Woolworths has debts of £385m but is trading on while administrators find buyers.
Suppliers to the 800-store chain include AE Roberts, Mr Fothergill's, Scotts and Porters Horticultural. They have orders ready to be delivered in January but fear they no longer have a buyer.
AE Roberts managing director John Gwynn, who supplies trees, fruit bushes, shrubs and climbers, said: "We were probably their biggest horticultural supplier. They were our biggest customer by a long way. But we are going to carry on as a company. We will survive, though it won't be easy. We can't put the crop we're growing for them on hold so we're working extremely hard to sell elsewhere."
Mr Fothergill's marketing manager Ian Cross said: "Woolworths has been a pretty significant customer for us. It is never good to lose a reasonably sized customer. But depending on what happens it's possible they will trade on.
"So many things were battling for space at Woolworths. We supplied a far from extensive range."
Merseyside-based bedding specialist Porters owner Robin Porter said: "It's disappointing. We had a £250,000-a-year contract but had dropped that down. We were forewarned and aware of their problems so we played safely. We didn't want to get exposed. We're not owed anything but we'll have to wait and see what will happen. Someone might buy them out and carry on but I doubt that very much."
Dutch grower De Ree supplied bulbs, perennials and roses. Many horticulture gift lines are believed to be sourced from China.
Suttons marketing manager Tom Sharples said his firm supplied seeds to Woolworths until seven years ago: "We could see the way things were going with cuts in gardening. They let Wilkinsons in but were a big name in horticulture."
Woolworths was the biggest gardening supplier in the UK for 70 years until the advent of DIY chains, then supermarkets, garden centres and high-street rivals such as Wilkinsons over the past 30 years.
Cross said he had been unable to contact his buyer at the chain because the buyer had been on holiday and then off sick.