The association's market information manager David Denny said: "The market has good short to mid-term prospects (with disruptive risks) but long-term demographic challenges."
Kantar statistics quoted in the report say renters spend 40-57 per cent less than homeowners on their garden.
The percentage of UK adults owning a garden has fallen from 80 per cent to 77 per cent since 2000.
The report says 10 per cent of spend on garden plants is made online (£100m), compared with around 20 per cent for non-plant garden ranges (£800m). It cites Saga as having a new online plant offer.
It warns on price competition between DIYs following Bunnings purchase of Homebase and from supermarkets such as Aldi and Lidl.
The report states: "Garden centre sales when adjusted for a nominal 1.5 per cent inflation since 2011 have also performed positively in the past five years. However within this garden plants have performed less well."
Kantar figures show plants have fallen from £1.6bn to £1.4bn annual sales in that time, while garden care is down from £1.7bn to £1.6bn and garden leisure from £1.2bn to £1.1bn.
Denny says "major forces affecting the market" are technology, environment, socio-cultural, demographics, economy and political.
- Regulatory complexity and opportunity
- Emerging opportunities on health and re-greening towns
- Increasing tax/levy burden on businesses
- Risk to consumer confidence
- Labour supply challenges
- High consumer confidence
- Medium term garden spend under threat among young consumers
- Pressure for businesses to become more productive
- Non traditional businesses competing for garden spend
- Need for labour-saving garden products/services
- Increasing regional variation in garden spend and needs
- Vintage and free-cycling
- Demand for bargains
- Taste for sustainable businesses and products
- Possible rise of renting garden equipment
- Securing water supply critical for growers
- High potential impact of plant disease outbreak
- Demand for container gardening may increase
- Variation in weather a risk driving diversification
- Increasing automation in businesses
- Challenges to find capital investment to automate
- £900m online garden sales
- Potential trend for cordless power tools