How Onions Affect Plants Around Them

Salad Vegetables

• Cabbage and its relatives (Brassicaceae) always profit from onions growing nearby. Onions deter cabbage loopers, as well as maggots, that can attack cabbage and broccoli. Also benefiting from the company of onions, carrots (Daucus carota) are protected from the carrot fly when grown beside onions. Since cabbage and lettuce (Lactuca sativa) attract hungry rabbits, including onions in the garden repels the pesky mammals. When interplanted with arugula (Eruca sativa), a leafy green of the mustard family, the onion benefits from a reduction in weeds while it keeps the arugula free from flea beetles.

Nightshade Vegetables

• The tomato (Lycopersicon esculentum), potato (Solanum tuberosum), eggplant (Solanum melongena) and sweet pepper (Capsicum frutescens) all fall under the nightshade classification, along with tobacco and belladonna. Planting with onions benefits each. The onion family deters red spider mites from tomato plants. An enemy of both potatoes and eggplant, the Colorado potato beetle is repelled by onions nearby. Finally, onions enhance the growth of both tomatoes and sweet peppers.

Interplanting with Herbs

• While onions often improve the lot of their companions, two common herbs bring rewards to onions. Chamomile (Matricaria chamomilla) enriches the growth and flavor of onions, when planted as a companion. Grow it sparingly, however, just one plant every 150 feet. Summer savory (Satureia hortensis) has the same effect on onions. Be sure not to grow onions near sage (Salvia officinalis), as they are detrimental to each other.

Companions Other Than Vegetables

• Onions planted close to peach (Prunus persica) trees can repel borers that might attack trunks and roots. Squirrel damage to pines (Pinus) and other conifers can be lessened by planting onions nearby. Roses (Rosa) likely benefit the most from onions' company. Protecting the roses from a number of pests and diseases, such as aphids, black spot and mildew, onions also cause the flowers to produce a stronger perfume.

Onion Enemies

• Although their friends far outnumber their antagonists, members of the onion family definitely have enemies. Never plant them near any type of bean or pea. Onions do not benefit from the legumes' fixing of nitrogen in the soil, and the alliums hinder bean or pea growth. Onions have a similar effect on asparagus (Asparagus officinalis). Even other onions do not always gain from close onion companionship. Since onion maggots easily travel from plant to plant, scatter your onions throughout the garden, rather than setting them in a row.