Companion Planting

Demonstration garden following the companion planting technique

Companion planting is based around the idea that certain plants can benefit others when planted next to, or close to one another. It is generally thought of as a practice for small scale gardening, but can be applied on a larger scale as well. The concept of companion planting includes a number of different strategies that increase the biodiversity of agro ecosystems (Kuepper & Dodson, 2001).

One of the benefits of companion planting is natural pest control, which eliminates the need to use chemicals and sometimes provides a higher crop yield. Sometimes, a neighboring crop may be chosen because it is more appealing to pests and distracts them from the main crop. An example of this is the use of collards to distract moths away from cabbage; this technique is called trap cropping. Another pest controlling technique is biochemical pest suppression; some plants exude chemicals from roots or aerial parts that suppress or repel pests and protect neighboring plants (Dancer, 2006). A good example of this is the African marigold; it releases thiopene, a nematode repellent which makes it a good companion for a number of garden crops. Strategically planting corn with squash or pumpkins will also deter pests; the presences of the prickly vines discourage raccoons from the corn.

The release of certain biochemicals may also be harmful in some situations; for example juglon, which is found black walnut can suppress the growth of several other plants. A positive use is the allelochemicals that leak from rye residue; they prevent weed germination, do not harm transplanted vegetables and can be used in mulch.

Physical and spatial interactions are an important element of companion planting. Tall, sun-loving plants are compatible with lower growing, shade-tolerant plants because they act as a blockade from the sun and increase the total yields from the land. This strategy is referred to as nurse cropping. Two plants will grow well together if their roots occupy different layers of the soil. Deep-rooted vegetables plunge their roots into lower soil, drawing nutrients and moisture from soil layers that shallow-rooted crops cannot reach; for example, potatoes and beans. Beneficial habitats, sometimes called refugia are another type of companion plant interaction. This is valuable when the companion plants provide a desirable environment for beneficial insects and other arthropods (Kuepper & Dodson, 2001).

Useful Tips

Nasturtium and rosemary deter beetles that attack beans
Thyme repels the cabbage worm
Marigolds and oregano near anything will repel insects, and will keep hornworms away from tomatoes
Garlic repels insects
Keep closely related plants separated. Vegetables of the same family group are more likely to share pests and diseases.