The report added that it expected the garden market would grow by more than 55 per cent compared with 2016 levels.
AMA estimated non-specialist retailers such as DIY multiples accounted for the majority share of the internet garden products in 2015-16, while grocery multiples and high street retailers had recently increased their presence in the market.
AMA said: "The garden centres sector has been slower than others in adopting online sales platforms, but this is now beginning to change. Growth within the overall market for garden products is likely to be fairly restrained in the medium term however, more impressive growth is forecast for the internet sector, which is less mature and at an earlier stage of development."
AMA added that market growth is expected to slow during the years leading up to 2020, as the sector continued to mature.
Companies analysed included: Amazon, Argos, Calgary Just Companies, crocus.co.uk, Dobbies Garden Centres, Garden Bargains, Garden Centre Online, Garden Direct, Garden Products Online, Garden Supply Direct, Garden4less, Gardening Express, Gerrico, Greenfingers Trading, Growhouse, Hall’s Garden Supplies, Jersey Choice, Keengardener, Kybotech, Lighting for Gardens, MBL Group, Meika, Moneta UK, MowDirect, Notcutts, Oxfordshire Garden Furniture, J Parker Dutch Bulbs (Wholesale), Pepe Garden Furniture, Simply Paving, Summer Garden and Leisure Building, Sutton Seeds, Taylored Investments, Taylors Garden Buildings, Tesco, The Garden Centre Group, The Garden Superstore, The Internet Gardener, Thompson & Morgan, Water Garden, Wyevale Garden Centres.