Last month's UK mean temperature was 7.9 degsC (4.1 degsC above average), making it the warmest December since 1910. Rainfall was 191 per cent of average, making it the wettest December in a century. These conditions were then followed by a cold snap in mid January.
Nursery consultant John Adlam said some growers would be experiencing problems caused by plants growing up to a month ahead of normal. "The weather has certainly changed the characteristics within which many pests and diseases would normally shut down or even die out during the cold, so we have got a situation where many problems such as box blight have been continuing far longer than they normally would and are giving quite a few problems in the amenity sector."
He added: "People get plants coming into flower far too early for their scheduled dispatch date for the customer." Adlam said lavender is an example of a crop coming into flower too early.
ADAS reported that the mild weather has also caused primroses to start to flower ahead of schedule when there is minimal demand and this may result in a shortfall in January.
Adlam added that some evergreens have failed to reach dormancy over the winter, with Christmas trees and conifer growers at greater risk from pests. He said some growers might not have seen the death of aphids and spider mites, and in outdoor shrubs and tunnel crops there is a higher proportion of overwintering pests ready to emerge than in past years, which will need "earlier and stronger attention".
Irrigation issues could arise because plants need watering so growers have not drained systems and are now vulnerable to cold snaps, he pointed out. Heavy rainfall has caused more nutritional leaching than usual so "2016 could be the year of top dressing to recoup losses caused by rain and higher growing-media temperatures". He said weeds such as groundsel, bittercress and chickweed have not stopped growing, which interferes with growers' routines.
Burston Nurseries managing director James Alcaraz said an end to the warm weather would help to kill off old bedding and generate demand for replacements.
ICL technical manager Andrew Wilson said the mild autumn followed by a sudden cold snap left many plants insufficiently prepared. He recommended keeping irrigation to a minimum and avoiding high-nitrogen feeds as well as late pruning, both of which encourage soft growth. Switching to higher-potassium feeds will promote hardy compact growth, he added.
Cornish daffodils began five weeks early in December, while Lincolnshire daffodils are three weeks ahead, leading to wholesale prices falling by 25 per cent and some wastage. RHS chief horticulture adviser Guy Barter predicted an increase in the numbers of slugs and snails in spring because of the mild weather.
Gardens Early flowering and strong visitor numbers reported
Warm weather has seen gardens flowering much earlier than usual, although the ground is too soggy for many to carry out planting and maintenance work.
Visitor numbers have been strong for those gardens that stay open over winter. Carol Adams, gardens team manager at Trentham Estate, said numbers have been very positive: "We have had lovely sunny weather in between snow showers so lots of snowmen appeared all over the garden as a result."
Otherwise mild ground conditions have been beneficial but have required changes to maintenance schedules, with herbaceous areas being cut back early to beat prematurely emerging bulbs.
Pests are emerging but many gardeners are adopting a wait-and-see approach in the hope of a turnaround in weather conditions.
Adams said: "Obviously we are going to have more aphids and the wet weather conditions could cause fungal diseases, but we’ve put a plant health policy in place and staff are all being vigilant at cleaning off kit and watching for new problems on site."
James Hall, head gardener at Myddleton House Gardens in the Lee Valley, said he has seen box tree caterpillar appearing for the first time — a concern for the gardens’ huge box topiaries. He is using the bacteria-based biological pesticide Dipel (Bacillus thuringiensis or Bt) in attempts to halt the pest.
Myddleton had daffodils flowering at the start of December and snowdrops well before Christmas, with the flowers perfectly timed to peak for the gardens’ Snowdrop Festival this weekend.
Chelsea Physic Garden also ran its Snowdrop Festival this week, two weeks earlier than planned. Head gardener Nick Bailey reported plants responding oddly to the weather, with some deciduous species staying semi-evergreen over winter in the garden’s sheltered microclimate by the Thames.
At Caerhays Estate in Cornwall, magnolias have been opening in record early time, said head gardener Jaimie Parsons. Caerhays will open on 22 February but the National Plant Collection of magnolias has several plants out in flower, including Magnolia campbellii ‘Strybling White’ and ‘Ethel Hillier’, M. ‘Todd’s Forty Niner’ and M. ‘Red Lion’.
"Everyone in Cornwall has got the same issue of the magnolias coming out earlier," added Parsons. "But there are over 800 in the collection and the garden is over 120 acres so there’s always going to be plenty to see. Just because there is a handful flowering now is not going to upset the gardens."