Weed preventing mulch

Asked March 3, 2016, 5:56 PM EST

I live in Sisters and my garden area already has grass growing in it. Every year I either spray the grass or yank it out. Every year it comes back. I tried landscape fabric but think it does little good. I'm considering rock as opposed to bark dust or wood chips as after a couple of years the organic mulch will decompose and become a very inviting environment for grass and weeds. I'm wondering what you think of my plan and have any suggestions. Thanks in advance, Robert

Deschutes County Oregon

1 Response

Hello Robert,

Thank you for your question about mulch as a means to prevent weeds in your garden. Organic mulch has many positive advantages for a garden. It helps to conserve water, improve the quality of the soil, and prevent weed growth. Unfortunately, no mulch can prevent all weeds from emerging. You may still need to treat and pull some weeds with any mulch you apply to your garden.

What time of year you apply the mulch may be helpful to limit weed growth. I have included a link an OSU Extension publication entitled, Mulching Woody Ornamentals with Organic Materials: http://extension.oregonstate.edu/deschutes/sites/default/files/ec1629-e.pdf.

This publication details the types of organic mulch, advantages of each, and how and when to apply mulch to reduce weed growth in the garden. It states:

“Fall mulch applications can smother winter annual weeds, decrease runoff, and increase soil water retention for the following summer.” Also, you may need to apply the mulch a bit thicker to exclude sunlight to prevent weed seeds from germinating. This article recommends a 3-inch layer of mulch to prevent weed growth.

Here is Central Oregon, Toni Stephan, Deschutes County OSU Extension instructor, recommends that fall mulch be applied in late November or early December, as it is important to let the soil cool down and the plants begin to go dormant for the winter. Also, mulch should not be directly placed around stems or trunks of perennials, shrubs or trees.

Some thoughts on using rock for weed prevention from Alison O'Connor, PhD, Horticulture Agent, Larimer County Extension in Colorado:

“Research has found that weeds tend to grow in the soil that settles in the rocks and the weed barrier is only effective for a short period of time. Further, the pores in the weed barrier can clog (from silt and soil) and actually prevent water from penetrating into the root system of plants. In addition, rock mulch can be very hot for some plants, leading to increased stress.”

Also, rock mulch can be very difficult to remove if you decide not to use it in the future.