Met Office forecasters said current colder-than-average conditions are set to continue, with central and northern Britain likely to experience frost on Saturday night and Sunday morning and widespread frost expected in the south on Sunday night.
Next week much colder conditions are expected to bring wintry weather to northern hills and more frost in places with clear skies.
April frosts are not unusual but the cold weather comes after the fifth warmest March since records began in 1910, which could have set plants off to an erratic start of the gardening season, the Met Office warned.
March's largely frost-free conditions may have lulled gardeners into a "false sense of security",Tim Legg, a member of the Met Office's National Climate Information Centre, said.
"With only 11 days of frost, March 2017 was the most frost-free since March 1998.
"In fact, England had fewer air frosts than in any other March since records of air frost began in 1961, with several stations in the south including Farnborough, Larkhill and Boscombe Down having had no air frosts at all during March.
"The low incidence of frosts across the UK last month may have lulled gardeners into a false sense of security as frosts will continue to feature prominently in the forecast for the next few days.
"Springtime frosts are a particular threat to gardeners who will be concerned about the impacts on tender plants," he said.
The Royal Horticultural Society's (RHS) chief horticultural adviser Guy Barter said: "Frosty weather at this time of year is always a worry for gardeners as fruit blossom in particular is vulnerable to damage.
"There is not much that can be done to protect apples, plums and other tree fruits but strawberries can be covered with cloches, curtains or a fleece at night, although care must be taken to allow bees to pollinate during the day.
"Happily, soft fruit - such as currants and gooseberries - are leafy now and the foliage shelters the flowers from frost."
There is an upside for gardeners to the cold nights, he said, as the chilly conditions hold back lawns so less mowing is needed, and they inhibit weeds so gardeners can get on top of the weeding.
The frosty weather could spell bad news for commercial fruit growers, too.
National Farmers' Union chief horticulture adviser Hayley Campbell-Gibbons said: "Soft fruit production is protected by polytunnels, which can handle frosts of up to minus two degrees, but a severe frost would still be dangerous, and the cooler than average temperatures will certainly slow down production.
"The biggest concern is outdoor fruit production - such as apple and pear orchards, and blackcurrants.
"Production is ahead of schedule in many parts of the country, which means trees are in full flower and very vulnerable to night frosts. A severe frost could significantly impact British fruit production."