There's moss killing my lawn. What can I do?
First of all, it's important to know the moss isn't actually hurting anything. Moss takes over when the lawn thins out due to shady and/or damp conditions. (Shade and dampness often go together, but not always. Moss will thrive in sun, given enough consistent moisture.) In conditions that are optimal for lawns, moss rarely is a problem because the lawn will effectively choke out the moss. Looked at this way, moss problems are best addressed by changing conditions in a way that tilts the balance back in favor of lawn. Attacking moss with chemicals or by mechanical removal is only fighting the symptom, not the root cause.
Chemical control. Iron sulfate is often sold as a moss control. It does kill the moss, but you'll be left with a bare patch of mud or dirt. Further, the moss will eventually return unless you've improved conditions to where the lawn can grow. Drainage. Lawns may stay excessively moist for several reasons, and this is the No. 1 reason moss invades. If the site it graded so that water doesn't run off readily, correct the grading, if possible. It may be that an unusually rainy or cloudy season has suddenly favored moss growth by keeping things wetter than usual. There's nothing you can do about the weather, but it's a good bet that when weather returns to normal, you'll see the moss recede. Shade. If the site is shaded, try to increase sunlight and air circulation. Sometimes this isn't possible, but look for things like low-hanging branches that can be pruned. Soil pH. Lime is often recommended as a control strategy for moss. It's true that moss prefers very low pH (and grasses do not) and that lime will raise pH, but this alone is not likely to resolve the problem. It is one part of the puzzle - not the entire picture. Try to maintain a soil pH between 6 and 7. Good lawn care. Anything that reduces a lawn's vigor can, possibly, thin it out enough to let moss get a foothold. So be sure you fertilize and aerate regularly, and keep your mowing height reasonably high (the exact height depends on what's appropriate for the type of turf, but mowing too low is one way to encourage moss). Re establishing turf. If you've succeeded in eliminating moss, your next challenge is to re-establish turf in areas that were previously taken over by moss. First, remove as much of the moss as possible. A steel garden rake is pretty effective in pulling up moss mats. Once you've exposed bare soil, you can reseed or sod in the usual manner. If the site is shaded, definitely use a shade-tolerant grass mix, such as fineleaf fescues. But be aware that some sites are simply too shady for turf to thrive. If this is the case, you should consider installing shade-tolerant groundcovers or perennials instead of lawn. You will encounter the same problem all over again if you cannot create conditions in which lawns thrive.