Keep Your Plants from Freezing this Winter

By Trisha Gedon, Oklahoma State University

First posted on 12-11-2014

Oklahoma has already dealt with several days below the freezing mark. So as you pull out that extra coat or throw another log on the fire to keep warm, remember to protect your shallow-rooted plants as well.

Kim Toscano, Oklahoma State University Cooperative Extension assistant specialist and host of “Oklahoma Gardening,” said mulch is a great way to protect your plants over the winter.

“As we all know, the weather in Oklahoma can come in extremes. It’s not unusual for it to be freezing temperatures one day and spring-like the next,” Toscano said. “The alternate thawing and freezing of soil as temperatures warm during the day and drop at night can cause the soil to heave or buckle.”

Heaving can lift shallow-rooted plants right out of the ground and expose the tender crowns to freezing temperatures. Heucheras, tiarellas and astillbes are plants with shallow root systems often affected by soil heaving.

These plants can be protected by applying mulch as winter sets in. Mulch has an insulating effect that reduces temperature extremes through the day and prevents soil heaving. Organic, easily decomposed materials are ideal for winter mulching. Pine needles, straw, chopped leaves and other organic materials provide winter protection.

“These materials decompose quickly in the spring as temperatures being to rise,” she said. “Wood chips utilize a lot of nitrogen as they decompose, so it’s better if they are already composted before using as mulch.”

Regardless of the material a gardener selects, the rule of thumb for winter mulches is to apply a two- or three-inch layer because you do not want to smother the plant. If you use straw, start with a four- to six-inch layer, as it will pack down to the desired final depth of two to three inches over time. After applying the mulch, gently pull it away from the stems or trunks of the plants. Mulching too closely to the trunk may provide optimum conditions for the development of insect and disease pests.

Keep in mind some plants are sensitive to wet soils. While mulch helps retain soil moisture, it can be a detriment to plants such as lavender that require good drainage to thrive. These plants can be mulched with a thin layer of gravel instead.

“You can wait until after a few hard freezes to mulch because this helps plants harden off for winter,” Toscano said. “Remember, plants still need water during the winter months. Irrigate plants one to two times per month if adequate rain hasn’t fallen.”