Companion Planting: A Compilation of Beneficial Garden Pairings

What is Companion Planting?

Companion planting is the practice of growing different plants together to enhance growth, health, productivity, and efficient use of space.

8 Benefits of Companion Planting

There are plenty of good reasons to plant certain crops together:

• Deterring pests: One of the biggest challenges for any gardener is pests. Sure, you can spray pesticides, but these are very blunt tools that have serious repercussions for nontarget species, including beneficial bugs. It's better to plant companions. Planting single crops in large blocks—which many of us do—makes it easier for them to be located and attacked by their main pests. However, overwhelming evidence supports the homesteader's hunch that intermingling crops make fewer nemeses.
• Attracting beneficials: Some plants also attract beneficial insects. For example, borage attracts pollinating bees and tiny pest-eating wasps.  Beneficial insect-attracting plants have received the thumbs-up from researchers, validating the efforts of gardeners who plant "pollinator strips" (rows of pollinator-magnet plants) within the garden to encourage pollinators and pest predators.
• Shade regulation: Large plants provide shade for smaller plants needing sun protection—for example, corn shades lettuce.
• Natural supports: Tall plants, like corn and sunflowers, can support lower-growing, sprawling crops such as cucumbers and peas.
• Improved plant health: When one plant absorbs certain substances from the soil, it may change the soil biochemistry in favor of nearby plants.
• Improving soil fertility: Some crops, like beans, peas, and other legumes, help to make nitrogen more available in the soil. Similarly, plants with long taproots, like burdock, bring up nutrients from deep in the soil, enriching the topsoil to the benefit of shallow-rooted plants. 
• Weed suppression: Planting sprawling crops like potatoes with tall, upright plants minimizes open areas where weeds typically take hold.
• Saving space: Planting early, short-season crops in the same beds as later maturing crops is a way to conserve space and grow multiple successions of plants in the same space. Examples include: planting lettuce, spinach, or basil early in the season, and transplanting peppers or tomatoes into the same bed as the early season crop matures. By the time the early season crop is harvested, the canopy of the later season crop will begin to fill in. This is not only efficient but can help with weed management and soil health by keeping living roots in the soil and a plant canopy above the soil surface.

Science Behind Companion Planting

Many studies have put specific plant-insect relationships under the spotlight. For example, researchers at the University of Nebraska found that radishes planted with pumpkins appear to repel squash bugs. Similarly, it has been shown elsewhere that nasturtiums grown around fruit trees can deter pests such as codling moths. This is because nasturtiums secrete water-soluble glucosinolates from their roots into the soil. The trees can then absorb these, which take on the same defensive superpowers.

Many brassica family plants, including mustards, horseradish, cabbage, and kale, also use glucosinolates to defend against pests. Some companion combinations work because one plant hides the presence of another, such as that inseparable duo of tomato and basil.

Analysis by the University of Nice Sophia Antipolis in France saw a reduction in tomato pests when basil was present—great news for gardeners looking to outwit thrips, a common carrier of the tomato-spotted wilt virus. Plus, while pest populations decreased, no increase in pest predators was detected as the cause, indicating that basil's strong scent disguised the tomatoes from pests. Leaf shape can also do a great job of obscuring a target plant.

Faced with a confusing array of leaves, a pest may fail to recognize its host plant amidst the melee. This works with thyme, another tomato companion. Scientists at Iowa State saw a reduction in egg-laying by adult armyworms when tomatoes were interplanted with the herb. Used as a living mulch like this, thyme presented small leaves that did a great job of confusing the moths.

Many flowers rich in nectar and irresistible to bees, butterflies, and other pollinators are also a big draw for insects that make a meal of garden pests. Hoverflies can't resist the sunny-side-up charms of poached egg plants (Limnanthes douglasii) and love a meal of juicy aphids, too. Pop poached egg plants near lettuces, and guess what? Fewer aphids!

Another example is borage, which attracts tiny wasps, the natural enemies of tomato hornworm caterpillars. Borage is also a well-known bee magnet, so with it, you get a "twofer": boosted pollination plus wasps laying parasitic eggs to purge notorious hornworm pests. It's the very best friend that any tomato could wish for!

A study published by the Kentucky Academy of Science in Louisville found that planting dwarf sunflowers around a field of corn attracted an army of pest-hungry ladybugs. Other research confirms the effectiveness of dill in attracting parasitic wasps to control cabbage worms and -loopers, while researchers at Rutgers University in New Jersey have shown that dill also does a great job of luring enemies of the Colorado potato beetle.

Examples of the Best Companion Plants

Here are some of the best companion planting combinations for your garden:

• Basil and tomatoes, as interplanted basil, repel thrips, as mentioned above. Basil also deters moths, which lay tomato hornworms and egg-laying armyworms; it also attracts bees, which improves pollination, tomato health, and flavor.
• Dill attracts ladybugs, which eat tiny garden pests such as aphids and spider mites.
• Borage pairs well with tomatoes, attracting pollinating bees. It also enhances strawberries' flavor and vigor.
• Garlic and garlic spray have a strong scent that deters many insects. Aphids can't stand garlic! Garlic also repels onion flies, ermine moths, and Japanese beetles. Plant garlic between rows of potatoes, alongside lettuces and cabbage, and near fruit trees, together with alyssum, to attract aphid-eating hoverflies.
• Mint deters aphids, ants, and flea beetles. Just be careful to plant mint nearby in its own pot or bed, as it is a very aggressive grower!
• Nasturtiums attract hungry caterpillars away from brassicas like cabbage, broccoli, and kale, so grow these pretty flowers close to those crops; nasturtium also lure black flies away from fava beans.
• Parsley attracts beneficial insects to protect and pollinate tomatoes. Plant this herb between tomatoes.
• Poached egg plants (a wildflower) draw in hoverflies, which control aphids on nearby lettuce. 
• Sage is a helpful herb that repels carrot flies. Also, plant it around a cabbage patch to reduce injury from cabbage moths.
• Sunflowers pair well with cucumbers and pole beans. They help provide support for climbing plants and shade for crops, which can become sun-stressed in hotter climates.
• Tansy is a real draw to pest-eating bugs such as ladybugs, ladybirds, and predatory wasps. At the same time, tansy repels many typical baddies, such as cutworm, which attacks asparagus, bean, cabbage, carrot, celery, corn, lettuce, pea, pepper, potato, and tomato plants. Tansy is a perennial, which means you only have to plant it once. What more could you want in a garden flower?

And finally...

Add more flowers! Growing calendula or cosmos nearby will attract tiny parasitizing wasps and aphid-hungry hoverflies. We also love marigolds for drawing in pest-hungry beneficial bugs.

These ideas should have given you the companion planting concept. Now, enjoy a colorful and productive garden!

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