Mulch perennials after ground freezes

Q. I noticed my neighbor applying mulch to many of the perennials in his garden. I remember reading somewhere that you should not apply mulch too early. Is that true? Why?

A. You are correct. Mulches should be placed on perennials after the ground freezes and remains frozen. This occurs when nighttime temperatures drop to the teens for several nights in a row. Many people incorrectly think that mulch keeps the ground and plants warm. The purpose of mulch is to insulate the soil and keep the ground evenly frozen until the final spring thaw. Mulch also keeps the plants from drying out by winter winds. In the Chicago area we often have repeated freeze/thaw periods in January and February. During these periods, plant crowns and roots can be damaged when the plants are heaved upward in the ground.

Roses

The best method for mulching roses in Northern Illinois is to “hill up" 10-12 inches of well-drained loose soil/compost mix around the base of the plants. If you grow hybrid tea roses you need to protect the graft union (planted 2-3 inches below soil level in our climate) from winter damage. After the mound freezes you can pile on straw, evergreen boughs, or leaves to keep the ground frozen and insulated. To keep soil and mulch in place during the winter, you may want to encircle each bush (18-24 inches diameter) by chicken wire forming a cage 18 inches high. After the spring thaw you can gently dig the mulch into the soil around the bushes. Be careful not to damage the root system when you do this.

You could also place Styrofoam cones over your roses. If you choose to do this, make sure you poke at least four 1-inch holes around the top of each cone to release heat and provide good aeration inside the cone. For best protection you should mound soil over the crown of the plant before you put the cone in place. Put a brick on top of the cone to keep if from blowing away.

Perennials

Straw, loose compost, and evergreen boughs are the best mulches for most perennial beds. Loosely pile 3-4 inches of mulch on the perennial bed after the ground is thoroughly frozen. Some gardeners successfully use 3-4 inches of leaves piled loosely over the plants. This method is controversial because the wet leaves could become matted and lead to crown rot of the plants. Additionally, because piles of leaves provide good insulation, they make attractive winter homes for mice and other small rodents.

Gently remove the mulch in spring when nighttime temperatures are no lower than freezing, usually mid-late April in this area. Be careful not to damage emerging plants when you remove the mulch. A leaf blower is a good way to remove mulch from around plants.

Provided by Mary Boldan, Mary Moisand and Donna Siemro, University of Illinois Extension Master Gardeners. Send questions to Ask a Master Gardener, c/o Friendship Park Conservatory, 395 W. Algonquin Road, Des Plaines, IL 60016, (847) 298-3502 or via e-mail to cookcountymg.com@gmail.com.