September generally gives maritime Northwest gardeners great opportunities for all the lawn care practices that grow good turf. Whether moss is a problem or not, this is the month to aerate, fertilize, and overseed to improve the turf. All these practices will also help with moss management.
Think of moss in the lawn as an indication that the turf is stressed by one or more common problems. Too much shade, poor drainage, low fertility, soil compaction, or thin spots where turf has died out all contribute to difficulties. Any one of these, or quite often a combination of these conditions, will generally be present where moss is a problem.
Another way of stating this is that moss is a symptom of lawn problems, not a cause of them. Mosses are distinctly different plants from turfgrasses. Though they are both green, the resemblance stops at that point. A true moss has root-like stem extensions underground. They are shallow and almost feather-like, not roots as gardeners recognize roots. Mosses do not have veins, or the vascular system that supports grasses. They do not retrieve water from soil and send it along veins out to leaves. Mosses don't achieve plant growth by photosynthesis combining light and plant nutrients. Mosses can and do survive in distinctly different conditions than grasses.
The obvious place to begin with a moss management program is to make sure that the turfgrasses have the best possible conditions for growth. Lawns need good light. They also need fertile, well-drained soil to support roots that can penetrate 6 to 8 inches down, or even deeper. Start your moss control program by giving the lawn good, thorough basic care, beginning with a clear analysis of the location and how the lawn is established.
Mosses are opportunists, like all weeds, moving into areas where the lawn has thinned out or died.
What about the soil conditions? Poor drainage means that the area around the grass roots stays soggy and wet. Turf requires water, but needs good drainage for root health. Any place in a lawn where water stands after rain or irrigation, or places where the ground is too compacted to drain well will be potential trouble spots for moss and weeds.
Sometimes lawns are installed over ground that's been compacted by heavy equipment, such as around a newly-constructed home. In many locations in western Washington, drainage is restricted by layers of hardpan, which is often nearly impermeable to water. A lawn planted over compacted soil or hardpan will never grow well, and cannot become thick enough to resist moss and weed invasions. Analyzing the problems may result in choosing to install drainage systems or lifting turf to drill through the hardpan. In these extreme situations, there are no easy fixes. Get a qualified turf professional to help determine what needs to be done.
If you are undertaking lawn renovation to remove moss and repair the resulting gaps in the turf, get rid of the moss first. Established moss can be removed by hand raking, though this method doesn't control moss effectively. A lot of rhizomes and spores will be left behind to regrow. If the moss is raked out but nothing else is changed in the turf situation, the moss will return almost immediately when weather conditions allow.
Most people who wish to control moss do it by using any one of several common chemical compounds registered for moss control. Most of these contain iron, in the form of ferrous sulfate, ferrous ammonium sulfate, or iron chelates. These iron-containing products are sold both in granular and liquid forms. The finer the particles, the better the coverage and the better the moss control. The moss will blacken with an iron application. Follow label directions carefully. Products containing iron will stain concrete and should be kept off sidewalks and patios.
Another type of moss control product contains potassium salts of fatty acids, essentially a soap-type compound. These cause mosses to yellow and brown out, not turn black. Some yellowing and discoloration of the turf as well as the moss will occur. These are more commonly used to remove mosses and algae from hard surfaces in the landscape than they are for applying to moss infestations in turf. Again, read the label to determine the best site of application and method of handling. Whatever chemical is used on existing moss, the result will be areas of dead moss that must then be raked out. This process will leave bare spots that should be aerated and over-seeded with appropriate turf grass seed. An orderly lawn care plan and an understanding of how moss grows can help reduce and manage this common Northwest lawn invader.
Thanks to Dr. Gwen Stahnke, Turf Specialist, Washington State University, for material in this column.
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