Companion plants as nature’s own “pesticide”
Home gardeners can play an important role in supporting local agriculture by using climate-friendly practices in gardens and landscapes. One important measure is to reduce the use of pesticides. Most pesticides are insecticides and kill beneficial insects such as pollinators. Even organic pesticides (e.g., Neem Oil) are moderately toxic to bees according to the University of California and therefore should be only applied in the evening when honeybees have returned to their hives for the night.
Ways to reduce the use of pesticides include: Planting lawn alternatives, prefer native plants, improve soil health, do not overwater, and introduce Companion Planting
Companion Planting involves growing two or more different plants in proximity for some benefit such as enriching the soil or growth support (example: “The three sisters”, beans, corn and squash. More info: https://www.nativeseeds.org/blogs/blog-news/how-to-grow-a-three-sisters-garden).
Another benefit of companion planting: Pest Control. Plants can help each other naturally, without the use of chemicals, based on a few general strategies:
Trap Crop: Use a decoy or sacrificial plant to lure pests away from plants you want to protect. Usually, these decoys are easy to grow low-cost plants, which must be removed and destroyed once full of pests. Sunflowers will attract stinkbugs, dill attracts tomato hornworms, nasturtiums and alyssum attract aphids, and radishes attract flea beetles and cabbage maggots. More info and list of trap plants for use in home gardens: https://underwoodgardens.com/trap-crops-organic-pest-management-for-gardeners/
Attract and kill: This method is also based on attracting the pest, but this time natural plant chemicals will be fatal for pests. Few plants have this ability. Marigold roots are toxic for some nematodes such as the root-knot nematodes, geranium is attractive but also toxic for Japanese Beetle and there are conflicting reports about Four O’clock, as to whether these flowers can do the same. Petunias attract aphids and other pests, which then get stuck on the sticky petunia stamen and die.
Attract beneficial insects that prey on pests: Not all insects are pests. On the contrary, there are many beneficial insects such as pollinators and species that we want in the garden because they will prey on the pests. Certain plants are very attractive to these beneficial insects and by increasing their number in your garden you will increase the number of natural enemies to the garden pests.
Examples of beneficial insects that prey on garden pests: Green Lacewing, hoverflies, lady bugs, ground beetles, parasitic wasps.
Pest insects they prey on: Aphids, mealybugs, whiteflies, mites, scales, beetles, and worms that damage certain crops such as potatoes, the tomato horn worm, or the cabbage worm.
Plants that will attract the beneficial insects: Dill, fennel, alyssum, Queen Anne’s lace, cosmos, clover, daisy, mint, yarrow, goldenrod, or milkweed.
More information and lists of plants that attract beneficial insects:
https://hort.extension.wisc.edu/articles/plant-flowers-to-encourage-beneficial-insects/
https://www.motherearthnews.com/organic-gardening/plants-to-attract-beneficial-insects-zl0z1005zvau/
https://www.gardeningknowhow.com/plant-problems/pests/insects/beneficial-insects.htm
Pictures of beneficial insects: https://www.planetnatural.com/pest-problem-solver/beneficial-insects/
Repel: Certain plants smell “bad” to pests and they avoid places where these plants grow. This can be used as protection in the home garden too. Many of these repelling plants are strong smelling herbs, for example basil, borage, mints, dill, garlic, rosemary, chives, thyme, or catnip, and some are flowers such as chrysanthemum, marigold, or lavender.
More info and lists of plants and which pest they deter:
https://gardentherapy.ca/deter-pests-naturally/
https://www.farmersalmanac.com/what-to-plant-to-keep-bugs-away-24734
https://en.wikipedia.org/wiki/List_of_pest-repelling_plants
Disguise and hide: The last strategy also uses strong smelling plants to protect the garden. However, in this case the strong-smelling plants are used to hide plants that are attractive to pests. Many insects find their target plant by smell. For that reason, avoid large groupings of plants that are attractive to harmful insects. Instead diversify and mix with strong smelling plants such as garlic, mints, marigold, or tarragon to hide the target plant.
More Resources:
Companion planting: https://www.hpfb.org/uploads/companionplanting.pdf
Where to buy organic seeds for plants that are a bit harder to come by:
Below a few pictures of plants that attract beneficial bugs, e.g. goldenrod, butterfly weed and sunflowers.
This article on Companion Planting was written by Christiane Turnheim, owner/farmer of Good Spirits Farm in Harvard, MA. She is also a member of the Harvard Agricultural Commission.
