Some Plants Like it Cold

ChiloquinNews article 1/3/2011

During winter, the chemical reactions that take place in plants slow down almost to a standstill, due to the cold temperatures. Surprisingly though, some important physiological processes take place in winter and actually depend on the cold temperatures. Dormant winter buds must experience a cold period to prepare for their awakening in the warmer days of spring. Depending on the plant, it can take just a few days of cold or it may take months of cold temperatures to overcome bud dormancy. This is known as the chilling requirement, and you will see it listed for many fruit trees. For example, apples require at least 1000 hours below 45oF. Special cultivars have been developed for many fruits that require less chilling and you will usually see them listed as ‘low chill’ varieties, or ‘suitable for the south’. We can ignore those ratings. We have a long enough cold spell for anything we’d like to grow.

It is similar with bulbs. In tulips, the next year’s flower bud is already present before winter arrives, but for the flowers to develop and open in the spring, the bulbs must first go through a period of 13-14 weeks with temperatures below 50oF. This is why we were never able to grow tulips or apples in Adelaide where I grew up. We just didn’t have enough cold. We would have had to dig up the tulip bulbs each fall and keep them refrigerated until spring. Now you can buy tulip bulbs that have been ‘prechilled’ if you live in a climate that will not do it for you. There are many bulbs with a chilling requirement. Onions, for example, can be prevented from flowering by keeping them at warm temperatures throughout the year.

Some seeds need a cold treatment before they will germinate, and in some plants, flowers are formed in spring only if the plant has been kept at close to freezing temperatures for several weeks.  Many biennials (those that flower in the second year) require a cold period and then shoot up a flower as soon as spring arrives. When this happens with our foxgloves, we are very pleased, but when it happens to a carrot or a cabbage that we left in the ground over winter and didn’t get dug up fast enough in spring, we are not so amused.

Those plants you see in stores that are flowering in mid-winter, have been given an artificial cold period and an artificial photoperiod to bring them into flower at the right time for say, Valentine’s Day. I wonder if coming back to reality for them is a bit like a trip to Australia for us—everything feels upside down.

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