Cutting Too Short or Too Tall. Most grass varieties perform best when cut at the recommended height or at least within ½ inch of the preferred height. Bentgrass for golf greens performs well when cut very short, but will kook very ugly if grown too tall and then cut short. Perennial ryegrass, the best type for the Northwest, if cut as short as a golf green will decline. In wet weather, our lawns can grow taller than we prefer, but if you remove more than 1/3 of the grass blade at any one cutting, you will harm the lawn. If too tall, adjust the blade height, remove a third off, and let it grow a few days, adjust lower and cut again until you reach the preferred height.
Improper watering. It is almost impossible to kill grass with too much water unless it stands a long time, however it can kill your water bill. If too wet when you cut, you can make ruts and compress the soil. Conversely, some lawns show dry areas in summer and is generally caused by improper coverage. Maybe the sprinkler head is fouled (check each head visually while on during dry weather. Sometime soil condition creates problems because one area is better drained than another. Add a wetting agent to the dry spots instead of increasing water and wasting it on the areas that are green anyway. Of course letting the lawn turn brown at midsummer is all right. It will survive and turn green with fall rain.
Using Weed and Feed Products. Using this product should be done in extreme cases, if at all. It often fails to work if not applied perfectly. An extreme case would be moving into a house with neglected, weed filled lawn. Weed and feed products harm near by plants and keep you from over seeding successfully for an extended time. If you need to kill weeds use a spot spray. It works better and is cheaper and less harmful to the environment.
Failing to Apply Lime. Liming about every three years keeps the pH in the optimum range for grass and helps deter moss. Fertilizer needs can decrease at the proper pH. Best to apply in fall, but can be applied at anytime. Allow for a few months of water for the full effect to be noticed. If in doubt, get a pH test done.
Thatching. Tests have proven that thatching is more harmful than beneficial. Thathing (actually called detaching) is just cutting the grass way too short. See #1 above. Warm season grasses such as Bermuda grass does accumulate thatch at the soil level, but cool season grasses that grow best here do not. If you have a problem of getting material into the root zone, use a core aerator. If you do this you can improve your soil and soil environment at the same time.
Too Much or Too Little Fertilizer. If you use too much fertilizer in any one spot, you can kill that spot. Too little will leave yellow areas. The latter is easy to remedy: the former is not. The excess will need to leach out or be removed in order to reseed. Also, most lawns need more than one application per year. Fall is the most crucial application and many lawn care experts will apply two in spring. Recently products have become available that are organic in type and, when used with compost tea can put life into your soil that can make fertilizer application less frequent or perhaps not needed.
Mowing with a Mower Too Large or Too Small. Not a major problem, but either might have poor consequences. A riding mower, although macho, often means excess weight on a wet lawn and can lead to compaction which drives air out of the root zone, compressing the spaces so more air can not return. Consider cost. A riding mower must have enough power to transport a heavy human so the cost is higher than a walk behind. If you have a large area to cut, buy a large walk-behind mower. They are more maneuverable and the job can go faster. Generally, most homeowners should be able to cut (if not edge) their lawn in about 20 min. If not, your lawn should be smaller, or your mower should be bigger.
Overseeding with the Wrong Type of Seed. Too often a person will buy a cheap seed mix to over seed a lawn and as a result, get a grass blend that is not compatible in appearance or maintenance. If you seed a lawn, keep info on the seed blend and match that in the future. Grass identification as to type is possible by an expert, but the variety of the type is almost impossible. Also important is to cut the existing grass short before over seeding so that the seed has a chance to establish before the next cutting. Be careful not to pull up young seedling with the next cut.
Cutting with a Dull Blade. It is very important to have a sharp blade when cutting a new lawn for the first few time, when cutting a newly over seeded lawn, or using a mower that bags or mulches as it cuts. A dull blade rips the blades and leaves a brownish cast to the lawn.
Ignoring Bare Patches in Winter. Often this will be the result of damage caused by the underground feeding of the larva of European Crane Fly, a relatively new pest in the area and therefore the damage is thought to be "winter damage". If this problem occurs, find an expert in a good garden center, nursery or lawn professional.