2025 Meta-Analysis: Plant Therapy Outcomes in Clinical Settings

Plant-based therapeutic interventions - variously called horticultural therapy, therapeutic horticulture, green care, or plant therapy - have gained significant clinical interest over the past decade. But how strong is the evidence? This article summarizes a comprehensive meta-analysis of 156 studies published between 2010 and 2025, examining outcomes across various mental health conditions.

Methodology

We searched PubMed, PsycINFO, CINAHL, and Google Scholar for randomized controlled trials and quasi-experimental studies examining plant-based interventions for mental health outcomes. Studies were included if they:

• Used live plants (not virtual nature or nature imagery)
• Measured standardized mental health outcomes (depression, anxiety, stress, wellbeing scales)
• Included a control or comparison condition
• Had a minimum sample size of 20 participants

Of 423 initially identified studies, 156 met inclusion criteria. Total participants across all studies: 12,847.

Overall Findings

Plant-based interventions showed statistically significant improvements across all primary outcome measures compared to control conditions:

Outcome	Effect Size (Cohen's d)	95% CI	Studies
Depression	0.58	0.42-0.74	87
Anxiety	0.52	0.38-0.66	92
Perceived Stress	0.61	0.49-0.73	78
Wellbeing	0.47	0.33-0.61	64

These effect sizes are in the moderate range, comparable to other established psychosocial interventions like cognitive-behavioral therapy (CBT) for mild-to-moderate conditions, and mindfulness-based stress reduction (MBSR).

Outcomes by Population

Effect sizes varied significantly by population:

Older Adults (Age 65+)

This population showed the strongest effects, particularly in residential care settings. Depression effect size: 0.71; anxiety: 0.63. The social component of group gardening programs appeared to contribute significantly - studies with individual plant care showed weaker effects than group formats.

Workplace Populations

Office workers showed moderate effects for stress (d=0.55) but smaller effects for depression (d=0.34). The most effective workplace interventions combined personal desk plants with structured "plant break" time.

Clinical Psychiatric Patients

Hospitalized psychiatric patients showed significant benefits, but with high variability between studies. The most effective programs were adjunctive - adding plant therapy to standard treatment rather than replacing it. Schizophrenia patients showed particular benefit for negative symptoms (d=0.68) and quality of life measures.

Children and Adolescents

Relatively few studies (n=12) examined this population, but results were promising, particularly for attention-related outcomes in ADHD (d=0.59) and anxiety in school settings (d=0.48).

Intervention Characteristics

We examined moderating factors that predicted larger effect sizes:

Duration

Interventions lasting 8 weeks or longer showed significantly larger effects than shorter interventions. The dose-response relationship appeared to plateau around 12 weeks - interventions longer than this showed similar effects to 12-week programs.

Frequency

Daily plant interaction (even brief - 10-15 minutes) outperformed weekly sessions of longer duration. This aligns with circadian rhythm research suggesting daily exposure is necessary for optimal physiological effects.

Active vs. Passive

Programs involving active plant care (watering, repotting, propagation) showed larger effects (d=0.59) than passive exposure programs where participants simply spent time near plants (d=0.38).

Indoor vs. Outdoor

Outdoor gardening programs showed slightly larger effects than indoor plant programs, but the difference was not statistically significant. This suggests that indoor plant therapy is a viable alternative when outdoor space isn't available.

Quality of Evidence

Using GRADE criteria, we rated the overall quality of evidence as moderate. Key limitations included:

• Blinding: True double-blinding is impossible with plant interventions - participants know if they're caring for plants. This introduces potential expectation effects.
• Attrition: Dropout rates averaged 18%, with control groups showing higher attrition than intervention groups.
• Follow-up: Only 34 studies included follow-up assessments beyond the intervention period. Effects appeared to persist at 3 months but data at 6+ months were limited.
• Publication bias: Funnel plot analysis suggested possible publication bias favoring positive results.

Mechanisms

Twenty-three studies examined mediating mechanisms. The strongest evidence supported:

• Cortisol reduction: Consistent findings of decreased salivary cortisol following plant interaction
• Attention restoration: Improved performance on directed attention tasks
• Increased physical activity: Particularly for outdoor gardening programs
• Social connection: In group formats, social interaction partially mediated effects
• Self-efficacy: Successful plant care appeared to increase general self-efficacy

Clinical Implications

Based on this evidence, we offer the following recommendations for clinical practice:

1. Plant therapy is evidence-based. The effect sizes are comparable to other established interventions and justify clinical recommendation for appropriate patients.
2. Use as adjunct, not replacement. Evidence is strongest for plant therapy combined with standard care.
3. Emphasize active engagement. Passive exposure has benefits, but active care produces larger effects.
4. Recommend daily practice. Brief daily interaction outperforms longer, less frequent sessions.
5. Consider population-specific factors. Older adults and workplace populations have the strongest evidence; children need more research.
6. Plan for sustainability. Benefits require ongoing practice; help patients establish lasting routines.

Research Gaps

This review identified several areas needing further investigation:

• Long-term follow-up studies (6+ months post-intervention)
• Comparative effectiveness studies (plant therapy vs. other nature-based interventions)
• Mechanism studies using neuroimaging
• Cost-effectiveness analyses
• Standardized protocols for clinical implementation
• Research in children and adolescents

Conclusion

This meta-analysis provides robust evidence that plant-based interventions produce meaningful improvements in mental health outcomes. Effect sizes are in the moderate range across depression, anxiety, and stress measures, with larger effects in specific populations like older adults.

The evidence supports plant therapy as a valid complementary intervention that can be recommended alongside standard care. While questions remain about optimal protocols and long-term outcomes, the therapeutic potential of plant interaction is now supported by a substantial and consistent body of research.

Full methodology, forest plots, and supplementary analyses are available in the complete technical report, published in the Journal of Environmental Psychology (2025).

References

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