The wettest January for a century has set landscape projects and maintenance back weeks and is set to lead to more root rot, canker and invasive species.
In the worst-hit areas 175-220mm of rain fell in the south and west last month, according to the Met Office. It issued new severe weather warnings this week with winds of up to 80mph and further rain forecast for the south and west coasts.
Hillier Landscapes did no domestic work at all in January because of the weather. Managing director Richard Barnard said: "This is the worst season I've seen in 45 years. It's expensive for us. Our order book is good but sales will be down by 40 per cent for January at least. The sad thing is that up to Christmas we were flying. Sales were up 18 per cent.
"We've kept off all of the land. It's incredibly wet. We can't do hard landscaping very easily because it has to be dry underneath. Private gardens clients don't want to see you if it's untidy."
He added: "We can plant trees but we'd have to go two or three foot down and move across somebody's lawn using heavy machinery. We've found it very awkward."
Glendale Managed Services managing director Andy Corcoran said some staff shifted to flood-response work. "The rain affects our landscaping business quite significantly as they've been unable to get on site to plant. I spoke to our South West land manager Mike Dennys last week - he's a bit down. He's got a lot of projects where he can't do the work.
"But in general it doesn't affect us too badly. We operate the North Somerset Council contract and we have response teams written in. Local authorities can call on us to do whatever is required - clearance work, debris, unblocking work.
"It shows we are able to be flexible. If we can't do work on the grounds and shrub beds because it's too wet, we should help out elsewhere. It benefits the industry."
Hambrooks managing director Norman Hambrook said he plans in his budget to lose four weeks' production a year for adverse weather.
At Hadlow College, garden management programme head Caroline Jackson said: "It's a disaster. You can't walk on growing areas as they are so wet, so the normal winter jobs are delayed. Also, some plants are rotting in the soil.
"Some plants are displaying signs of stress because of the high water levels and high relative humidity. There have been tree losses, partly due to wind but aggravated by a high water table. We have had cool temperatures rather than the usual winter cold, which may mean a greater number of pests overwintering. There may also be an impact on flowering and fruiting because some plants need a substantial winter cold period to flower."
ADAS ornamentals consultant David Talbot said: "Anything field-grown is very wet, particularly if you have heavy clay soil. When it's submerged, the water pushes air out and the roots need air to respire, and that can lead to anaerobic conditions conducive to root rot. There's not an awful lot you can do about that except for longer-term improving of soil structure."
He said Christmas trees and conifers generally are most at risk from root rot, adding that domestic gardeners could not get out despite the mild weather. "We need it to stop raining as soon as possible because it will take three weeks for most soils to dry out and get back to workable condition." There is potential leaching of top dressing for container growers but "they're mostly concerned about the rain's impact on sales".
BALI technical director and Ground Control group training manager Neil Huck said the floods pose a danger in terms of invasive species. "Species such as Japanese knotweed have got to sites that are too wet to treat. It will be a problem in spring when it starts to grow. Floating pennywort and Crassula helmsii could spread fast because of the flood water."
Flooding Adaptive commute and top tips
RHS Chelsea Flower Show designer Cleve West is having to travel to work by boat.
RHS principal horticulture adviser Leigh Hunt advised gardeners to "hold back on borders and lawns" to avoid compaction. Appearing on The One Show on BBC1, Christine Walkden told viewers to avoid working sodden garden soil.
B&Q horticulture category manager Joclyn Silezin said: "One of the largest risks this year is the prolonged flooding and prolonged impact on people's gardens."
English Apples & Pears chief executive Adrian Barlow said widespread waterlogging could increase the incidence of canker.