Therapeutic horticulture as a potential tool of preventive geriatric medicine improving health, well-being and life quality – A systematic review

For the purposes of this article, health is, as stated by the WHO in 1948, ‘a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity’. According to this definition, a ‘healthy person’ is one in the state of overall well-being and able to function independently on a daily basis. Positive outcomes are interpreted not only as the amelioration of symptoms of a chronic illness but also as the presence of positive and lack of negative emotions. The term ‘gardening’ is used to describe growing, cultivating and otherwise taking care of plants in a non-commercial setting. Garden maintenance, care of house plants and floristry are also included in ‘gardening’. In this study, horticultural therapy was considered as a form of gardening that engages clients in gardening activities and aims to improve their physical, physiological and social well-being.

The databases searched were PubMed, ProQuest and Google Scholar (the first 10 pages), using the keywords horticultural therapy, therapeutic horticulture, therapeutic gardening, elderly, and older adults. The combinations of the terms in the keyword search are presented in Figure 1. Only the articles published between January 1, 2010, and December 31, 2021, were taken into consideration. Due to the lack of agreement on the actual definition of ‘horticultural therapy’, articles on horticultural therapy, therapeutic horticulture and therapeutic gardening were also included. Titles and abstracts of the identified published articles were reviewed for relevance. For all substantial articles, references were manually searched for inclusion in other significant articles.

Figure 1

Studies were included if they met the following criteria.

To ensure consistency between the studies, data extraction sheets were developed. The extracted data for each study included (if possible) the author and study design, year of the publication, country, settings, participant characteristics, description of the intervention and control activities (examples of horticultural activities performed during the studies), treatment regimen, outcome measurement methods and results.

The quality of the included articles was evaluated using the Johns Hopkins Nursing Evidence-Based Practice Levels of Evidence guidelines (JHNEBP), which is a clinician-focused model granting quick and appropriate application of present research and best practices. The model objective is to ensure that patient care is provided with the use of the latest research findings and best practices. It encompasses five levels of evidence that show if the studies are of high, good or low quality based on the study design, sample size, control group type, consistency of results and ability to draw conclusions (Dang and Dearholt, 2019).

The CDSR search revealed that no similar systematic reviews were being conducted. The search strategy identified 5731 results, of which we selected 81 for further review. The excluded articles included studies on virtual horticulture and studies in which the participants were <60 years with systematic reviews. After full-text evaluation, a further 30 articles were excluded due to a lack of description of horticultural activities, missing participants’ characteristics and/or their age or due to being literature reviews or database duplicates. The next 33 articles were excluded as duplicates between databases. Finally, 35 papers were deemed eligible, including 17 identified following a manual search of references. The search process is summarised in a flow diagram in Figure 2.

Figure 2

An overview of 35 case studies is presented in Table 2. Over half of the studies (19) were published from 2019 to 2021, which shows the increasing interest in researching TH for healthy older adults. Another 16 articles were published between 2010 and 2018. There were no studies between 2011 and 2013 (Figure 3).

Figure 3

Most of the studies (21) came from Asian countries: Korea (6), China (4), Taiwan (5), Singapore (4), Indonesia (1) and Japan (1). The rest came from Europe (5), including Italy (2), the UK (2) and Sweden (1), and from other countries: the USA (4), Canada (1), Australia (3), and New Zealand (1) (Figure 4).

Figure 4

The study groups were highly diverse in terms of size and the age of the participants, although all of them were deemed healthy older people. The sample size ranged from 10 to 3,237 participants, and the participant age from 53 years to 103 years. The research analyses included the elderly residents of nursing homes (13) and independently living seniors (22) (Table 2).

The intervention types included HT programs (15 studies), gardening activities (13 studies) and passive contact with nature (7 studies). The interventions lasted from 1 day to 1 year, but most commonly were from 6 weeks to 12 weeks (15 studies), with the duration of sessions ranging from 5 min to 120 min, but, sometimes, that depended on the participants (Table 2). The studies yielded a wide range of health outcomes of physical, psychological and sociopsychological nature.

Our analysis identified two main types of intervention, i.e. passive or active people–plant interactions. The active interventions were applied in 28 studies and the passive ones in 7 studies.

Active interventions included gardening, garden design and planning, plant-related art, plant-related crafts, floristry, garden parties with plant-related cooking, and an environmental volunteering program (Table 3). Most of the studies combined more than one active intervention. Gardening was the most popular of all interventions and was used in 24 studies. Gardening activities were more often held outdoors (14 studies) than indoors (10 studies) and included actions such as seeding, propagating, planting, replanting, watering, weeding, fertilising, trimming, compost making, harvesting, plant care, and garden bed making. The second-most popular activity was plant-related art which included making name tags and fans using dried flowers, flower pockets, grass dolls, pressed flower frames and cards, sun-catchers, origami flowers and nature art collage. It also covered rock art, leaf printing and sketching, jigsaw puzzles of weeds and decorating flower pots. Plant-related art activities were used in eight studies. The remaining activities included floristry (eight studies) – flower and plant arrangements, making praising and dry bouquets, wreath making, planting succulents in bottles, kokedama and terrarium making; making plant-related items (five studies) such as potpourri, lavender soap, herbal heating pads, body scrubs, anti-mosquito herbal repellents, multi-functional herbal creams, monopoly maps of plant landscapes of the community and herb tea; garden design and planning (four studies); garden parties with plant-related cooking (four studies) and an environmental volunteering program (one study).

Passive interactions were differential and included garden visits (two studies), visual stimulation with plants (two studies), outdoor walking group (one study) and green exercise (one study) (Table 3).

Three groups of specified test measurements were used for this research. Thirty-three different psychological tests were used in 21 studies, and 32 different physiological and functional parameters were assessed in 23 studies. Also, 16 studies used questionnaires developed by the investigators or interviews, of which the most popular were the individual interviews which were used in 11 studies (Table 4).

The studies focused on psychological effects and explored depression, anxiety, well-being, life satisfaction, quality of life, self-respect, self-esteem, happiness, stress and attitudes to ageing. Most commonly, the psychological tests used in three or more of the studies were the Geriatric Depression Scale (GDS), Self-rated Health and Quality of Life (SF-36), Mini-Mental State Examination (MMSE), Friendship Scale (FS), Lubben Social Network Scale (LSNS) and the Attitudes to Ageing Questionnaire (AAQ).

Depression and anxiety were investigated in eight studies using the GDS, Zung Self-Rating Depression Scale (SDS), Zung Self-Rating Anxiety Scale (SAS) and State-Trait Anxiety Inventory (STAI). All studies showed a decrease in depression and anxiety levels after the completion of HT programs. Masuya et al. (2014) obtained scores of great significance on the GDS-15 scale that showed the benefits of their HT program on short-term depression. This may be attributed not only to the participant’s ability to take care of plants but also to share their experiences with each other. Chen and Ji (2015) registered a lowering of the depression scale from 8.1 before the treatment to 3.6 after the study (p < 0.001). Also, Sia et al. (2020) reported that SAS scores significantly developed after the intervention. The TH interventions induced improvements in mean SAS scores at time points 2 and 3, by −2.18 ± 0.90 (p < 0.05) and −1.87 ± 0.60 (p = 0.03) in comparison to the baseline. That supports the assumption that HT reduces the depression and anxiety of older residents in nursing homes. Only one study (Park et al., 2016) showed enhanced depression levels in pre-post measurements in the control group (not participating in any gardening or other activity), from normal before gardening to slightly depressive symptoms by the end of the study.

Well-being, quality of life and life satisfaction were measured in 11 studies. The outcome scores for well-being included the Personal Well-Being Index (PWI), Ryff’s Scales of Psychological Well-Being (SPWB), World Health Organization Well-Being Index (WHO-5) and Warwick Edinburgh Mental Well-Being Scale (WEMWBS). The outcome scores for the quality of life and satisfaction with life studies included Quality of Life (QoL), Satisfaction with Life Scale (SWL), Self-rated Health and Quality of Life (SF-36), Life Satisfaction Index (LSI) and Meaning of Life Scale (MLS). Four of the studies revealed a significant increase in life satisfaction (Masuya et al., 2014; Tse, 2010; Yao and Chen, 2017; Gagliardi et al., 2020). Masuya et al. (2014) stated that high QoL scores may be due to the commitment to caring for plants daily and the ability to share this experience with others. As people age, they tend to lose interest in their surroundings, and these results are of great significance for further investigation. An Italian study (Gagliardi et al., 2020) demonstrated a boost in satisfaction with life from 7.2 in the pre-intervention tests to 8.0 in the post-intervention ones. These findings are of huge importance, as Italian national insights show that older people are generally less satisfied with life than the entire population, and the national mean value of life satisfaction is 7.6. The other studies did not report any significant changes in well-being and life satisfaction between the intervention and the groups without intervention.

Other psychological outcome measurements included the Positive Self-Respect Scale (PSRS), Subjective Happiness Scale (SHS), Chinese Happiness Inventory (CHI), Rosenberg Self-Esteem Scale (RSES), AAQ, Herth Hope Index (HHI), Profile of Mood States (POMS), Perceived Stress Scale (PSS), Connectedness to Nature Scale (CN) and Perceived Restorativeness Scale (PRS). All investigated variables improved after nature exposure and HT activities. Yao and Chen (2017) reported a significant increase in the sense of happiness after HT activities, which they correlated with the ability to express creativity during intervention sessions and satisfaction and joy at completing the work. Scott et al. (2019, 2020) indicated that gardeners show a more positive attitude towards ageing, and this facilitated the development of necessary resilience to health and physical changes. They discovered that the physical benefits obtained from gardening and the restorative benefits of a garden were associated with positive self-perceptions of ageing. Wood and Smyth (2020) revealed that an essential role in predicting physiological health outcomes during adulthood may be played by CN, but to develop such a connection to nature, it is essential to be exposed to it as a child. The study also showed a significant relation between nature connectedness and levels of stress and heart rate variability (HRV) during sleep. Those with lower CN scores were approximately more stressed and had a lower HRV during sleep, both of which are signs of poorer health.

Cognitive functions were measured in nine studies using the Montreal Cognitive Assessment (MoCA), Visual Analogue Scale (VAS), MMSE and the Clinical Dementia Rating (CDR). Significant improvement in overall cognitive function after the HT program was shown in five of those studies (Bassi et al., 2018; Park et al., 2016; Park et al., 2020; Sia et al., 2020; Wu et al., 2020). Meaningful changes in MoCA were observed by Wu et al. (2020). The mean of the pre-test (M = 15.06) was significantly lower than that of the post-test (M = 21.18), demonstrating improvement in cognitive functions of the participants following the HT program. Also, Park et al. (2016) reported significant improvement in the cognitive functions of elderly women after a 15-session gardening intervention, with an increase in K-MMSE scores from 22.6 ± 4.2 to 23.6 ± 2.8 (p < 0.05).

Social effects were investigated in eight studies. The most commonly used tools were the FS, LSNS and UCLA Loneliness Scale. The other tools included the Social Engagement Scale (SES), Positive Relations with Others Sub-Scale (PROS) and Interpersonal Intimacy Scale (IIS). Half of the studies showed improvement in social connections after plant-related interventions (Chen and Ji, 2015; Ng et al., 2018; Tse, 2010; Yao and Chen, 2017). Chen and Ji (2015) and Tse (2010) mentioned that the horticultural activities offered the participants new subjects for conversations. New horticultural experiences and an increase in personal knowledge contributed to socialising and the formation of meaningful connections with others. Ng et al. (2018) demonstrated a significant increase in the mean scores of positive relations with others by 2.14 points (95% confidence interval (CI) 0.52–3.76) in HT participants (p = 0.001). Also, Yao and Chen (2017) established that HT was conducive to making supportive relationships amongst older adults in nursing homes and that HT, when implemented as a group activity, encouraged interpersonal relations among the participants. They observed that with each session, the participants were not only more engaged in gardening activities but also helped their neighbours more. In the last sessions, the formed bonds were strong enough to get the participants to share their thoughts and cheer for others.

Physiological effects were investigated in 13 studies using a variety of indices. The most common ones included HRV, blood pressure, electroencephalography (EEG), brain nerve growth factors (BDNF, PDGF, VEGF) and different types of biomarkers. The nerve growth factors are implicated in several molecular processes in the central nervous system and play an important role in neurogenesis. The status of cognitive health in humans is characterised by their levels (Park and Poo, 2013). BDNF is present in the hippocampus and its content increases during participation in physical activities. This stimulation contributes to the growth of new nerve cells in the brain for learning and long-term memory, as well as other cognitive functions (Cotman and Berchtold, 2002). Physical activity also affects VEGF and PDGF but a greater association is shown with vascular formation, permeability and connective tissue changes, which can promote cell proliferation and growth as well as neuronal development and function (Czarkowska-Paczek et al., 2006).

Only four studies showed significant differences in the physiology of the participants. Tu et al. (2020) reported decreased salivary amylase activity (SAA) and pulse rate after a horticultural program. Pulse rate and SAA, which represent the current state of mental stress and physical fatigue, decrease in a state of relaxation. Based on these results, it can be assumed that horticultural activities in this study lead to the relaxation of older adults.

Hassan et al. (2019) observed a drop in systolic blood pressure, which is the indicator of calmness in older adults, and variations in high alfa and beta brainwaves, after visual stimulation with a money plant (Pilea peperomioides). Different mental states are linked to different waves in the human brain, and relaxation is indicated by alpha waves and activity by beta waves. During the visual stimulation with a money plant, higher alpha waves were observed when compared with the control, revealing improved calmness after viewing the plant versus viewing the control stimulus. The significant increase in beta activity indicated alertness and a higher activity state in the participants. Park et al. (2019, 2020) observed a significant increase in the levels of BDNF and PDGF following a gardening activity.

Nine studies examined the physical and functional effects of active or passive interventions. The majority of them reported no significant changes in the Modified Barthel Index (BI), Lawton’s Instrumental Activities of Daily Living scale (IADL), Fried Frailty Index (FFI), General Self-Efficacy Scale, Activities of Daily Living Scale (ADL-20), Senior Fitness Test (SFT), International Physical Activity Questionnaire-Short Form (IPAQ-SF), Physical Activity Scale for the Elderly (PASE) or Scottish Physical Activity Screening Question (Scot-PASQ). Yao and Chen (2017) found a significant increase in BI and ADL-20 after their HT program. The elderly that joined the HT activities became more self-sufficient in everyday tasks like eating meals. Chu et al. (2021) observed a significant decrease (from 33.56 s to 25.38 s) in the time needed to perform The Cup Stacking Test in the experimental group and no significant change in the control group. These observations confirm improvement in the hand–eye coordination after horticultural activities. In addition, HT can provide physiological adjustment, the enhancement of small and large motor skills and improve self-care amongst the elderly. The results foster the theory that suitable horticultural activities can improve the ADL of elderly people living in nursing homes.

Self-developed questionnaires and interviews were used in 16 studies. The most common were individual interviews (11 studies), but their duration and structure varied between the studies. The second most popular were garden questionnaires (four studies), which were different in every study. All of them were used to gain qualitative data, such as the participant’s opinion on the HT program, their attitude to gardening, their favourite gardening activities and gardening-related problems that cropped up with age. Most of the participants showed interest in gardening activities, recognised them as pleasant and were willing to continue them after the HT program finished. The enjoyment of their horticultural experiences and gaining personal knowledge allowed new discussions and even caused more of a team effort while performing other institutional activities. Nature was reported as a high value with an affinity to life. Some seniors perceived plants almost as people and their gardens as a really important element of their life with gardening activities as a part of their daily schedule.