While gardeners are enjoying the mild winter we have had in Michigan this year some have expressed concern over what the weather is doing to our garden plants. Because we don’t know what the rest of the winter will bring weather-wise to our Michigan gardens we can’t really assess yet what damage plants will have in the spring.
While it seems that mild early winter weather would be great for plants it can cause problems. Plants need to go into deep dormancy to best survive a Michigan winter. When the temperature doesn’t go down and stay below freezing for long periods of time plants may not go into deep dormancy, especially those in protected locations such as the south side of buildings. When the weather suddenly becomes very cold, the plants can then become damaged or killed.
Normally plants are exposed to a long period of slowly decreasing temperatures and decreasing day length that start the process of hardening off – or preparing for winter. Once the plant has hardened off the plant has a greater resistance to cold. Mild weather during the time when plants are supposed to be going into dormancy is worse than a mild spell that occurs later in the season- at least until we start getting to spring equinox when plants are being signaled by the longer days to resume growth with just a small increase in temperature.
Plants like roses, heuchera and boxwood, which tend to be semi-evergreen in Michigan, are some of the most likely to be affected plants. Many retain leaves in a protected location or when fall weather has been mild. During warmer periods they may resume growth, and then that new growth is killed during the next bout of colder winter weather. The branch and tip dieback may be much more extensive than when we have colder weather. In most cases there is little we can do to help this situation except possibly to shade such plants so they stay cooler.
Freezing and thawing soil can have other consequences. As soil freezes and thaws it sometimes pushes shallow rooted plants out of the ground, and then the roots die when exposed to really cold air. Strawberries are a plant where freezing and thawing can heave the plants out of the ground. Several other shallow rooted perennials are also affected. On one of the mild sunny days a walk around the garden might reveal plants which are being popped out of the ground or plants that mulch has blown off and which are now exposed. The good news is that if the plants are replaced in the soil and additional mulch added to bare areas the plants may be saved.
One thing not to worry about is spring flowering bulbs, at least not yet. If the tips of the leaves break ground and then are exposed to snow or cold air little beyond the yellowing of the leaf tips will occur. At this point the flower bud is still immature and pretty well protected. However if our mild weather continues another month and buds actually start to swell well above ground, some flower death could occur if temperatures then get very cold. The bulbs will generally survive to bloom again another year. You could add some mulch over growing tips of bulbs; remember to rake some of it away when spring does finally settle in.
Grass that hasn’t gone dormant and is still green when a sudden cold spell hits may have the foliage killed, but that probably won’t kill the plant, especially if we get snow. If extreme cold (0F or below) hits suddenly, and if there is no insulating snow cover, then grass plants may be killed. Green grass that gets covered with snow, especially if it’s a little long, may be more susceptible to snow mold in the spring.
Since breaking dormancy in plants is also tied to increasing day length in many plants, the longer our mild spell holds before returning to regular winter weather the more damage we may see in the landscape. However if winter continues to be mild some plants may actually have better than normal condition in the spring. Unfrozen soil allows broad leaved evergreens to take up water and suffer less from drying out. And some marginally hardy plants in our zone may come back more robustly and bloom earlier than usual.
Many gardeners are hoping for a short mild winter that our plants survive with few problems. But if cold weather is going to make a sharp return let’s hope an insulating cover of a few inches of snow arrives with it. It can be frustrating knowing we can’t control the weather and that in most cases we can’t do much to help our plants survive nature’s capricious ways. But even if some of our precious plants don’t make it, most will, and there will be beautiful gardens in Michigan next spring.