How a plant knows that spring is here

Light, temperature and hormones all play a part in it

In The Garden

March 23, 2003|By Nancy O'Donnell | Nancy O'Donnell,New York Times News Service

Have you ever wondered how a plant knows it's time to wake from its winter slumber? Or how all those spring flowering bulbs you planted last fall know it's time to begin that journey upward, especially when buried beneath snow?

To understand how this spring phenomenon happens, we have to know what causes the plant to go into dormancy in the first place.

A plant's dormant cycle begins in autumn, when daylight hours shorten and temperatures begin to drop. These key environmental changes cause a bit of hormonal upheaval in the plant, resulting in a slowing down to an almost virtual standstill of its metabolism.

During this slowing-down process, the food manufactured and housed in the leaves during active growth relocates to the root zone, converts to starch and is stored for winter. At the same time, physiological changes are occurring inside each cell to keep it from freezing. Once all these stages are complete and the temperature remains near freezing for a particular length of time, the plant is said to be dormant.

So, if those key environmental changes were to reverse themselves, it stands to reason that the plant would break dormancy, correct? Not quite. The catalyst for breaking dormancy is that particular length of time spent in the "chill period."

Each plant species has its own chilling requirement; once reached, the plant is then ready to stretch its buds.

It's believed that during the fall, a growth inhibitor hormone called ABA builds up in the plant as it goes into dormancy, causing growth to halt. Once the required number of chill hours are reached, ABA begins to break down.

For the bulb planted underground and under all the snow, this is precisely how it knows to begin inching upward. The growth inhibitor is no longer present in a high enough quantity to stop growth.

Ever wonder how a particular perennial could emerge so quickly once the frost leaves the ground? The same principle applies: The growth inhibitor is no longer present.

To complete the breaking of dormancy, as the snow melts and the surrounding soil and air begin to warm, more hormones kick in. These are the growth hormones known as gibberellin (responsible for stem elongation) and cytokinins (responsible for cell division). For all plant material -- bulbs, perennials, trees, shrubs and germinating seeds -- these hormones are crucial for the development of stems, leaves, roots and flowers.

You can fool Mother Nature into thinking spring has sprung.

Many spring-blooming shrubs, such as forsythia, quince, deutzia, honeysuckle, pussy willow, flowering almond, bridal veil spirea and viburnum, can be forced into bloom earlier because they only need nine weeks of chilling time. You can also force trees such as flowering crab apples, fruit trees, shadblow and dogwood.

Simply cut a branch and pound the cut end with a hammer before placing it in a bucket of water. This is a trick used by florists to increase the water uptake of woody stemmed plants by breaking the cambium (water vessel) layer.

Remember, your house is far less humid than the great outdoors, so either mist the branches frequently for about a week or submerge the branches over-night in a tub of tepid water (then return to a bucket of water). With these techniques, you are helping the buds and stems absorb moisture and begin to break dormancy. A low humidity will cause the buds to fall off, so this step is essential.

Finally, you need to imitate the milder spring temperatures by placing the bucket in a location where the temperature is somewhere between 60 to 65 degrees and where it will receive bright, indirect sunlight until the buds break. Temperatures that are too cool will slow the bud break, but those that are too warm will cause the buds to develop too quickly and not open properly.

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