What is the effect of mild winter weather on plants?
The mild weather is certainly having a curious effect on plants across the country. In my garden, in Lincolnshire, red campion, knautia and lychnis, usually summer performers, are still doing their thing and last week we heard about daffodils blooming all over the place.
Amongst gardeners on Twitter the talk is of cherries, clematis and rhododendron springing into life. As Christmas approaches delphinium spires are ascending, hydrangea buds a-swelling and Japanese anenome shoots a-shooting.
Mild Decembers are not entirely unknown of course but the recent weather has been truly exceptional, and with current temperatures as high as 16C forecasters are, yet again, predicting records to tumble.
The unseasonal weather has not gone unnoticed by plants and Ray Townsend, Senior Horticulturist at Kew said: “I have been here for nearly 42 years and I have never seen this kind of flowering in December before. The warm weather means that we are seeing a number of plants which normally flower in early spring making an appearance now. For example, we are seeing daffodils, snowdrops, iris, ash and hollies in the Gardens at Kew.”
So what will it mean, particularly if the weather continues this way? Ray Townsend strikes a reassuring note: ‘This will have no effect on spring flowering, other than bulbs which only flower once a year.”
Guy Barter, chief horticultural officer at the Royal Horticultural Society agrees: “Plants should return to expected behaviour when the weather returns to 'normal' so there is not likely to be any knock-on effect. It is after all not dissimilar to the climate in mild regions such as Cornwall, Brittany and the Channel Islands.”
In fact, far from being of concern, the mild weather has some advantages: “It is ideal for getting on with gardening jobs so in general very useful for the gardener,” says Guy. “It’s also an opportunity to exercise our natural horticultural curiosity, largely free from any concerns about plant damage.”
“Having said that, some flowering times are very much dependant on sufficient winter chill. Blackcurrants are a classic example where in the absence of enough cool (typically 3-12C) flowering can be delayed and sporadic and the crop diminished as a result. “
The chill factor, known as vernalisation, affects plants differently. Some need a prolonged period of cold to enable them to flower while others are more influenced by day length. Many fruit varieties, such as apple, are well known to need lots of ‘chill’ hours (the number of hours a year below, in the case of apple, about 7C) but other trees and shrubs are affected too.
The effect of the chill factor is also not limited to woody plants. Spring-flowering perennials such as the pasque flower, aquilegia and astible require vernalisation (in contrast to many late-flowering perennials such as rudbeckia that are more influenced by day length), while the seed of some annuals, such as yellow rattle, also need the effect of cold.
Apart from the possible effect on some plant’s flowering, many gardeners will be hoping for cold weather for other reasons. Without the cleansing effect of some good frosts "there is the risk of more pests and diseases later down the line" said nursery owner James Coles in Horticulture Week recently. And then, of course, there’s the dilemma of how long to hold off planting your tulips.