Question: There is moss growing all around my house—in the lawn, in the garden beds, on the trees and even the brick wall. I’m wondering, how did it spread to all of these areas, and how might I control it?

Answer: Moss, it’s everywhere! It’s that bright green, cushion-like mound of a plant found growing on just about any surface in our Pacific Northwest yards. To understand why that is, and how one might control it, it is helpful to understand how moss grows, and the conditions it prefers.

We typically think of a plant as having roots growing in soil with stems, or a trunk like a flower, shrub or tree that grow from seeds. Moss is a Bryophyte, a non-vascular plant, in which the leaves are mostly one cell thick; they have no true roots, stems, flowers or fruit. And instead of seeds, they produce spores, which are similar to a flowering plant’s seeds, but are single celled and more primitive than a seed.

Moss grows in a form that looks like a thick green carpet with very thin stems (seta) supporting a brown capsule; the spores are housed in the capsula and are released as they ripen. If the spores land in an area with enough moisture, they will grow, thus our area has the perfect conditions for moss. In addition to spores spreading through the air, mosses will also spread simply by pieces breaking off and getting moved by wind or water.

But how is it growing on walls, trees, walkways, rooftops? Mosses do not need soil to grow, as they are able to take up nutrients just from the atmosphere and water flowing over them. So they can easily grow in the cracks of sidewalks, the nooks of tree bark, between roof shingles and stone walls; you can find them on both vertical and horizontal surfaces.

Mosses are at their best in the winter when there is plenty of water, more shade and cooler temperatures. In the summer, they will dry out and become dormant; they can tolerate a complete loss of water, and then rehydrate when the moisture returns and become a healthy, growing plant again.

If you are looking to control the growth of moss, the best way is to prevent the conditions that are favorable to its growth by reducing moisture and increasing exposure to sunlight. Options to consider:

  • Trim tree and shrub limbs to allow more sunlight into the area; complete removal of trees or shrubs may be needed in areas that receive less than 3 to 4 hours of direct sunlight, or 6 to 8 hours of filtered sunlight. This is the amount of sunlight that is required for most lawn grasses to grow well.
  • Add sand or soil to low-lying areas; this will increase the elevation and allow the area to dry out.
  • Contour or trenching in the wet area will allow excess water to drain off.
  • Install French drains or surface drainage tiles where water builds up.
  • Increase the aeration of the soil; heavy or compacted soil encourage the growth of moss. In a lawn core, aeration or aerification is a process that removes cores of soil from the lawn either manually or with a power-drive aerator.
  • Remove thatch (dead organic matter) from lawns; this will increase grass vigor and decrease water retention on the surface. Moss in a lawn is an indication that the lawn is not healthy, and that the conditions are beneficial to moss.
  • Incorporate several inches of organic matter into the soil to improve texture and drainage.
  • Physically removing the moss to prevent it from returning and/or spreading (i.e. digging it up, scrubbing off pots, power washing walkways).

Although you might not want moss in all areas of your yard, there are many positive benefits to adding mosses to the landscape. Mosses can be attractive additions to rock gardens, stone walls and other bare surfaces.

In the garden, they can help as erosion control, as well as increase moisture; moss will help store moisture and improve the soil’s nutrient holding capacity. And moss is a self-planting, low maintenance, easy to grow, green groundcover!

Do you have a gardening question? Please e-mail, call or visit the Douglas County Master Gardeners Plant Clinic at douglasmg@oregonstate.edu, 541-236-3052, or 1134 S.E. Douglas Ave., Roseburg. Douglas Country Master Gardeners are trained volunteers who help the OSU Extension Service serve the people of Douglas County.

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(1) comment

Mogie

What about some natural recipes for moss killers? How do we combat moss growing in or on our trees? I was told that moss usually doesn't grow on healthy trees is that correct?

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