PLANTS KEEP CHANGING THROUGHOUT THE ICY MONTHS TO PERFORM WELL IN THE SPRING AND SUMMER
Under its seasonal covering of snow or frost, the winter garden appears dead and lifeless. Brave splashes of colour come from witch hazels, viburnums and a few other shrubs. But the majority of the garden seems to be sleeping, waiting for the warmer weather and longer daylight hours.
This is an illusion. The garden never sleeps, and all winter there are subtle but vital changes taking place. Under the soil, the cold is working its magic on seeds, bulbs and roots. Even the chill winds have a role to play to ensure flowers and fruits appear later in the year. Without the winter cold, gardens would be less beautiful in the summer.
A period of cold weather is essential to many plants and crops. Without it, some would struggle to grow at all, while others would not flower or produce crops.
One example of crops that need a cold spell are fruit trees such as apples, plums and pears. If temperatures remain high, these trees would not come into growth in spring, nor produce flower buds. In a process known as vernalisation, the reduction in daylight initially induces the trees to go dormant. They shed their leaves, which would require too much energy to maintain in winter when they make less food. Then the trees must be exposed to a certain number of days with a minimum temperature, usually less than 7°C. Only when this has happened are they ready to burst into growth and bloom when temperatures rise. This process is designed to ensure this happens in spring, and not in autumn.
The amount of days of cold required is expressed in chill hours. Some require longer periods of cold than others, although the reason for this is not currently known. However, the plants are believed to store the necessary information and pass it on in their genes.
Bulbs in winter
Hardy bulbs need different weather conditions at different times of the year to grow and produce flowers. Flower bulbs are formed in summer, as a result of heat in a process called baking. Root growth is stimulated by the cooler and wetter weather of autumn. However, it is the cold of winter that is needed to stimulate stem growth. Temperatures of 10°C or below trigger the elongation of the flower stem.
Attempts to grow hyacinths or daffodils in the home often results in hyacinths with a clump of flowers crowded in the neck of the bulb or daffodils on dwarf stalks. This is the result of ignoring the bulbs’ need to spend at least 10 weeks in cool conditions, ideally outside below 10°C. There are exceptions to this such as ‘Paperwhite’ and ‘Soleil d’Or’. Native to warm Mediterranean regions, they do not need a period of cold for their stems to grow. It is autumn rain, rather than winter cold, that spurs on these daffodils’ growth.
Getting ahead in the race for light
In winter, herbaceous plants stop growing and die back. Instead, they store the carbohydrates they make from water and carbon dioxide from the air in their roots. This provides the plants with a reserve of energy, designed to give them a head start in spring. They store the carbohydrates as starch because this is more concentrated in energy (calories) than simple sugars. Starch is not water soluble, making it difficult to move around the plant in the sap.
The onset of cold weather, however, triggers enzymes in the root to convert the starch back into soluble sugar. This means it can be moved to the growing tips of the plant, ready for early spring’s surge of growth. Once the conditions are right, they are able to push their shoots to the sunlight ahead of surrounding plants such as annuals. This stops the new growth from being smothered by the hundreds of annual seedlings which are germinating at the same time. Peonies and dahlias are both examples of the wide range of plants this benefits. It is also the reason why parsnips taste sweeter after they have been frosted.
Saving the next generation
When grown from seed some plants need a cold spell before they will flower. These include biennials such as wallflowers, aquilegias, sweet Williams and onions. Blooming in late spring, they release their seeds in summer. If the seeds germinated immediately, they would flower and set seed as soon as they were big enough. These new seeds would not have time to ripen before winter arrived. The cold weather would kill them, with the loss of a whole generation. Instead the plants make healthy clumps of foliage the first year. Then they wait for a sufficient winter chilling before producing flowers. No matter how early in spring these seeds are sown, they will only produce leaves in the first year, never flowers.
The necessary cold period to stimulate flowering can be very short. Several biennial plants, such as parsnips, carrots, beet and onions, are grown as vegetables. The starches and sugars stored in their roots provide valuable food when eaten. If these vegetables are sown too early in spring, there is a risk of short cold snap while they are growing in April or May. This could fool them into thinking that winter had come and gone, and it was time to flower. At this point the plants ‘bolt’, sending up flower stems and the crop is lost to the gardener.
The biennials above require winter chilling to make them flower. There are other plants, however, that need their seeds to be vernalised before they germinate at all. The seeds undergo a period of dormancy. In some cases, simply the softening of the hard coat by frost and weathering action will allow the seed to germinate. This applies to some lathyrus species including sweet peas.
In other cases, this weathering is linked to the need for a cold, moist period. This triggers the seed’s embryo to grow and expand. It breaks through the softened seed coat seeking the sun and nutrients. Seeds do this because if they germinated in autumn, the seedlings would be unlikely to survive the winter. If frost did not kill them, grazing animals, slugs and snails would eat them in the absence of other food. Delaying germination till spring gives every seed a better chance of survival to maturity.
All these processes show that winter is not just a time of frosty beauty. Its chilling weather creates more than a snowy landscape, it plays an essential role in ensuring that the following season is as productive and beautiful as the last.
Words: Geoff Stebbings Photography: Alamy