The mercury hit -10°C this week as parts of Britain experienced their coldest January since 1996.
Nursery consultant John Adlam said winterkill, or "physiological drought", happens when growing media in pots is frozen but daytime sun means evergreen plants are transpiring. This leads to drought symptoms because plants cannot draw water from frozen roots.
Adlam recommended fleecing plants so they don't warm so quickly during daytime. He warned that losses could increase if conditions continue over the next fortnight.
Buckingham-based irrigation firm Flowering Plants owner Francis Richardson said: "The last time this happened, in 1986, our garden centre customers lost about 2,000 plants per centre."
Meanwhile, Cumbria-based Grasmere Garden Centre glasshouse supervisor Jane Thomas said a broken boiler meant all the centre's houseplants had died: "All our houseplants are going in the bin. We'll lose a lot of stock. Even the primroses have been hit. Virtually everything has gone. We keep mainly hardy stock but that usually means hardy to -3°C. It was -7°C last night."
But Lothian-based New Hopetoun Gardens general manager Lesley Watson said: "We won't know if we have any losses until spring. It's not colder in Scotland than usual but it might be in southern Britain. Centres there might be trying plants that may not get through. We guarantee all our plants for two years."
Wiseweather said January temperatures will average 0.7°C below average. It forecast mid-month frost and freezing fog, along with sleet or snow in central parts at the end of the month and frosts in the South East.