Saturday, December 20, 2008

Assessing Cold Damage to Plants

It’s natural for Pacific NW gardeners to worry about their plants during this unusually long cold snap, and there’s no doubt that it’s likely to damage some of our garden plants. The lack of previous hard freezes during the fall caused many plants to remain in their active growth phase rather than going into their usual dormancy, so the sudden drop in temperature was a big shock. Also, for the past few years many gardeners around here have been “pushing the envelope” with much success, in terms of growing plants that are not normally hardy in this zone. This weather may be the end of those tender beauties.

However, it takes time for the real degree of damage to become evident, and many plants that look absolutely pathetic after a freeze like this can bounce back completely and be just as beautiful as ever. This means patience on your part! Waiting can be difficult, but it’s the best thing to do before assuming the worst and deciding to give one of your shrubs or perennials the heave-ho.

Knowing the ways that plants respond to a hard freeze, and how they recover, can help you assess cold damage and make good decisions about what to do:

Rolled-up leaves. The moisture in the air drops very low when we have day after day of near or below freezing weather. Plants conserve moisture by keeping their stomata closed, which are little “breathing holes” on the underside of their leaves. Many evergreen plants also curl or roll-up their leaves, which makes them look like they’re dying. Rhododendron, Camellia, Sarcococca, Evergreen Clematis, Choisya, and other shrubs like this should perk up again once our typical mild and wet winter weather returns, though they may not bloom as profusely as normal.

Brown leaves and stems. Some evergreen plants, such as Hardy Hebes and Phygelius (Cape Fuschia) can suffer dieback of their outermost branches and leaves, which makes them look dead. However, the roots and crown of these plants are likely to be just fine, and will send out a bounty of new growth once spring weather returns. Even though they look unsightly now, leave the old dieback just as it is until you see the tiny green buds emerge. Then cut back the branches to just above those new buds, and by summer they’ll look as good as new.

Snow itself can be a good thing when the temperature is below freezing, because it can insulate flower and leaf buds from the cold and wind. However, the incredible amount of snow we've received can also pose some problems for evergreens with flexible branches, like the plants mentioned above. The weight of the snow, especially as it begins to melt or if rain falls on it, can bend the whole shrub to the ground and sometimes even break branches completely. To safely knock off the snow, use a soft tool such as a kitchen broom, and gently push the branches up from below so that snow falls off the lowest branches first. That way, the lower branches won't be overloaded when you knock the snow off the upper branches.

What about container plants? Because they are much more exposed to cold air and wind than if they were in the ground, the roots of container plants can freeze and die. However, many of these plants can survive the cold and bounce back completely. In general, the roots of most plants are much more cold hardy than their branch and leaf canopy. To prevent as much damage as possible, wrap the pot with bubble wrap or an old blanket, cover them with Remay or an old sheet, and move the pot up close to the house or into the garage until the deep freeze is over.

Weather like this is one reason I always advise clients to wait until spring to cut back their grasses and perennials, because the dead foliage helps protect the crown of the plant from cold damage. It also helps provide a place for beneficial insects to overwinter in your garden, where they will be ready to gobble up any spring pests that arrive. Last but not least, the spent flowers and leaves of many of these plants are a very attractive feature to the winter garden.

There are other factors that can affect the amount of cold damage to plants, such as water logged or parched soil conditions, exposure to wind, and the depth of snow insulation. Because we do tend to have these deep freezes every few years, I usually don’t include many marginally-hardy plants in my garden designs. But if you have questions about cold damage to your specific garden plants, I’ll be glad to provide advice and guidance.

Many plants look beautiful in the snow!