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A diverse mix of vegetable and flowering plants will lead to fewer pest problems.
Most people think of plants as passive organisms and assume that, because they can’t move, plants are defenseless against pests. Nothing could be further from the truth. Science is beginning to understand what observant gardeners have realized for centuries—many plants have complex chemicals in their leaves and roots to ward off invaders and, sometimes, they even work together for their common defense.
This is the notion behind companion planting, a gardening technique that calls for the grouping of plants that are mutually beneficial to one another. Today there is renewed interest in this strategy, particularly when it comes to vegetable gardens.
Insect pests often use smell to find their preferred food plant. You can confuse those pests by planting strong scented plants. Herbs like catnip, basil, cilantro and mint and members of the onion family such as onions, garlic, chives and leeks, all produce aromatic compounds that may either repel insects directly or simply mask the scent of the pest’s preferred food. For example, basil planted near tomatoes is said to repel tomato hornworms, and nasturtiums surrounding squash may fend off squash bugs.
You may also use companion plants to attract beneficial insects to your garden. To do so, plant flowers—lots and lots of flowers. Many beneficial insects, predators and parasites of pest insects, feed on nectar and pollen from flowers. The best flowers for this purpose are small and produce lots of nectar or pollen. Plants in the carrot family are excellent for beneficial insects. These include dill, fennel, cilantro and Queen Anne’s lace. Asters, daisies, black-eyed Susans, goldenrod and yarrow, all members of the sunflower family, make excellent food sources as well. The mint family with plants like rosemary, lavender and salvia are also very good. Don’t be afraid to intersperse these plants within your vegetable garden. Not only will they attract beneficial insects, they will also make your garden more colorful and interesting.
Companion plants benefit each other in ways beyond just protection from pests. Sometimes they create a more favorable environment. Legumes, for example, extract nitrogen from the air and with a little help from beneficial bacteria, naturally fertilize the soil. That’s why gardeners have long planted pole beans alongside corn. The beans provide nitrogen to the corn while the corn provides structure for the beans to climb.A third way to manage insect pests is by using trap crops. A trap crop is a sacrificial planting that a pest prefers over the main crop. The idea is that the pest will be attracted to the trap crop first. By regularly monitoring the trap you will detect a pest infestation before it becomes widespread. Research has shown that diamondback moths will often lay their eggs on collards before cabbage. Thus, collards could be used as a trap crop for diamondback moth caterpillars, protecting the cabbage from damage. Buckwheat is another good trap because it attracts stink bugs, which ordinarily feed on many different vegetables and fruits.
There are many other possible plant combinations. Just remember that companion planting is all about diversity. Experiment in your garden, take notes, and share what works with your neighbors. That’s how gardeners have done it for centuries.