There are many herbs available for use as companion plants but I have picked these three due to their ability to repel many insect pests.
Coriander grows 1-3′ tall and matures quickly in cool weather. It is a fast growing annual. The upper leaves look like dill and the lower leaves resemble Italian parsley. Both the leaves and the seeds are used as a seasoning. It is a member of the carrot family with umbels of white or very pale lavender flowers. Native to Eurasia, the plants can take full sun for at least a half of the day, if not more. Actually, coriander tolerates more shade than most herbs. Coriander repels aphids, Colorado potato beetle and spider mites. Bees love it!
If you want to harvest the seeds, wait until they turn brown. Cut off the branches and hang upside down in a paper bag to catch the seeds as they fall off. Remove any of the hulls and make sure the seeds are dried well before storing.
Borage is an annual and needs to be replanted each year, although the seeds will self sow. Direct sowing seed in the garden is recommended. The seeds are large and grow quickly but should be pre-chilled in the refrigerator a week or so before planting. Cover the seeds as they need darkness to germinate.
Borage is a good companion to beans and corn and it is said to improve the flavor of tomatoes and cucumbers. It will repel many common pests such as cabbage worms from cabbage and will repel tomato hornworms. It grows to about 2′ tall, prefers cooler weather and has star shaped, electric sky blue flowers with hairy stems and leaves. It is a great favorite of bees of all types. There are only (3) species, all native to the Mediterranean region. It prefers average soil on the dry side and at least 1/2 day of full sun.
The plants will falter in hot, humid summers. Both the leaves and the flowers are edible and the new, young leaves add a cucumber flavor to salads. They can be fertilized with 5-10-5.
Tansy is closely related to Chrysanthemum. It is a great companion for beans and attracts beneficial insects to control cabbage pests. Also, they help to deter and repel many flying insects. The plant has been grown since ancient times to repel a variety of bugs.
The plants can spread invasively through underground runners so it may be better to keep in containers throughout the garden area.
Tansy is a hardy perennial herb with attractive flowers. It grows to 3-4′ tall and tolerates poor, dry soil but needs to drain well. Better to be kept loamy and moist. The foliage is strong scented and used as an insect repellent. It has a “peppery-camphorous” aroma. It blooms all summer with flat clusters of lots of yellow flower heads that look like gold buttons, about 1/2″ wide.
Brew a tea from tansy leaves and spray it on other plants to repel insects. As a companion plant, it repels green peach aphids and squash bugs from susceptible garden crops, along with many beetles, flies, cutworms and ants.
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