Issue 5, May 23, 2016


Some folks spend time and money mixing up concoctions to get moss to grow between bricks and stone pavers in sidewalks and patios.  Others fills concrete or hypertufa troughs with all sorts of loose soil mediums to raise moss gardens on rocks and boulders, spraying them daily with moisture.  Books have been written on developing your own moss garden.

On the other hand, more people get frustrated with moss taking over their lawns, choking out the turfgrass.

Moss is classified as a bryophyte, one of simplest forms of plants.  They absorb water and nutrients through their leaves.  They differ from most of our typical weeds in that they reproduce by spores instead of seeds, and don't have a vascular system – the xylem, cambium and phloem we learned in elementary school.    Not having the vascular system means that food and water flow freely within the plant.  It also means the plant doesn't have the ability to produce tall stems, with plant growth just bunched on top of each other creating a layered effect.

Moss as groundcover at Missouri botanic garden.

Moss prefers a loose growing medium which is why you find it growing on the sand between brick and stone pavers, on shake shingles, and on rock scree under trees in foggy, tree-lined mountains.  However, moss will grow on the thatch, organic layer, sitting on top of a poorly-drained clay soil.

Moss is generally an indicator of site condition problems.  Fix those conditions, and the moss problems usually subside, though they may not be totally gone.  Just killing the moss without changing the site conditions will only allow moss to re-grow.  Conditions favoring moss generally don't favor turfgrass.

Moss thrives in shade and moisture, though the plant still needs some light in order to manufacture food for growth so you won't find it in dark, shaded conditions.  If moss is growing under a tree canopy, careful and professional thinning of the branches of trees and shrubs will bring in more direct light which it turn will thin out the moss.  However, you can't easily move buildings if they are causing the shade.

Moisture may be more difficult to change, though not impossible. 

Aerating the soil can help with drainage, though it may require several years of fall aeration to really make a difference.  Adding organic matter may increase drainage initially, but would need to be added every two or three years as it breaks down.

French drains can funnel water away from low-lying areas if aeration doesn't work.  In large areas, underground tiles may work.

Of course, watch your irrigation.  If moss is unwanted, avoid watering those areas when irrigating.  Reposition sprinklers or choose a type that allows you to direct the water to certain areas and avoid others.  There's not much you can do to control rainfall. 

Herbicides are limited because the plant has no vascular system, which is why grass and broadleaf weed killers provide no or limited control.  Instead, products containing iron in the form of ferrous sulfate or ferrous ammonium sulfate compounds seem to provide the best control.  There are some herbicidal soaps, usually potassium salts of fatty acids, that also claim some control. Though with all products, if the site conditions aren't modified, the moss usually returns. (David Robson)

David Robson

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