Although not a plant collector Forsyth made a major contribution to British horticulture publishing early works on arboriculture and the management of fruit trees. He was one of the founder members of the Royal Horticulturual Society.

Forsyth went to London as a young man in 1763 to work at Syon House and later to Apothicaries Garden in Chelsea eventually becoming it's Curator in 1771. Under his direction many areas of the garden were replanted and he started to exchange plants and seeds internationally. In 1774 he built the first ever rock garden.

Later he became gardener to George III at St James’s and Kensington Palaces where he turned his efforts to the cultivation of fruit and vegetables. He found many of the old fruit trees to be diseased and had to remove several cankerous growths. This often left the trees with large wounds and therefore open to more disease. Forsyth created a 'plaister' - which consisited of cow dung, lime, wood ashes and sand which he coated on to the wounds and which he claimed catalysed new healthy growth.

In 1789 England was at war with Napoleon, and required oak for building war ships. There was great concern over the state of the royal forests as much of the timber was defective. Forsyth received a letter enquiring of his 'plaister' as the royal oaks required 'a complete remedy'. A committee was formed from both Houses of Parliament to study the improvements claimed by the 'plaister'. The committee had to report to the Treasury. They agreed that the mix did seem to be beneficial and recommended Forsyth receive a grant of £1500 to make the composition of the mix available. There then followed some controversy over the efficacy of the 'plaister' in a horticultural publication.

Plant Material to Represent William Forsyth:
Forsyth made no major plant introductions however the Danish Botanist Martin Vahl named the genus Forthysia in his honour.

• Calluna vulgaris ‘Glenfiddich’
• Calluna vulgaris ‘Inshriach Bronze’
• Calluna vulgaris ‘Kinlochruel’
• Forsythia ‘Arnold Dwarf’
• Forsythia ‘Beatrix Farrand’
• Forsythia ‘Golden Nugget’
• Forsythia europea
• Forsythia giraldiana