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Companions in the garden: Neighboring plants can help each other

Linn County Master Gardener

OSU Linn County Master Gardener

Companion gardening – choosing who you garden with? Companions who are mutually beneficial, allies that provide protection or improve one’s growth – or enemies that cause detrimental effects. 

That’s how plants consider their neighbors. Companionable plants are able to act as nursery plants, repellant plants, decoy plants, or attractant plants. (Just like people we know?) 

“Nursery” plants produce sugar and amino acids that attract predators and parasites of plant pests. 

“Repellant” plants discourage pests by spines, toxins, or odors that insects reject or get confused with. 

“Attractant” plants act as a diversion, offering an alternative feeding spot for pests, away from the cultured crop. 

“Decoy” plants cause death or decreased reproduction to pests nutritionally.

According to researchers at a USDA research laboratory in Florida, attraction and rejection of pests happens at a molecular vibration level, too. From the vibes of a plant the insects, tuning in with their antennae and hairs, can tell which plants they want to eat and which they don’t. Plants respond to vibrations of insects by producing odors, toxins, or in some cases projecting their pollen. 

For example, marigolds stink and confuse insects, while tomatoes and squashes throw their pollen from the flowers when they experience the vibration frequency of bumble bee wings.

Allelochemics is the study of interactions between different organisms. All plants and animals affect their neighbors by chemicals they give off or contain in their tissues. The chemicals affect growth, health, behavior or population of different species. 

Chemical agents can be released from plant leaves or directly into the soil from the roots, affecting neighboring plants positively or negatively. Chemicals exuded in the soil can repel some underground insects or kill harmful fungi. Fragrances from the leaves can repel certain insects. Mosquitos are repelled by scents of citronella, rose geranium and cedarwood, for example.

Interplanting, integration of different plants in the garden, can be used to increase quality and yield of vegetables. Planting crops so their leaves overlap and roots intermingle can cause them to interact in physical and chemical ways. 

The key to successful interplanting is learning which combinations are beneficial. 

Length of growing period, above-ground growing patterns, below-ground rooting patterns, light needs and nutritional needs are all factors of compatibility. 

Long-term plants can be happy with short-term plants that will be done and gone before the space is needed (radishes with cucumbers). 

Tall plants can provide support for vines (corn and beans), and can provide shade for shorter plants that need less sun (lettuce under tomatoes). 

Plants that require lots of nitrogen, those that feed lightly and those that actually manufacture nitrogen can be beneficial to each other. Soil builders, like beans and peas, can store nitrogen in the soil for later use by heavy feeders like leafy greens and the cabbage family. 

Fruiting vegetables often require more phosphorus, leafy vegetables suck up nitrogen, and root vegetables use more potash. Combining species of these three categories reduces nutrient competition. 

Space-efficient root combinations, planting fibrous-rooted plants with tap-rooted plants allows them to share the soil space at different levels.

There is a lot to think about when combining plants in the garden. Research on companion planting has been limited but is becoming more important in agricultural production. Most “research” has been by experimenting in home gardens, putting friends together and keeping
enemies apart. 

There are some good books and videos available accounting for personal garden experiences, but little published by universities or institutions like the USDA’s Insect Attractants, Behavior and Basic Biology Research Laboratory, a source used by Rodale Press in The Gardener’s Guide to Companion Planting.

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