Grasses often are planted between rows of perennial crops such as fruit trees. The grass alleys cool the soil, prevent erosion, improve water penetration, exclude weeds, and harbor beneficial insects. Reducing pest damage Most plants produce defensive chemicals that help fend off insects and diseases. These chemicals may be insect poisons, feeding deterrents or have fungicidal properties. The roots of some French and African marigolds contain a substance, which is toxic to certain types of nematodes.
Nematodes are soil inhabiting microscopic roundworms that damage many species of plants.
Certain nematodes can be eliminated from a site by growing a thick crop of marigolds for one season prior to planting the vegetable or fruit crop, or by interplanting marigolds between crop rows. Destructive insects often locate their food by smell. Many plants, especially culinary herbs, produce strong scents which may confuse insect pests looking for a host to feed on. Garden vegetable plants such as garlic, onions, chives, and herbs such as catnip, horehound, wormwood, basil, tansy, and mints all produce scents which seem to repel insects or mask the scents which attract insects. A certain level of insect protection can be achieved by carefully interplanting some of these as companions to vegetables.