Human Health & Plants Research: Effect of Horticultural Therapy on Mental Health

Dr. Charles Guy

Gardening has been anecdotally touted to benefit the gardener’s mental health for more than 200 years.  However, in the last five years, a number of randomized controlled trials (RCTs) of the effects of horticultural therapy on participant mental health have begun to emerge in the peer-reviewed scientific literature.  Randomized controlled trials are the gold standard for evidence-based medical research.  Moreover, science is most robust when research evidence and outcomes are independently observed and confirmed.

At the National Chung Hsing University in Taiwan, Hung-Ming Tu conducted a systematic search of the literature and a meta-analysis of eligible RCTs that investigated the effect of horticultural therapy on one or more aspects of participant mental health1.  A meta-analysis is a statistical approach that combines the results of multiple scientific studies.  Meta-analysis can be performed when multiple independent scientific studies evaluate similar questions.  The objective is to use statistics to derive a pooled estimate of treatment effects and outcomes and gauge potential therapeutic benefit.  Meta-analysis is regarded as the most trustworthy source of evidence in the evidence-based medical literature.

Tu conducted keyword searches of the vast scientific literature using a framework known as PRISMA, with the words horticultural therapy or therapeutic horticulture and randomized controlled trial.  The searches retrieved 1,056 candidate articles from which 18 eligible RCTs involving mental health were identified and included in the meta-analysis.  Studies of leisure gardening not involving horticultural therapy were not included in the analysis, or any studies if sufficient information for meta-analysis was not available.  The quality of each included RCT was evaluated on ten experimental design characteristics.  The internal or statistical validity for 13/18 RCTs was judged to be moderate experimental quality.  The Comprehensive Meta-Analysis Version 3 software was used to perform the meta-analysis.

The characteristics of the collected RCTs reflected the broad diversity of the horticultural therapy research literature.  Four studies had mean population ages greater than 60, 4 had ages over 80, nine studies had middle-aged populations, and one study was with children aged 10.  Clinical populations ranged from dementia, stroke, neurocognitive disorder, schizophrenia, maladjusted, psychiatric illness, to caregiver to healthy participants.  Ten studies had experimental treatment sample sizes that were primarily small (<20), while the largest sample size was 51.  There were 30 different psychometric assessment instruments used, and 24 were used by only one study each.  Just two assessment instruments were used in three separate studies.

Tu’s meta-analysis revealed that the horticultural therapy treatments had a significant and positive impact on participants’ mental health compared to the control groups.  This held true across the individual studies as the number of treatment sessions ranged from as few as eight up to 36.  The overall treatment effect size was medium effect (0.55) on mental health, not small or large effect.  Based on the meta-analysis findings, Tu recommended that “horticultural therapy be integrated into medical, healthcare and community settings to improve mental health.”

This meta-analysis study further justifies major funding initiatives for large-scale multi-institutional RCTs to establish horticultural therapy’s precise treatment and therapeutic outcomes.

1Tu, H. M.  (2022).  Effect of horticultural therapy on mental health: A meta‐analysis of randomized controlled trials.  Journal of Psychiatric and Mental Health Nursing.

Charles Guy, Emeritus and Courtesy Professor
Department of Environmental Horticulture, University of Florida