Sunday 25 February 2018

Unusual weather sets off precocious growth

A lot of people have been surprised to see daffodils and snowdrops appearing exceptionally early in recent weeks and have wondered how this has come about and whether it will affect the growth of these plants next spring.

It is unusual but not rare to see some plants jump the gun and flower out of season. This happens when plants react to unusual weather conditions. Plants know when it is spring and time to unfold their flowers because they get clues from the weather. If the weather patterns are a bit unusual, there can be a false dawn.

Daffodils and snowdrops, which are closely related, form their flowers for next spring in the late spring and early summer of the previous year. As soon as the flowers fade, the plant begins to lay down the foliage and flowers for the following year, and, by mid-summer, the foliage has withered away and the bulb is in summer dormancy.

What stops the bulb from sprouting again in summer? It needs a cold period to trigger new root development, leaf extension and eventual flowering. In the normal way, this cold period comes in autumn and flowering in spring.

This year, the summer was exceptionally cold in July and August and some bulbs reached the threshold of cold needed earlier than normal.

The cool summer was followed by a very warm autumn and this triggered early flowering.

But it is worth noting that only some bulbs and plants reacted in this way. Others may have started into growth a bit more than usual but did not flower. There were no whole swathes of daffodils in flower, just a few of the kinds that have a low cold threshold.

It is possible to select within a population of plants those that have a low threshold for triggering growth. For instance, the winter flowering cherry, in flower just now, is a selection from the type of species that flowers in fits and starts from autumn to spring, even in a normal year.

A warm autumn can often trigger pear and apple trees to flower out of season, also rhododendrons, roses and magnolias. The ornamental pear variety 'Chanticleer' commonly throws some flowers in autumn.

The question is asked whether this early show, or late show to look at it the other way, will affect plants when spring really arrives?

The effects are negligible as a few frosty nights put a stop to early flowering and the normal rhythm of the seasons resumes. And this just a few days after the shortest day of the year!

Sunday Independent

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