I love growing brassicas. In fact, in our garden, broccoli, kale, cabbages, and arugula reign supreme. That’s partially because we live in a colder, northern climate and brassicas tend to be more cold hardy than melons, cucumbers, and tomatoes. If you love them too, you can protect your brassicas by planting a “trap crop” like a mustard plant nearby.
We plant ours early, and we continue to plant our favorite varieties throughout the growing season. In fact, kale is one of the first and last plants in our garden. It’s strong enough to withstand an early October snow storm, and delicious enough to get a place of honor at the Thanksgiving table.
Of course, these beloved brassicas aren’t without their own issues…
Hardy as they may be, all members of the brassica family have some devastating enemies. Flea beetles, cabbage worms, and aphids can quickly decimate your cauliflower or Brussels sprout beds.
After all, there’s nothing worse than seeing your hardiest plants withering away under the jaws of pests. If you were planning on stocking the freezer with homegrown kale, or canning up a few jars of broccoli cheddar soup or sauerkraut, the invasion is devastating.
But what can you do to protect your plants? Sure, there are a lot of options ranging from neem oil to chickens and guinea fowl, but all of them take time, energy, and focus. In contrast, companion planting is a great, no-fuss option for deterring insects.
These types of companion plants do far more than just support one another. Sometimes, companion plants repel invasive insects, but you can also use one plant to act as a “trap crop” to lure away pests instead.
What is a Trap Crop?
In simplest terms, trap crop is a snare for pests. It lures destructive insects away from your primary crop—in this case, brassicas. They’re incredibly attractive to pest insects, and keep your favorite plants safe by taking the brunt of the invasion onto themselves.
Some trap crops go the extra mile and do double duty with enticing and killing pest. They draw them in, and then fail to offer them an environment they need to thrive and reproduce.
When you plant a trap crop, you don’t need to worry about pesticides. Additionally, you won’t need to actively defend your primary crop against invading insects. All you have to do is sit back and watch the companion plant take care of your invaders.
Some gardeners take extra precautions by luring pests with a trap crop, and then spraying a bit of neem oil on the trap to disrupt the insects’ life cycle.
How Does it Work?
Insects have preferences, just as people do. Just like us, they don’t always prefer the plants that are the healthiest and best for them to feed on. As such, a trap crop is often called a sacrificial crop. It’s a species that’s planted, but not intended for harvest.
A trap crop grows for the sole purpose to call away the pests from your food. This crop needs to be attractive enough to your pests to lure them away from the plants you really love. Therefore, the trap crop you choose depends on two factors: the pest you’re trying to trap, and the plants you’re trying to protect.
Your trap crop also has to be close enough to lure pests from the primary one. You don’t want to plant it too far away, or the pests might not notice it. But you also don’t want to plant it too close or the pests won’t stay away: they’ll just eat everything.
For brassicas, adding a row of Indian mustard plants between two rows of broccoli, cabbage, or kale is ideal.
Indian Mustard Plant
Indian mustard (Brassica juncea) is an ideal trap crop for our favorite brassicas because it’s part of the same family.
Insects that are drawn to broccoli and cabbage will also be drawn to mustard. In fact, pests that feed on our favorite brassicas prefer mustard. They’d much rather devour a row of Brassica juncea than a row of broccoli. We don’t know why, but both laboratory and farm tests have proven this preference again and again.
We don’t grow a lot of Indian mustard in American gardens. It can be very spicy, with a mustard-horseradish flavor. Although it has some culinary uses, they’re not as common in the west as they are overseas. The leaves, seeds, and stem of Brassica juncea are edible. Additionally, the seeds of the Indian mustard plant are used to make brown mustard—our popular yellow mustard’s spicier cousin.
The leaves of Indian mustard are common in African, Nepali, and Punjabi cuisine, but they’re rarely eaten in America. Furthermore, Indian mustard is an ideal plant for detoxifying the soil. Like sunflowers, mustard plants can pull heavy metals from the ground. Plant them if your garden is near an old house or barn with chipping lead paint.
If you plant Indian mustard as a soil detoxifier, don’t harvest or consume any of the plant. Wait until after the growing season is over, then harvest and then burn these plants.
Indian Mustard as a Trap Crop
With its wide, lush leaves and pretty, yellow flowers, Brassica juncea is decidedly attractive to hungry insects. It’s a new and exciting brassica, so all of the cabbage-eating caterpillars, aphids, and beetles find it irresistible.
No-one seems to know for sure why insects would rather munch on mustard leaves than cabbage. It may be the wide leaves or the spicy flavor, but whatever the draw, it’s a crowd pleaser.
As a trap crop, mustard attracts insects but doesn’t always damage them. While some trap crops will give your invasive pests a false sense of security and devastate their life cycle, mustard’s effects are milder. It draws them in and it holds their interest all season long. As a result, your kale and broccoli plants will have the time and space to produce a successful crop.
Most pests that attack garden brassicas will choose to devour the mustard instead. The mustard plant won’t do any damage to those pests, but it will hold their interest. A cabbage white butterfly caterpillar that enthusiastically chomps down on mustard will still grow to be a healthy little butterfly.
It’ll come back and lay its eggs on the row of mustard as well, so the pest life cycle will continue. If you want to affect the life cycle itself, use mustard as a snare crop and them follow up with a natural pest control technique.
Natural Pest Control with A Trap Crop
Because mustard lures brassica pests, it gives you a specific area to focus on in pest control. The problematic creatures are all gathered together on Brassica juncea. Now you can swoop in with some spritzes of neem oil or insecticidal soap and wipe them out.
While you’re fighting off the pest population with natural remedies, the plants you actually care about are thriving. Whether it takes you 2 or 10 applications of neem oil to drive away all the pests, the mustard plants are sacrificed to save your Brussels sprouts.
A Mustard Plant is a Brassica’s Best Friend
Indian mustard plants are proven companion plants for common brassica varieties. If you’re growing a row of cabbages or a bed of kale, you should really consider adding a row of mustard plants nearby. Planting cabbage alongside mustard isn’t just an old wives’ tale either. Companion planting is a well researched gardening method that you can trust to get your insect invasion under control.
In particular, a mustard plant will attract moths like the diamondback and the leaf webber. These pests love to lay their eggs on cabbage plants and other brassicas. But, given the choice between mustard and cabbage, the moths will overwhelmingly choose mustard.
This means that all the moths who would have raised hungry larvae in your cabbages are moving next door.
Cabbage aphids also fall prey to the lure of the mustard plant. Try planting a row of mustard about a week after sowing broccoli or cabbage seeds. It isn’t just aphids that are drawn to mustard plants, but their natural predators as well. Syrphid larvae devour cabbage aphids, and they’re more likely to grow and develop on mustard plants.
Timing is Everything
Remember that since Indian mustard is a part of the brassica family, it has similar nutritional needs. You might be worried about planting mustard so near your brussels sprouts because the mustard might compete for nutrients.
We definitely don’t want a sacrificial crop stealing food from our tasty Brussels sprouts! As a result, the trick to protecting your favorite brassicas from competition is to plant the edible crops first.
Seed in your broccoli, kale, arugula, and cabbages about a week or two before seeding in your mustard plants. If you’re planting seedlings, sow a row of mustard only a day or two after planting your brassica seedlings.
This will give your primary crop a chance to establish itself before its hungry cousins grow.