In the spring, when grass is lush and soil is moist, mushrooms, puffballs and toadstools may appear in the lawn. Sometimes they are in a circle, called a fairy ring. In medieval times people believed the rings were where fairies danced at night.

Usually the mushrooms are not harmful to humans, says Dr. Charles C. Powell, Ohio State University plant pathologist. But he suggests that children be taught not to eat them.

The mushrooms are the spore bearing parts of fungi that grow in the ground. They live on organic matter buried in the soil, such as wood and stumps, and do no real harm to the grass.

It is best to pick them and put them in the trash can. Some can be eaten, but it is dangerous. Many poisonous species grow side by side with edible ones and there is no simple way to distinguish one from the other. One person may eat at certain mushroom with no ill effects while another may have serious nausea and intestinal distress.

Fruit trees tend to overbear. So do grape vines.

The tree or vine can nourish and properly ripen only a certain amount of fruit. With too big a crop, ripening is delayed and the fruit is not as large and flavorful as it should be.

To prevent overbearing, specialists recommend thinning. Most peaches should be thinned just after the so-called June drop so they are spaced 6 to 8 inches apart.

Apples also should be thinned so they are no closer than six inches to each other.

If the trees are small, thinning can be done by hand. If they are large, you can do a lot of it with a pole. This involves knocking the fruit off with a piece of thick-walled rubber garden hose on the end of the pole for about six inches. This must fit snugly, leaving a flexible, free end to knock off the fruit.

Mowing the lawn appears to be a rather routine job. You do it yourself or get a neighborhood kid to do it for you. But in fact, the way the grass is cut is even more important than fertilizing and watering.

Cutting the grass too low often badly damages the lawn. The green blades of the grass produce the food for the plant by photosynthesis. The raw materials come from the soil and air.

If no more than one-third of the grass leaf area is removed at a time, the grass can establish a good root system. It can withstand heat and drought and is less likely to be seriously damaged by turf diseases.

Most new varieties of Kentucky bluegrass and fescue are better adapted to low moving than the older kinds. Both with all kinds, if too much of the green part is removed, it drastically reduces the ability of the plant to produce food. CAPTION: Picture, no caption