One of the biggest reasons gardeners look up companion planting, specifically organic gardeners, is for insect pest control. It seems like there is always something in the garden that wants to eat our plants before we get the chance! What we wouldn’t give to find a way to keep them at bay or otherwise control them with natural means. Enter companion plants and trap crops.
Companion Plants
Things like aphids, nematodes, whiteflies, and cabbage moths are all candidates for control with plants. Certain flowers, like marigolds and nasturtium, and many herbs, like basil, have earned reputations for being great plant companions with numerous benefits: repelling insects, attracting beneficial predatory insects, camouflaging the crop, or acting as bait. Sometimes that reputation is well-researched and well-deserved and there is a science-based reason for the phenomenon. There have been studies that show, for example, that specific marigold varieties control specific plant-parasitic nematodes. And there are numerous studies done on aromatic herbs that show there are pest repelling properties to many of them. Other times, though, it’s simply a matter of correlation and the effect of the plant combination was just a coincidence. So how do you know which plants work and which combinations are just garden lore?
The good news is that the simple act of planting more than one crop together, regardless of what it is, reduces the number of parasitic and plant-predating insects. This is because a monoculture, one single crop all by itself, is way more vulnerable to insect pests than a di-culture or a polyculture. If you’ve got one crop growing all by itself, the predatory insects can home in on it very easily. But if you have different plants growing together that don’t all attract the same pest, it helps camouflage each crop from their individual predators. The plants are masking each other’s chemical cues that the insects use to find their target; essentially each plant is a host plant surrounded by other non-host plants that hide it.
For another layer of protection, mix in plants that help attract the natural enemies of the pests, like ladybugs, lacewings, and parasitic wasps. Plants proven to attract beneficial insects include: the carrot family, like coriander (cilantro) and dill; asters, like coreopsis and sunflower; legumes, like clover and alfalfa; brassicas, like mustard and sweet alyssum; and the verbena family, like lantana and all the verbenas. Intermix these among the plants you’re growing for food and you’ll add one more layer of natural pest control to your garden.
Trap Crops
The other way to do prevent insects in the garden is with a trap crop. For example, aphids seem to be especially attracted to nasturtium. So interplanting nasturtium among plants you typically battle aphids in may be a way to go. It keeps the aphids on the nasturtium and away from whatever else you’re growing.
You can also use a crop similar to the one you’re growing as a sacrifice crop. This has been particularly beneficial in my garden with brassicas. 
I’ve planted dinosaur kale or mustard seeds early enough to give them a good head start before I transplant my broccoli, cabbage, and other brassicas. After I transplant, I’ve and covered the cash crop tightly with insect netting but left the kale or mustard exposed. The moths and butterflies gravitate to the uncovered easy targets of the kale and mustard and stay away from the broccoli and other things I’ve got covered up. I can then chop down the sacrificial crop and feed it to my chickens or burn it or otherwise dispose of it, and the eggs and larvae of the cabbage worms that are now present on that crop, keeping my other crops safe. Just be sure if you’re planting trap crops with other veggies that are in the same family you have some mechanism in place to keep the away from the plants you are growing for food. Whether it’s distance or exclusion methods, like insect netting, you want some way to lure the insects away from the plants you’re trying to protect and to keep them there by removing or otherwise destroying them.
And while it may do no harm to plant most things together that seem like they would benefit from each other, even if there’s no scientific proof to the benefits, we also need to look at the possibility there could be harm done. Unless you are using the crop as a trap crop like I just described, you generally don’t want to intercrop things in the same plant family together. For example, you don’t want to plant carrots and dill together. They’re in the same family and they attract the same pests. This goes with intercropping your kale with your cabbage unless you have something else in there that will deter the cabbage moths. So, when you see lists of plants that are supposed to be good companions and the list of bad ones, understand why it’s a bad companion. Do they have the same nutritional needs and will compete with each other? Do they attract the same pest? Does it give off a chemical that may stunt the growth of certain plants?
Click here for a free quick cheat sheet to the most common garden plants and families and their best and worst companions!
As you plan your garden this season be sure to include companion plants and trap crops. Making space for these plants and interplanting them among your other fruits and vegetables will help keep those insect pests away and give you a much better harvest.
For more resources, here a couple podcast episodes related to this topic:
Ep. 118 - Cover Crops in the Home Garden
And an article about how cover crops can also help with attracting beneficial insects: Cover Crops: Four Surprising Benefits for Home Gardeners
Your Friend in the Garden,
