Yard Moss: Nasty and Obstinate

     There are thousands of species of moss, a very simple plant that lacks the a leaf, shoot, root, and seed-forming systems. Unfortunately, at least if the stuff is growing in your lawn, moss is one of the hardiest living organisms on the planet! The stuff reproduces either sexually (spores) or asexually (breaking off into smaller pieces that divide and multiply).
    Regular grass cutting won't do anything to get rid of it. There are several underlying conditions that allows moss to come out ahead of regular lawn grasses. These conditions most frequently are some combination of: 

1) poorly drained, persistently wet soils
2) acidic soil conditions
3) medium to dense shade
4) repeated “scalping” of turf on uneven terrain
5) compacted soils 

The stuff thrives in these conditions! Lawn grass doesn't! 

     Another factor that helps moss is lack of sunlight because many species of moss prefer moist soil in the shaded areas. Removing trees though can be unacceptable, not to mention expensive. Tree pruning can help though. 
     If moss occurs in sunny spots, mower “scalp”(cutting the grass too short...don't do it!) may be the cause of the problem. Scalping severely weakens lawn grass and allows moss to thrive.
     Compacted soil is another cause and using a core aeration machine (either rent one or have a lawn care service do it) can also help. I can attest that physically removing moss with a rake is essentially impossible because you simply can't get rid of every single piece and since there are millions of spores already in the soil it will just continue to grow. The best thing to do is remove as much vegetation as possible then reseed. 
     Sprays, etc may help, but the stuff won't work on many kinds of moss and often when it does, it leaves very ugly yellow or brown spots when the moss dies. 
     One irritating thing is that if you don't fix the above 5 problems, the moss will come right back. You see, the moss is not killing the grass, but rather failure of the grass to thrive allows the moss to do so.  As one article stated, if the conditions are right for moss then significant renovation may be required to get the grass to thrive, but...there are no guarantees. 
     Moss growing in the yard is an indication of less than ideal conditions for growing grass. The potential causes behind the problem are: 

Low soil pH Lack of necessary nutrients in the soil 
Poor drainage 
Excessive shade 

So, just removing the moss is only the first step. THEN you have to some detective work to determine why the moss is growing to begin. If you can't determine the cause then the moss will simply return. It's recommended that you have the soil analyzed. Some local cooperative extensions will do it for you. so that they can test it for you. Let them know that you need to find out what the soil pH is and whether or not your soil contains the necessary nutrients for growing a healthy lawn. This way you can discover the root cause of the problem. 
     Your grass may need a more alkaline soil to compete effectively against moss. If this is the case, you will need to apply lime. If the ground lacks the nutrients, you will have to fertilize the lawn on a regular basis. 
     Bad drainage is another problem because if the lawn does not drain very well and retains excessive moisture, this condition is good for moss to grow. Soil with a high clay content is a red flag. Water tends to percolate slowly through overly clay soils and that can lead to puddling. Happily, there is a very simple test you can conduct to determine what type of soil you have. If you see standing water on your lawn after a rain then you have drainage issues. If clay is the source of the problem then something like humus has to added to the dirt. If the whole lawn is poorly drained then that may be a whole other issue! Good luck!