A chilly spring night can be a recipe for disaster, as we are talking about gardens in northeast Wisconsin. While there are not many nights with forecasted temperatures below freezing, frost can still form, and that frost is likely to severely damage or even kill many sensitive plants.
Frost can develop when we see surfaces cool to less than 32 degrees. This allows for water vapor within the air to freeze into tiny ice crystals. This leaves plants vulnerable as they also quickly lose heat in the overnight hours.
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The real threat is what’s happening within the plant itself. Plants contain water in their cells, so when temperatures drop below freezing, that water can freeze as well. As water freezes, it expands as ice; this expansion often leads to the rupturing of cell walls. Once the sun begins to rise and temperatures begin warming, those ruptured cells collapse, leaving your plants wilted and in a damaged state.
More tender plants, such as tomatoes, peppers, flowers, and many types of other new vegetation, are at higher risk because they contain softer tissue with higher water content. Select plants have cold-conditioned adaptations that allow them to do a better job of tolerating cooler temperatures.
Frost is typically most common on clear and calm nights because the heat that is built up throughout the day quickly escapes into the atmosphere following sunset. Remember that warm air rises and cold air sinks due to the heavy and dense profile of colder air. Windy or cloudy conditions can actually help in limiting frost formation as it aids in keeping temperatures mixed and trapping some heat towards the surface.
Gardeners can take many steps toward protecting plants on nights where frost may be a concern:
- Covering plants with lightweight blankets prior to sunset.
- Bring potted plants indoors.
- Use mulch around plant bases as this helps insulate roots.
- Water the soil during the afternoon as moist ground holds warmth better than dry ground.
- Prioritize low lying areas, cool air will settle here first.
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It’s also important to keep in mind that forecasted temperatures are about 6 feet above the ground. Temperatures at the surface level or plant level can run several degrees cooler, hence why you see frost form at a forecast of 35 or 36 degrees.
As spring planting ramps into full force across Wisconsin, keeping an eye on overnight forecasts and frost advisories can help gardeners avoid potentially costly damage and ensure plants stay healthy throughout the season.
