Garden Detective

Garden Detective: Sheet mulching can shrink violet problem

Some people like the flowers of the species Viola odorata but the plants can be tough to eradicate.
Some people like the flowers of the species Viola odorata but the plants can be tough to eradicate. Bigstock

Experts tackle readers’ garden questions.

Q: How do I get rid of violets? I’ve used Roundup and also dug up roots all summer. I’ve pulled them out and cut them down with electric shears. Still, they come back. Any other ideas would be appreciated.

Marian Schieck, Sacramento

Master gardener Lorraine Van Kekerix: Some violets can become pests because they spread by above-ground runners and produce large amounts of seed. Despite your efforts to get rid of them, the violets can grow back from small pieces of above-ground runners or roots, or germinate from violet seeds in the ground.

(Editor’s note: There are several species of wild violet – all members of the Viola family – that behave this same way. A waxy coating on their leaves makes them resistant to most herbicides.)

If your violets have been in place for many years, there is a very large seed bank from which new violets germinate and grow. To get rid of future violets that may germinate as well as kill off existing plants, one of the most effective and low-cost methods is to sheet mulch the area.

Sheet mulch is a method of layering that can smother unwanted plants (such as those wild violets). A layer of cardboard or newspapers is covered by 2 to 5 inches of organic mulch (bark, wood chips, compost, etc.) that prevents light from reaching the soil surface. Plants under the sheet mulch cannot get the light they need to grow, so they die. Seeds under sheet mulch cannot get the light they need to germinate and grow. Sheet mulch also will help conserve water.

Measure the area to be sheet mulched to determine the amount of cardboard or newspaper and mulch needed. For example, a 3-by-3-foot patch of violets is 9 square feet in area.

A large piece of cardboard can be cut to cover all 9 square feet. If you use smaller pieces of cardboard, or stacks of six to eight pieces of newspaper, overlap interior edges by 3 to 4 inches to create a 9-square-foot layer. The overlap is needed as the cardboard and newspaper can shift as they get wet and dry over time. The overlap prevents light from getting through the sheet mulch.

There are a variety of mulches. Mulch is available in bags from garden centers or in bulk from sand and gravel suppliers, large garden centers and some public utilities. Bagged mulch is usually sold in 1- or 2-cubic-foot bags. One cubic foot of mulch will cover 3 square feet with 4 inches of mulch.

To install the sheet mulch, lay out the cardboard or newspaper, being sure to overlap the edges. Since wind can blow cardboard or newspaper, it is easiest to install a small section at a time.

The existing plants covered by the sheet mulch should die off within a growing season. To kill off a large seed bank in the soil, the sheet mulch will need to stay in place for several growing seasons.

To replace the violets with other plants, wait until the existing plants die. Then, plant the replacements in the sheet mulched area. It’s not necessary to remove the cardboard or newspapers. Scrape back the mulch from an area larger than the replacement plant’s pot. Cut an “X” through the cardboard or newspaper layer and fold the edges back, then dig the hole and plant the new plant. Fold the cardboard or newspaper towards the new plant. Then, move the mulch back around the new plant, keeping it a few inches from its stem or trunk. Add more mulch around the new plant, if needed, to prevent weeds and conserve water.

For more details, check out “EHN 94: Sheet Mulching,” available on the “Publications” page at ucanr.edu/sacmg.

Lorraine Van Kekerix is a UC Cooperative Extension master gardener in Sacramento County.

Garden questions?

Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to h&g@sacbee.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:

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