Around 68 staff at the Oswaldwistle, Lancashire-based company, founded in 1887, are being made redundant over the closing period.
About six of them will move to Lincoln-based William Sinclair, which bought Gem from the UK's largest privately owned fertiliser and compost company Joseph Metcalf in January for almost £3m.
Gem supplies the professional horticulture, amenity and consumer gardening markets with composts, feed and weeds and related products. In April new Sinclair managing director Danny Adamson said Gem was "performing well" after chief executive officer Bernard Burns led the buy-out (HW, 17 April).
A Gem representative said: "Gem dies a death this season." He added that the company had been losing "a lot" of money: "It's been a poor season because of economic constraints and lousy weather. Trading has been difficult.
"People generally buy on price now rather than brand loyalty. They will buy supermarket deals - there's a lot less brand loyalty now. We had the potential to be made successful but Sinclair's plan to invest £5m in us had problems with planning and greenbelt issues."
Sinclair began an ongoing period of consultation on the closure of the Oswaldtwistle factory in June.
Gem-branded products that Sinclair will continue to sell are sodium chlorate, grass seed and garden drop spreaders.
A Sinclair representative said: "Additional peat volumes from Joseph Metcalf gives us a strong competitive advantage."
- William Sinclair chief executive Bernard Burns has said growing media prices will be 30 per cent higher next year. He blamed low demand on adverse weather, early Easter, poor harvests a month ago and oil and fertiliser price hikes.