and increase in alpha waves in the brains of the subjects – both of these parasympathetic responses are indicative of a decrease in overall stress (Hassan et al. 2018). This trend of increased mental wellbeing associated with plants could extend even to schools or workplaces. In one study, female high school students who were placed in an experimental group with plants present in their classrooms demonstrated significantly increased happiness scores over a control group not exposed to plants in the classroom (Najafi & Keshmiri 2019). This is an important finding because it suggests that the mere presence of – not just the act of tending to – plants indoors can have a demonstrably positive impact on mental wellbeing. In fact, the positive psychological impact of ornamental indoor plants can be so significant that it can have tangible benefits on physical health. In a study conducted among hospital patients recovering from surgery, patients in rooms with ornamental plants during the recovery period experienced more positive psychological responses, such as lower systolic blood pressure, pain, anxiety, and fatigue, as well as a subjectively positive experience based on comments about a more pleasant environment and stress reduction (Park & Mattson 2009). These psychological benefits of passive interaction with indoor plants are worth further investigation as it could be a simple and effective way to improve mental wellbeing for many without regular exposure to nature.

Our society does not seem to be moving away from spending copious amounts of time indoors in favor of reconnecting with nature any time soon. Our modern way of life has been trending further and further away from the natural world; a trend that doesn’t seem to be slowing down, regardless of the detrimental effect it has on our mental and emotional health. It is therefore a worthwhile endeavor to discover whether indoor plants can be an effective measure to increase overall happiness for those with limited access to green space in their day to day lives. In this study, I hope to find out what kind of impact indoor plants can have on the mental health of Bloomington residents, especially as winter approaches and quality time spent outdoors becomes more rare.

 

Methods

In order to find out what kind of effect, if any, indoor plants have on the livelihood and mental health of people, I intend to survey members of the Bloomington community on their plant ownership and habits, as well as other relevant information to their mental health. My plan with the survey is to establish a baseline understanding of their mental wellbeing without making them go into too much detail, and then use other questions about plants and other information to determine if a correlation can be found between the plant ownership and mental wellbeing. Surveys have been demonstrated to be an effective data collection method in this field, as demonstrated by Pérez-Urrestarazu et al.’s comprehensive evaluation of home plant ownership during the COVID-19 pandemic (2021). This survey will be distributed via a web-based survey creator like Survey Monkey. While paper surveys do tend to yield a higher response rate than web-based surveys, the relative advantage of being able to reach much more respondents with a web-based survey should outweigh this shortcoming, and there is no difference in content of responses between the two (Hayslett & Wildemuth et al. 2004). The questions I’ll be asking in this survey and answer choices are as follows:

1. What is your gender?
   1. Male, female, other (free response)
2. What is your age?
   1. Free response
3. On a scale of 1 to 10 (1 being least stressed, 10 being most stressed), how would you rate your level of stress in the past 30 days?
   1. Multiple choice from 1 to 10
4. On a scale of 1 to 10 (1 being most unhappy, 10 being most happy), how would you rate your level of happiness in the past 30 days?
   1. Multiple choice from 1 to 10
5. In the past 30 days, have you experienced problems falling asleep or staying asleep?
   1. Never, rarely, somewhat often, often, very often
6. How many indoor house plants do you own?
   1. 0, 1-3, 4-6, 7+
7. In the past 30 days, when indoors, about how much time have you spent in a room with plants per day?
   1. Less than 1 hour, 1-3 hours, 4-6 hours, 7+ hours
8. In the past 30 days, how much time have you spent per week tending to house plants? (Watering, repotting, etc.)
   1. None, 10-30 minutes, 45 minutes-1 hour, More than 1 hour
9. In the past 30 days, how much time on average have you spent outdoors per day?
   1. Less than 1 hour, 1-2 hours, 3-4 hours, 5+ hours
10. In the past 30 days, when indoors, about how much time have you spent in a room with windows with a clear view of green space (any natural landscape, e.g., not a parking lot)?
    1. Less than 1 hour, 1-2 hours, 3-4 hours, 5+ hours
11. Do you own a garden or outdoor plants?
    1. Yes, no
12. If you own indoor house plants, how would you say that they impact your overall happiness and mental health?
    1. Free response

As you can see, I wanted to make this survey relatively short and easy to answer. According to Kota and Correa de Rosa, decreasing the length of a survey maintains reliability while also increasing both response rates and completion rates (2018). This is also why I made most of the answer choices multiple choice, making the survey easier and quicker to answer.

The first two questions are just questions about the respondent’s demographics. These are just used to identify whether there is a trend associated with a particular gender or age group.

You’ll notice that for a lot of the questions I asked, I specify the respondent to answer based on the “past 30 days.” Time-bound retrospection in self-reporting surveys is widely used in behavioral health research because it forces the respondent to think critically about his or her experience over a specific time period, as opposed to making a general assumption based on a larger time scale. For example, someone who is not usually under high amounts of stress may read the question without the specified time period and respond with a 3 or 4, but if they’ve had a particularly stressful few weeks recently, they might respond with a 6 or 7 to the question that specifies a time period. It also ensures that everyone is answering the question based on the same parameters. It’s important to note that subtle differences in wording may have a significant effect on answers; more specific wording like “past 30 days” as opposed to “past month” may yield more precise and specific answers (Gryczynski, J. et al. 2015).

To establish a baseline for respondents’ mental health, I used the 12-item General Health Questionnaire, which is widely used as a diagnostic tool for psychological wellness (Hystad & Johnsen 2020), as a general guideline. The GHQ-12 is shown below.

Because I aimed to make my survey quick and easy to increase response rate, I tried to summarize this 12-question questionnaire in three simple questions. I believe the GHQ-12 can be broken down into two categories: those that assess a person’s feelings of stress and anxiety and their ability to cope with those feelings (1, 2, 4, 5, 6, 8), and those that assess a person’s sense of happiness and fulfillment (3, 7, 9, 10, 11, 12). For this reason, I broke it down into two simple questions: are you stressed, and are you happy? I added the third question about sleep habits because it can be an important indicator of mental health and is specifically addressed in the GHQ-12. Poor sleep quality has been strongly associated with poor mental health, especially symptoms of anxiety, depression, and overall psychological distress (Rezaei, M. et al. 2018). For this reason, the question serves as a sort of subconscious reporting on mental status; a person might not feel consciously aware of their stress, but poor sleep quality can indicate that there is a problem.

Questions 6-8 aim to assess the respondents’ ownership of and time spent with indoor plants. I felt that it was important to distinguish between time spent passively in the presence of indoor plants and time spent actually interacting with them because some previous research has established a link between direct interaction and positive psychological effects (Hassan et al. 2018), while some indicates that the mere presence of plants is enough to have a benefit (Najafi & Keshmiri 2019, Park and Mattson 2009).

Questions 9-11 are used to identify possible extraneous variables that could contribute to mental health status other than indoor plant ownership and exposure. Because so many factors play a role in a person’s psychological state, it’s impossible to rule out all possible outside variables; these questions simply hope to address some common ones that are closely related to plant ownership. Spending time outdoors and exposure to green space have both been demonstrated to have a positive impact on mental health, and tending to a garden or outdoor plants is close, but not quite what we’re hoping to assess with this survey.

Finally, question 12 is a subjective response on how people perceive plants to impact their mental health. It’s important to assess this because a subjective feeling that plants have a positive impact can actually have a positive impact itself.

With these questions answered, I hope to use regression analysis to find links between some of the variables that this survey assesses and indoor plant ownership and exposure. Regression analysis aims to determine whether or not there is a significant correlation between a variable (number of plants owned) and an outcome (more positive mental health). This will allow me to assess which variables are the most effective predictors of the respondents’ mental health and hopefully find a pattern that definitively answers the question of how plants effect mental health.

 

Results

I conducted my survey over a period of about one week to try to minimize any confounding variables associated with seasonality. Though the research is incomplete and conflicting, there is some evidence to suggest that seasonality may play a role in mood and mental wellbeing (Schwartz et al. 1996, Kurlansik et al. 2012).  I was able to collect 54 responses, which I consider an acceptable sample size given the time constraints and lack of incentive to encourage respondents. This survey was mainly distributed to undergraduate students at IU Bloomington. Unsurprisingly, as a result, the demographic of the respondents of this survey was very uniform in age, with an average age of 21.1 years and only four respondents falling outside the range of 18-22. More unexpectedly, the gender of respondents was 74.1% female, 24.1% female, and 1.9% other or prefer not to answer, despite the survey being sent to a relatively equal numbers of each gender. So, the demographic of this survey is predominantly female college students. A uniform demographic does have advantages because it eliminates potential confounding variables that could result from varying age groups, genders, and occupations. Below are the graphical representations of answers to some of the questions of my survey.

Figure 1

Figure 2

Figure 3

 

 

 

 

 

 

 

Figures 1 and 2 show that the average respondent is pretty stressed out, with a mean rating of 7.3 out of 10, but considers themselves more happy than unhappy, with a mean happiness rating of 6.1 out of 10. Figure 3 displays a fairly high prevalence of sleep issues, with 66.7% reporting difficulties falling or staying asleep at least somewhat often.

 

Figure 4

Figure 5

Figure 6

Figures 4, 5, and 6 give a good idea of respondents’ plant ownership habits. The majority of respondents own at least one plant, with just over a quarter of respondents reporting no plant ownership. However, only 40.7% of respondents reported spending at least 4 hours per day in the presence of plants. Respondents also do not spend much time on average tending to house plants, with 40.7% spending no time watering them or tending to them in any way.

 

Figure 7

Figure 8

Figure 9

Figure 7 shows that the average respondent spends about the same amount of time outdoors as the average American (1-2 hours). Figure 8 shows that time spent with a view of green space is fairly evenly distributed, with the most common answer being 5 or more hours. Finally, per figure 9, the majority of respondents do not own a garden or outdoor plants.

To determine whether or not there was a significant correlation between reported mental wellbeing and any of the associated factors I asked about in the survey, I graphed the results with the variables on the x-axis and happiness or stress on the y-axis and constructed a trendline to determine the R² value, which measures the strength of the relationship between the two. The results can be seen below (positive R² values denote a positive correlation; negative R² values denote a negative correlation):

Table 1

Participants were also asked to describe subjectively how they felt indoor plants affected their happiness and mental health. While reading through these responses to get an idea of how people felt about plants, I categorized responses into three main groups: positive effect, negative effect, and neutral or no effect. I did not count the 14 respondents who said that they did not own plants in this assessment. The results are below:

Figure 10

Discussion

The first important note based on these results is the fact that no relationship that I examined had an R² value that would be considered statistically significant, including trends that have been previously established as reliable mental health predictors. While one possible explanation for this could be that the data set was too small to establish a clear trend, the biggest issue that I believe contributed to this was my survey taking method. I only allowed 4 or 5 response choices per question, using ranges to make answering the questions easier for respondents, because most people are unlikely to be able to reliably estimate the amount of time that they spend in rooms with plants or taking care of plants. What I failed to recognize ahead of time is that this significantly limits the variety of responses and therefore clumps the data together along the x-axis, which makes it much more difficult to establish a linear trend. You can see this from the graph to the right: The data is clumped into 4 categories along the x-axis. Allowing a greater variety of responses would spread the data out more along the x-axis and possibly allow for a more clear linear trend.

That being said, I will evaluate the observed trends based on their relative strength compared to each other, with the caveat that these results shouldn’t be considered statistically significant independently. The three strongest trends that were observed based on this survey were happiness vs. time spent indoors with a view of green space (0.0853), happiness vs. frequency of sleep problems (-0.0766), and stress vs. frequency of sleep problems (0.0571). As mentioned earlier in this blog, closer proximity to green space has been shown to be a significant indicator of improved mental health (Nutsford et al. 2013), so it is not surprising to see this trend. It has also been discovered previously that natural outdoor environments may provide enhanced psychological restoration compared to simulated indoor environments (Kjellgren & Buhrkall, 2010). This suggests that there may be an important component of true nature that cannot be equally replicated indoors by simulations or houseplants. It is also unsurprising that an increased frequency in difficulty falling asleep or staying asleep is correlated with decreased happiness and increased stress because poor sleep quality has been established as an indicator of poor mental and physical health; specifically, among this study’s demographic of college students, it results in reduced learning capacity and is associated with a decrease in grade point average (Friedrich & Schlarb, 2017). This leads me to conclude that previously known predictors of improved or reduced mental health like proximity to green space and sleep quality have a greater effect on a person’s overall mental wellbeing than indoor plants.

What I did find surprising is that the next strongest trends were happiness vs. time spent tending to plants (0.0319), happiness vs. number of plants owned (0.0242), and happiness vs. time spent with plants (0.0213). While none of these relationships are very strong, they all had a higher R² value than stress or happiness and time spent outdoors. Spending more time outdoors in natural environments has been shown in the past to be beneficial for human mental health (Triguero-Mas et al. 2017). The fact that three trends associated with indoor house plants were stronger indicators of increased mental health than spending time outdoors demonstrates that there is potential for indoor plants to be beneficial for the overall emotional wellbeing of humans. Also of note is that more intimate interaction with plants seems to be more beneficial than passive interaction. As listed above, when it comes to a participant’s reported happiness, time spent tending to plants was more beneficial than the number of plants they owned, which was more beneficial than merely spending time in the presence of plants. This suggests that much of the emotional benefit that comes from plants is a result of direct interaction with them, rather than simply looking at them. This is consistent with previous research discussed earlier which directly links working with plants to improved mental health (Hassan et al. 2018); however, what is interesting is that Hassan et al.’s study specifically found that interaction with plants decreased physiological stress responses, and across the three types of plant interaction discussed in my study, all of them saw a greater connection to happiness than to stress.

Because happiness and spending time tending to plants was the strongest plant-related relationship that I observed, I decided to perform a t-test to determine if happier participants spent a statistically significantly greater amount of time tending to plants. I separated respondents into two groups: those who reported a happiness level of 5 or below, and those who reported 6 or above. I then gave them a score of 1 through 4 depending on their response to how much time they spent with plants and compared their means. The results are below:

Table 2

The two-tail p-value is 0.0378, which is below the typical significance level of 0.05, which means that we can conclude that happier respondents spend significantly more time tending to plants than less happy respondents. This contrasts with happiness and plant ownership, whose t-test yielded a two-tail p-value 0.215, well above the accepted value of 0.05, indicating that owning more plants is not strongly associated with increased happiness. Owning a garden or outdoor plants was an even poorer indicator of happiness, with a two-tail p-value of 0.536.

In regard to the free response question, of the 40 participants who indicated that they own indoor house plants, 27 of them (67.5%) reported that plants impacted their happiness and mental health positively, 10 (25%) reported a neutral impact or no noticeable effect, and only 3 (7.5%) indicated a negative impact. Along with this overwhelmingly positive attitude toward plants, the content of these responses was noteworthy as well. The word “stress” was mentioned only 4 times across all 40 responses. 3 of these were negative, indicating that plants added a stressor to their life in some way, while only one reported stress reduction thanks to plants. The word “happy,” on the other hand, was mentioned in 17 of the 40 responses, all of which stating that the plants made them happy; two people mentioned that their plants dying makes them sad, one of whom stated the overall effect was positive, while the other reported a net neutral effect. This reinforces the prior findings that plants add happiness to people’s lives more than they reduce stress.

 

Conclusion

The results of this study indicate that proximity to green space and sleep abnormalities are both stronger indicators of mental health than indoor plants; however, there is a statistically significant relationship between increased happiness and time spent tending to plants, indicating that direct interaction with plants is beneficial for a person’s overall happiness. Those who own plants also overwhelmingly believe plants to have a positive impact on their mental health, especially by increasing their happiness. It’s estimated in the sustainable happiness model that approximately 40% of our happiness is a result of intentional activity (Sheldon & Lyudbomirsky, 2019), so these small regular interactions with plants that respondents categorize as making them happy play an important role in boosting their overall level of happiness, even if the individual moment of happiness is minimal and fleeting.

For future study of this topic, I believe that experimental analysis may yield a clearer picture of causality between tending to indoor plants and improved happiness. I’d recommend a study in which a group of participants spends time both with plants and without to see if there is a significant difference in their happiness with and without the plants. This would help to eliminate many of the extraneous variables that I couldn’t fully account for in this study.

 

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