Human Health & Plants Research: Meta-analyses of Published Controlled Research Studies on Horticultural Therapy and Cognitive Function

Dr. Charles Guy

Meta-analysis is a pooled quantitative study design to systematically evaluate a group of research studies to provide greater insight and a more precise determination of an overall experimental treatment’s effect(s). Given meta-analysis integrates the results of multiple independent studies, it can play a crucial role in the practice of evidence-based medicine.

Tu and Chiu’s meta-analysis1 was published in September 2020 in Scientific Reports following a systematic search of the literature using (PRISMA2) methods and individual study experimental quality analysis based on (MINORS3) criteria. The meta-analysis included ten non-randomly controlled studies of horticultural therapy (HT) interventions on participants’ cognitive function across a variety of populations with dementia or schizophrenia. Sample sizes of the included studies ranged from 18 to 63 participants, yielding an overall total of 340 participants. Average ages in the individual studies ranged from 42 to 89. While the included studies differed in many ways, including cognitive assessment questionnaires and horticultural activities, Krippendorff’s alpha coefficient indicated good reliability and agreement across the studies. Evaluation of the reported effect sizes failed to reveal significant outcome heterogeneity between the included studies.

Very few gold-standard randomized controlled studies have been published in the field of HT. There are many more non-randomized controlled studies that introduce the potential for bias in the experimental outcomes. The meta-analysis by Tu and Chiu focused on non-randomized controlled studies using a random-effects model. Several statistical tests suggested an absence of publication bias that would potentially diminish the treatment effect size estimate from the meta-analysis. The analysis revealed that HT treatments significantly improved cognitive function, and the overall treatment effect size was large. Based on the higher quality evidence of the present meta-analysis, Tu and Chiu suggest that an HT program can improve cognitive function, when including more than one treatment session per week, and continuing for at least eight weeks. However, an HT program’s frequency and duration for cognitive function improvement should be further studied and confirmed.

1Tu, H-M. and Chiu P-Y. Meta-analysis of controlled trials testing horticultural therapy for the improvement of cognitive function. Scientific Reports. 10, 14637. (2020).

2PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analyses. See

3Slim K., Nini, E., Forestier, D., Kwiatkowski, F., Panis, Y., Chipponi, J. Methodological index for non-randomized studies (minors): development and validation of a new instrument. ANZ Journal of Surgery. 73(9):712-716. (2003).

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