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The Garden Scoop

'Vernalization' Why Plants Need Winter

For those of us who mentally X out each day as January passes and proclaim the best thing about February is that it isn’t January read on... With temperatures falling, schools closing and car batteries dying you may be wondering, “Is winter really necessary?” Being a warm weather person, I would say NO, except for one little thing… without winter there are some plants that would just not perform the way we want come spring. There is a reason why Florida is not known as the apple state.

The scientific term is ‘Vernalization’. Basically it is a cold period where specific plants not only go dormant(they hit the pause button until the weather warms) but must be exposed to a certain number of days with minimum temperatures or they will not flower. Scientists are not certain why some plants need longer periods of cold than others but they believe plants store this information and pass it on genetically.

Come fall, do you like to go apple picking? According to the University of IL, apple trees need winter so they can "rest" for a certain number of days below 45 degrees in order to flower and set fruit properly. According to Texas A&M, in years where the winter temperatures are too mild, flower buds fail to fully develop and blooms are insufficient to produce a good crop. Leaf buds will show late canopy development. This is so stressful, over time it can actually kill the apple trees.

In the Midwest, if temperatures warm too quickly flower buds open too soon and can be hit with a killing frost, wiping out that year’s crop. Peach trees are especially susceptible to early frost damage. Basically it is better for plants if it gets cold and stay cold until spring has arrived and decided to stay.

Does the sight of tulips, daffodils and hyacinths blooming bring a smile to your face? In order for these to flower we need those cold temps. When we have an abnormally warm winter, come spring you may see sad looking tulips or hyacinths that are stunted with shorter stems or smaller flowers.

Planting garlic outdoors in mid-October should ensure a good crop next summer however if you plant garlic in the spring you will only get greens… no garlic bulb. Winter wheat must also be exposed to cold or it will not flower come spring, hence the name. Many seeds rely on winter temperatures, moisture freezes and expands the seed helping to break its hard shell so it can sprout come spring.

In the case of biennials, the plant’s life cycle occurs over two years–the first year you only get a rosette of leaves and the following year flowers. Foxgloves, Hollyhocks, Sweet William, Canterbury Bells will not flower unless the plant undergoes those winter temps. Many of our herbs and vegetables such as parsley, turnips, carrots and kale that are grown as annuals are actually biennials and would flower the second year if not harvested. Different varieties require different lengths of cooling time.

As with many things in nature, scientists have found ways around Vernalization. There are varieties of apple trees that will produce in the south. Spring bulbs can be purchased- ‘pre-chilled’. Grower’s mass produce flowers by lowering greenhouse temperatures and tricking plants into thinking they have experienced winter. People in the Midwest have created spring break.

In Illinois, I mark the winter days off, one by one, reminding myself, the spring bulbs I planted in October, need winter to make my garden spring to life.