Moss develops and flourishes in Saint Augustine lawns for three primary reasons. The affected lawn area where the moss is growing will need to be continually moist, will need to be growing in a high level of shade, and the lawn where the moss is growing will need to be weak, meaning the Saint Augustine grass will need to be in a weaker state than the invading moss in the lawn.
Saint Augustine grass should be one of the very best grass types for defeating moss and stopping its establishment and further growth. This is because Saint Augustine grass is a partial shade tolerant lawn type, and as such Saint Augustine will flourish and grow and outcompete moss in many shaded areas.
While St Augustine does have very good shade tolerance levels and can defeat moss in some shaded areas, we must remember that not even Saint Augustine grass can grow strong and healthy under shade conditions which exceed 70%. So in environments where there is very high levels of shade, and excess moisture, the Saint Augustine grass will still grow weak while the moss grows strong. So we can see we're still going to be facing moss problems under these conditions.
Killing and Treating Moss In Saint Augustine Grass
To kill or treat moss in Saint Augustine grass lawns, we must make the affected area highly favourable to Saint Augustine grass, and highly hostile to the moss. Which involves addressing all of the main environmental factors which encourage moss growth and which simultaneously makes the lawn weak.
Excess Shade On Lawn
If we have moss in Saint Augustine grass then we must have excessive shade in that same area, otherwise the moss wouldn't grow and the lawn wouldn't be weak.
Remove excess shade over the affected lawn area by pruning all surrounding foliage from plants, bushes and trees. If the excess shade is coming from garden sheds, fences or other man made constructions, then we should look at removing these wherever possible if we still want the lawn to continue to grow in this same area and without the moss.
If however we still want those structures which are causing the shade issues for the Saint Augustine lawn, then it would be best to remove the grass in the affected area, and replace it with a garden path or a new garden bed with shade tolerant plants.
Moisture In The Lawn Soil
Healthy moss growth also requires high levels of moisture to be present in the soil, and these same high levels of moisture combined with shade will not only weaken St Augustine grass, but make the turf much more susceptible to lawn disease as well. So we must remove all excess moisture from the affected area.
If irrigation sprinklers can be adjusted for the affected area then we should do so immediately, or otherwise stop or drastically slow down the watering in this area if using a hose and a sprinkler.
Aerate the lawn soil properly and effectively. and the very best way to do this is with the use of a lawn coring machine which will punch holes into the St Augustine turf and soil and remove plugs of the same matter in the process. These plugs should be removed from the turf after coring and the holes filled in with a free flowing sand, which will enable for greater drainage of the lawn soil. Aerification will not only need to be done once, but will need to be repeated as often as is necessary, which may be once a year or once every few years depending on the level of drainage at the affected site.
Apply wetting agents liberally twice a year in Spring and Fall. Wetting agents allow for much better free flowing of water throughout the soil profile, which should allow excess moisture on the surface of the soil to more freely flow away down into the water table.
Replacing Lawn Soil
If the soil beneath the St Augustine grass naturally holds onto excessive amounts of water, such as with soils with high clay content, then the soil can be removed to a depth of 8 to 12 inches and replaced with a sandy loam soil. Sandy loam will more freely flow water around the entire roots of the Saint Augustine lawn, but will also allow the water to flow away when it is not used by the turf itself.
Increasing Lawn Health
Finally in this battle between Saint Augustine grass and moss, we want to make the Saint Augustine grass as strong as possible so that it can more easily defeat the moss, killing it, outgrowing it, and smothering it. So lets increase the health of the lawn as much as we can.
We've already increased sunlight, removed excess moisture, added some lawn loving oxygen into the soil from aerification, now it’s time to look at nutrients.
Begin a good year round fertilization program for the Saint Augustine turf, using a high quality lawn fertilizer with trace elements, and applying to manufacturers recommendations (or less (never more)). This lawn food will greatly enhance the quality of the entire Saint Augustine lawn, as well as making it so much stronger in the shade too, which in the end should easily outcompete and kill off moss in our Saint Augustine grass lawns.