Expert shares tips to protect plants from spring frosts

Wednesday, April 6, 2016

Freezes at 27 degrees or less can damage tender plants and fruit trees

Keeping plants safe in between seasonal weather fluctuations can be tricky for gardeners, and even though Spring has officially arrived, plants are not out of the woods yet when it comes to unexpected freezes.

Bouts of spring frost are common in the Ozarks, but knowing a few facts can help even novice planters ensure their new plants survive, and thrive, for the season.

Robert Balek, southwest horticulture specialist, who covers eight counties, including Barry County, said on the average year for the Cassville area, the last frost expected would be around April 20.

"In an average year, they'll pretty much be frost-free after that," he said. "This year is warmer than usual, so they can probably hedge their bets and put things out early."

However, Balek said, unexpected frosts can still happen.

"The area is not completely frost-free until May 8," he said. "Once we get past our frost-free date, we can say a freeze has not been documented, so there's less chance of it occurring. By May 8, we have less than a 10 percent chance of freezing in the Cassville area."

If frost is forecasted, residents can cover their plants, especially tender ones like tomatoes.

"[Frost] could impact them at 32 degrees, but as far as long-term damage to the crops or trees, the number to watch out for is 27 degrees or below.

"They need to cover the plants, especially if they are tender annuals," he said. "That's to hold the heat in because the ground is their heat source. Some plants do just fine. People don't have to worry about tulips and hyacinths, or spinach, cabbage and kale, those winter crops. They'll take a late frost. It's when the temperature gets down in the 20s, because at 27 degrees is when it starts to affect trees. That's when we get the issues with the trees and shrubs.

"That's when we'll start to see peach and apple trees dropping their fruits. A freeze like that could cause damage to the trunk and twigs. When the air gets down to a 32 degrees, as opposed to a hard freeze where it can actually freeze the ground, which usually starts happening around the 27 degree mark, we might see cracking. That is the magic line for fruit trees. If it's a small tree, they can cover it. Some orchards use kerosene-fired heaters and try to heat the air. We will probably not get to that this year."

Balek said it's safe to put out cold-weather crops now, like spinach, cabbage, kale and peas, and flowers like pansies. But cautioned planters to hold off putting out tender summer plants like tomatoes, peppers, sweet corn and squashes, until certain there will be no more freezes.

"Cucumbers and squash should wait even longer," he said. "Those can be put out after that first week of May. The cold crops like a soil temperature of about 40-50 degrees, and the summer crops like soil temperatures in the 60s-70s."

One plausible reason for the unseasonably warm winter the county has seen, Balek said, is due to the tropical climate changes caused by El Nino in the Pacific ocean last fall.

"Now, I understand it's dissipating," he said. "Anything can happen with the weather, but on an average scale, we should be free of frost by May 8."

EJ Adams, Barry County Master Gardener, also shared some tips for protecting plants.

"I prefer to cover tender plants with old sheets, lightweight blankets or burlap," she said. "Do not lay plastic bags or plastic sheets over plants. They do not hold the heat. If plants are not very sturdy, you can put some stakes in the ground or set some pots on the ground to support the covering so that it does not crush the plants. If covering a larger area, using stakes and twine to create some support for the cover might be necessary.

"Other covers such as cardboard boxes with an open side, plastic jugs with the top cut out, or glass jars can be placed over the plants. Flower pots will work if the drainage hole is covered and newspaper might work for a small area. The covers should be removed the next morning when the temperature has warmed and the frost has melted. Leaving the covers on through the day creates too much heat and can damage the plants. Many gardeners mulch and gently cover the plants with loosely packed leaves or straw which is also helpful."

Adams emphasized that watering also plays a role protecting plants.

"Well-watered plants will be less likely to freeze than those in dryer soil," she said. "The soil will release moisture into the air during the night which will help to keep the plants a bit warmer. Water before the air temperature has dropped and the sun has set. If the location allows, and there is no danger of rain during the night, an electric fan can be used to keep the air moving which will prevent the cold air from settling on the ground around the plants during the night."

Other extension suggestions include moistening the soil, which Adams explained, or starting seedlings indoors.

For more information about protecting plants from freezing temperatures, residents can call the Barry County University Extension office at 417-847-3161, or visit the extension website at http://extension.missouri.edu/.

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