In the Garden: Surviving the Cold

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The winter of 2017-2018 is finally over.  How well did your garden survive the “New Year’s Freeze,” as well as the rest of the winter? And what can we expect in the upcoming spring?

I can hear some of you right now: “Wait a minute! According to my calendar, winter’s not over until March 20. In fact, spring 2018 won’t begin until 12:15 p.m. Eastern Daylight Time on that date. We still have almost three weeks of winter to get through!”

You’re right. Sort of. In fact, they’re actually several definitions of winter. Most of us are familiar with the astronomical, or calendar-based reckoning, based on the date of the vernal equinox, with winter ending either March 20 or 21. Meteorologists and climatologists prefer a more accurate system that divides the year into seasons based on average temperatures. In much of the Northern Hemisphere, the three coldest months of the year are December, January and February, so those entire three months constitute winter. June, July and August are summer, and so on. Just to confuse things a bit more, in some European countries they have a “cultural” winter based on traditional religious feast days.

Looking back, we had a very cold ten-day period ending on January 7. According to the records of Gazette columnists Heidi Sonen and Roscoe Shaw, the average temperature during that stretch was 21.1 degrees. Certainly quite cold, but only ranking fifteenth coldest for ten-day periods since 1900.  During this period the temperature topped out at only 34 degrees, and was generally much lower, with an absolute minimum of zero. The ground would have been frozen for most of that time, and therein lies the stressful part for plants.

Evergreen plants retain their foliage throughout the year and are subject to moisture loss through stomata, microscopic openings on the bottom of their leaves. Water is drawn up from the soil, conducted through the plant’s vascular system and exits via the stomata. But if the soil is frozen, water is locked up and unavailable to the plant. On warmer days, especially sunny ones, the plant may actually be in a physiological drought. And we definitely had warmer days soon after the big freeze; on January 12 the temperature rose to 67 degrees.

Plants cope with these conditions with varying degrees of success. Deciduous trees and shrubs drop their leaves and handily avoid the problem of water loss. Coniferous evergreens such as pines and spruces also take the cold in stride for the most part, although the ubiquitous Virginia pine (Pinus virginiana) will turn somewhat yellow in winter.

Broadleaf evergreens from cold climates are also able to deal with the freezing temperatures that they typically encounter in their native range. The rosebay rhododendron (Rhododendron maximum) reacts to cold in a distinctive way: the leaves droop and curl up. The plant may look miserable, but the strategy works. The drooping reportedly reduces exposure to bright sun light, while the curling creates a more humid microclimate around the underside of the leaves.

Plants from warmer climates—camellias and the southern magnolia, for example—may not handle extreme cold as well. Leaves will turn bronze in color, especially in direct sun. Such damage can be highly localized: leaves on the shady north side of the plant may show little damage, and the same goes for leaves that are in the plant’s interior. The fix for damaged foliage is simple: time. New growth soon appears and covers up the blemishes.

Beyond damage to foliage, exceptional cold can also injure buds and stems. Loss of flower buds is particularly common with spring-blooming hydrangeas, one reason that re-blooming varieties have been developed. On more tender shrubs, cold can also kill a portion of the stem and its buds. This becomes evident when new foliage flushes; the upper portion of the injured stem will show no growth. At this point, prune back to just above the uninjured portion of the plant. If it appears that the entire above-ground portion of the plant has been killed, cut it to the ground and wait. Usually new growth will appear, although it may take quite a while.

While damage from this past winter might have been significant, injury during the upcoming spring is also a threat. In a typical March, average low temperatures range from 30-39 degrees; last March the minimum temperature at Charlottesville airport was 21 degrees. Yet during the same month, temperatures ranged well into the seventies—and even the eighties—bringing some plants out of dormancy. There’s not a whole lot you can do about that except cross your fingers and pray. If a plant is very small, you may be able to cover it in some fashion, but this usually isn’t practical except for veggies in a hoop house. When a late freeze damages leaves or flowers, just grit your teeth and practice patience. Blackened or mushy tissue will soon drop off.

There are some measures you can take to lessen the chance of winter and spring damage when buying and installing new plants. Unless you’re a risk-taker, seek out hardier selections. You might really crave a plant that’s listed as hardy only to zone 7B, but accepting the fact that we live in colder zone 7A is more prudent. Also, plant breeding has given us some better options: some varieties of camellias and magnolias have been bred to bloom a bit later, giving them a better chance to escape frosts. Finally, correct siting is important. Plants located under high shade or on the north side of your house are less susceptible to damage from winter sun and premature warming.

Here’s hoping that most of your plants came through the winter in good shape. And ditto for the next few weeks.

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If you’re currently suffering from allergy symptoms, the not-so-obvious culprit might be all around us. The cones of male Red Cedars (Juniperus virginiana) are out and dumping clouds of pollen. The cones are tiny but extremely numerous, giving the entire tree a yellow-brown cast. The female trees are innocent; following pollination they’ll develop attractive bluish berries.

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