Companion planting is the idea that some kinds of plants can help each other grow. Plants that are compatible together generally have similar needs in terms of nutrients, soil and moisture levels.

Advice about which plants are compatible is sometimes based more on tradition than proof. But experts say there is evidence to support certain combinations. These can improve harvests, reduce disease and help with pest control by attracting helpful insects.

For example, some kinds of soil bacteria take nitrogen from the air and make it into a form that plants can use. The plants hold the nitrogen in their roots. Legumes are especially good at this nitrogen-fixing. Then any crops that share the same space as the legumes can get the nitrogen as the roots decompose.

Beans and potatoes can also share territory well because their roots reach different levels in the soil. Deep-rooted vegetables get nutrients and moisture from lower down in the soil, so they do not compete with plants with shallower roots.

But some plants placed together may harm each other's development. For example, tomatoes do not like wet soil but watercress does, so you would probably want to keep them separated.

Some kinds of produce should be kept apart even after being harvested. This is because of ethylene gas. Ethylene is a plant hormone that can cause some foods to ripen too quickly. Apples release ethylene gas. Apricots, melons and tomatoes also release a lot of ethylene. Some vegetable are easily affected by ethylene, including asparagus, broccoli, cabbage and cucumbers.

This risk of ripening too soon is why markets often separate high ethylene-producing foods from those that are sensitive to the gas.

But if you put an apple in a bag with a green banana, the ethylene from the apple will make the banana ready to eat sooner.

What about peaches, plums and nectarines that are too firm to eat? These kinds of fruit do not need any help from an apple — they release enough ethylene to ripen themselves. But that does not mean there is no way to get the job done faster.

Growers suggest placing the fruit in a fruit bowl or in a paper bag with the top folded over. Keep the fruit at room temperature, out of direct sunlight. Once the fruit is soft enough to your liking, either use it or put it in the refrigerator to keep it from getting too ripe. Yuck.