Bob Filburn
Over the past several weeks, this weather has been noticeably colder, even to a Colorado transplant like me. With a burr and a shrug, I leave my house a little before my usual time to give my old diesel a little extra time to warm up.
Waiting for the billows of exhaust to subside, I glance at the neighbor’s dogwood tree, which despite the morning nip, seems to be tired of its winter rest and is eager to pump life back into the delicate buds which adorn the branch tips. A question arises; does this dogwood recognize the same chill which causes me to pull my hood over my head?
While many people recognize that with the leaf drop in fall, our plants enter a state of dormancy, few understand that the developmental state of “rest” which follows can only be broken with a “chilling requirement” that is specific to each cultivated plant. Even if the environmental conditions are favorable – increased light levels and warmth, without this necessary cold period, the plant cannot wake from its winter slumber.
Mike Parker, Extension specialist at NCSU explains, “temperatures effective in satisfying the chilling requirement normally range from 32 F to 50 F, with the optimal temperature being approximately 45 F.”
To put this into perspective, let us consider Saturday evening’s change in temperature; at 7 p.m. the thermometer read 42 degrees and dropping, the following morning the mercury began to rise and crested 45 degrees at 7 a.m. This evening’s weather event enabled the magnificent dogwood and all of our fantastic spring bulbs to “bank” 12 chilling hours.
As a plant accumulates its necessary chilling units, it moves ever closer to breaking dormancy and sending forth a burst of flower or foliage. In the event that spring temperatures rise quickly, with the total chilling requirement yet to be fully met, leaf and flower bud development can occur, but noticeable physiological symptoms often follow. Initially foliage emerges as tufts of growth at the terminal ends, but the plant is devoid of leaves closer to the main stemming. Further leaf development sometimes occurs later in the season, but often the plant is weakened and suckering growth emerges from the base of the plant. As with the vegetative growth, flowering is often delayed or absent and the flowers that do form exhibit abnormalities that result in poor or reduced fruit production. While this disruption of flowering can pose a major disappointment for those of us eager for spring, it has a huge impact on plants producing edible fruits. Blueberries and peaches are two crops that can be severely affected by insufficient chilling.
For the grower or gardener who has inherited an existing orchard, patch or flower bed, control is in the hand of Mother Nature; however, for those planning a new or renovated planting, proper variety selection should be first and foremost on your mind. Although North Carolina typically receives in excess of 1,000 chilling hours annually, selecting varieties with a minimum of 750 chilling hours is suggested. While many of the peach varieties will fall within this category, purchasing an appropriate blueberry variety will require a higher degree of selectivity. "Morrow," "Reveille," "Croatan," "Murphy," "Bounty," "Bluechip" and "Jersey" are rabbiteye type varieties which have an appropriate chilling requirement and perform well in our climate and soils.
The flower gardener also needs to practice caution when selecting spring bulbs for this area. Although beautiful, tulips are often utilized for only one season because they require supplemental chilling, often in a cooler, for adequate flower development.
Charlotte Glen, Pender County Extension interim director, suggests experimenting with daffodil varieties which have performed well in this region: large trumpet types: St. Keverne, Carlton and Ice Follies; or tazetta and jonquilla types: "Quail," "Baby Moon," "Minnow," "Geranium," and "Avalanche." A combination planting of daffodils and grape hyacinth, Muscari neglectum, can be a real show stopper in the early spring.
Bob Filburn is an Edgecombe County Extension Service agent specializing in horticulture. Look for his Garden Guide each month on the Community page.
