Garden calendar | For the week of March 24
Wacky weather: If you are feeling a bit of weather whiplash, you are not alone. Some plants have leafed out or are blooming a few weeks earlier than usual due to the record-breaking warm weather in February and early March. Now, Mother Nature as she often does, is doing a bit of a course correction and we are back to more seasonal winter weather — and in fact, lower-than-normal temperatures. So, what will happen to the plants?
There are a lot of factors that go into whether flowers, flower buds, leaf buds or leaves, roots or plant vascular tissue experience cold damage.
For woody plants, last year’s weather can affect the likelihood of cold damage. Drought-stressed plants are more susceptible to winter injury. Watering your woody plants, especially evergreens, into late fall can help prevent winter burn or vascular damage so plants go into winter well hydrated. Damage can result if we have a long, warm fall, then an extreme cold snap. When this happens, plants have often not “shut down properly” and gone sufficiently dormant to avoid damage. Lack of snow cover at this time can cause root damage as well. Woody plants are usually fully dormant in December and January, unless temperatures have been too warm. While fully dormant, they won’t break bud, even with warmer weather, because the buds haven’t received enough chilling hours for them to break dormancy. Chilling hours on average are about six to eight weeks below 40 degrees F, but some plants need even colder temperatures for longer periods. During March and April, most woody plants enter “quiescence,” a sort of resting state, where various plant hormones cause the plants to start to deacclimate from dormancy, preparing for warmer weather. This can make cold damage more likely.
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For this spring, woody plants, as long as the buds are not far open, should be fine (barring drought issues or dormancy issues). The buds are protected by bud scales covering the tissue that will become leaves and flowers. A light frost, 32 degrees F to about 29 degrees F, won’t damage most perennial and woody plants, even if flower buds are showing color. Once flowers open though, they are more vulnerable. When temperatures drop into the mid-20-degree range or lower, then it is likely that some damage to flowers will occur. But other factors also play into it. Still, windless nights have lower temperatures, especially under clear skies. The duration of the low temperature, and the genetics of the particular plant also factor in. Lenten rose (hellebore) and corneliancherry dogwood, which typically bloom in March, are more resilient than plants that typically bloom in April, but are blooming or leafing out now due to the early warm weather.
So, forsythias that have started to bloom may have floral damage as they would normally bloom later — as well as having flowers higher up in the air than a ground-level bloomer that may get some residual heat from the ground. Topography matters too — since cold air sinks, gardens in low areas, often called frost pockets, may experience more damage regardless of whether a flower is closer to the ground, than gardens in more upland areas. Urban areas surrounded by buildings and asphalt that give off heat at night might protect plants. However, heat off those same buildings in daytime may have pushed plants to grow more even earlier, so they may be further along and more likely to experience damage than plants in cooler, more rural, areas.
As a protective measure, if your plants are in bloom, and you can cover them with a sheet or bucket, this may help as night temperatures dip into the low double digits. We will really just have to wait and see.
— Lisa Johnson, Dane County UW-Extension horticulture educator
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