You'll want to look at the following review: The benefits of gardening for older adults: A systematic review of the literature.

doi: http://dx.doi.org/10.1080/01924788.2013.784942

By Wang, Donna; MacMillan, Thalia

Activities, Adaptation & Aging, Vol 37(2), Apr 2013, 153-181.

This article systematically reviews evidence for gardening as a beneficial activity for older adults. The authors reviewed 22 articles that assessed the benefits of gardening for both community- dwelling and institutionalized older adults. Through various research designs (quantitative and qualitative) and measurements utilized, the results reveal that gardening can be an activity that promotes overall health and quality of life, physical strength, fitness and flexibility, cognitive ability, and socialization. The implementation of various aspects of gardening as health-promoting activities transcend contexts of practice and disciplines and can be used in urban and rural communities as both individual and group activities. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

Gardening as a mental health intervention: A review.

doi: http://dx.doi.org/10.1108/MHRJ-02-2013-0007

By Clatworthy, Jane; Hinds, Joe; Camic, Paul M.

Mental Health Review Journal, Vol 18(4), 2013, 214-225.

Purpose: The number of gardening-based mental health interventions is increasing, yet when the literature was last reviewed in 2003, limited evidence of their effectiveness was identified. The purpose of this paper is to evaluate the current evidence-base for gardening-based mental health interventions and projects through examining their reported benefits and the quality of research in this field. Design/methodology/approach: Studies evaluating the benefits of gardening-based interventions for adults experiencing mental health difficulties were identified through an electronic database search. Information on the content and theoretical foundations of the interventions, the identified benefits of the interventions and the study methodology was extracted and synthesised. Findings: Ten papers published since 2003 met the inclusion criteria. All reported positive effects of gardening as a mental health intervention for service users, including reduced symptoms of depression and anxiety. Participants described a range of benefits across emotional, social, vocational, physical and spiritual domains. Overall the research was of a considerably higher quality than that reviewed in 2003, providing more convincing evidence in support of gardening-based interventions. However, none of the studies employed a randomised-controlled trial design. Research limitations/implications: There is a need for further high-quality research in this field. It is important that adequate outcome measures are in place to evaluate existing gardening-based mental health interventions/projects effectively. Originality/value: This paper provides an up-to-date critique of the evidence for gardening-based mental health interventions, highlighting their potential clinical value. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

Nursing home quality of life: Study of an enabling garden.

doi: http://dx.doi.org/10.1080/01634371003741482

By Raske, Martha

Journal of Gerontological Social Work, Vol 53(4), May 2010, 336-351.

The purpose of this study was to conduct an in-depth evaluation of the impact of the construction and use of an enabling garden on resident quality of life in a rural nursing home. This qualitative study used interviews with residents, family members, staff members, and community volunteers who built the garden. Findings suggest the garden had positive effects on resident quality of life, particularly in terms of meaningful daily activities, enjoyment of daily life, resident relationships, and functional competency. Implications for research and practice are discussed. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

Therapeutic effects of an indoor gardening programme for older people living in nursing homes.

doi: http://dx.doi.org/10.1111/j.1365-2702.2009.02803.x

By Tse, Mimi Mun Yee

Journal of Clinical Nursing, Vol 19(7-8), Apr 2010, 949-958.

Aims: To explore the activities of daily living and psychological well-being of older people living in nursing homes and also to examine the effectiveness of a gardening programme in enhancing socilaisation and life satisfaction, reducing loneliness and promoting activities of daily living for older people living in nursing homes. Background: Life in nursing homes can mean very limited physical and social activity, leading to further decline in function for many older people. Design: This was a quasi-experimental pre and posttest control group design. Methods: Older people from nursing homes were invited to join the eight week indoor gardening programme (experimental group), while older people in other nursing homes were treated as the control group; they received regular care without the eight week indoor gardening programme. There were 26 older people (25 female and one male; mean age 85 years) in the experimental group and 27 (20 female and seven male; mean age 82 years) in the control group. Demographic data including age, gender, educational level and financial situation were collected, in addition to information regarding life satisfaction, loneliness, physical activity and social network situation, before and after the eight week indoor gardening programme for both the experimental and control groups. Also, details of experimental group subjects' experience of the indoor gardening programme were elicited using open-ended questions. Results: There were significant improvements in life satisfaction and social network and a significant decrease in perception of loneliness for older people in the experimental group after the eight week indoor gardening programme, while the activities of daily living were unchanged for both groups after the programme. Relevance to clinical practice: Given the positive effects of gardening activities, it is suggested that they be promoted more widely among nursing home residents. (PsycINFO Database Record (c) 2016 APA, all rights reserved)

I guess that this study includes both dementia and non-dementia subjects:

Yao & Chen (in press) Effects of horticulture therapy on nursing home older adults in southern Taiwan. Qual Life Res.

I work in a State Supported Living Center for older adults with developmental delay and many have other disabilities.  I work with blind and visually impaired individuals including those with multiple disabilities.  We use gardening in differing levels to based on the physical and sensory needs of our individuals.