Everyone who grows food deals with pests. For organic growers, there’s not much you can do. I accidentally stumbled on an effective technique this year and absolutely had to research it out
All onions and garlic contain allyl-epropyl-disulfide, which is an effective insect deterrent. By growing onions or garlic in between garden rows, you can see a significant reduction in insect damage in your garden. As little as 1-row of onion or garlic per 4 rows of other crops shows a significant difference in pests.
That’s a general idea, but there’s still some technique to it. There are several factors as to why it works, and there are a few simple things you can do to increase the effectiveness of intercropping with onions or garlic.
Do Onions Keep Bugs Away?
Onions are very seldom the victim of an onslaught of mindless droves of garden pests. In fact, I can’t think of one single type of bug that’s really known to plague anything in the Allium family (onions, chives, and garlic). On occasion, I will hear of a couple of onions that were nibbled on, but it’s pretty rare.
Onions, chives, and garlic all reduce problem bugs in their immediate growing area. All three of them seem to be equally effective too. After my personal positive experience and the information I learned in my research after that, I’m sold on onions in my garden forever!
All the members of the onion family (Allium) exhibit this same phenomenon. I started growing onions mainly because of this fact. I had lost all my cabbage to cabbage worms and had an infestation of stinkbugs, AKA squash bugs, in my squash and zucchini. I planted onions only because I was sick of bugs.
Well, the onions never had any bug problems. This summer, I noticed a cool thing which is what spurred me to research this topic for the next two weeks. My crop rotation schedule called for squash next to my perennial onions this year. The squash patch had very few stinkbugs in it for once.
The squash nearest the onions had literally zero bugs. I couldn’t believe it! I also noticed significantly fewer Plant Bugs on my ground cherries on the other side of my onions. Intercropping strong scented herbs with more pest-prone plants isn’t a new thing. I’ve tried it before with different herbs to no avail.
I tried mints, peppermint, chamomile, and thyme in my garden to help deal with pests and none of them ever made a noticeable difference. But the onions sure did! So now, let’s talk about the science behind it, how it works, and how to maximize the potential.
Do Onions Kill, or Repel insects?
Onions won’t kill bugs or pests, they act more as insect repellent or deterrents. The primary reason why this works is a chemical compound called allyl-epropyl-disulfide, also known as the spicey onion smell.
Allyl-epropyl-disulphide is a volatile compound found in all Allium plants. A volatile compound basically means that it’s very fragrant. It easily disperses into the air. Albeit, it’s not the most pleasant odor. But bugs don’t appreciate it very well either.
This compound has two primary modes of function for keeping pests away from our garden crops. First, it’s an irritant. Bugs, especially soft-bodied grubs and caterpillars, are very sensitive to any irritating substances. It’d be akin to grabbing an electric fence. You won’t want to be there for long.
According to one study, the irritating action disrupts the primary actions of several common pests. Basically, it affects them enough to not stick around, not lay eggs, or not even stay for a second course. The Onion aroma is irritating to bugs and most decide to look for a more pleasant target.
The irritation factor is the primary function of onions and garlic keeping pests at bay. It’s probably 70 or 80 percent of the reason why Allium works to deter pests. Still, there are a few other ways they help in your garden.
Besides the irritation, onions act as a cover scent. In hunting, we sometimes use a cover scent such as pine oil to cover mask the human scent. That way, a deer is less likely to smell you coming. With bugs, it’s the same principle.
If you plant a strong-smelling plant like onions or garlic, the smell is thought to mask that of your tomatoes or cabbage and confuse potential pests. That’s the theory anyway. It’s really hard to be sure about it without asking the bug.
“Mr. Cutworm, why didn’t you eat that cabbage? Honestly Mr. Gardener, I just didn’t know what to do. I was so confused!”
Spacing for Onions as Pest Control
So now you’re like me, you’re going to plant garlic or onions in your garden to keep from sharing your bounty with ungrateful little bugs. The next question is, how many to plant, how far apart, and what type works best? A lot of questions, but I have answers.
You can either plant them intermingled in rows, or you can plant alternating rows of onions and other crops. There are downsides to the first method. Having one densely planted row of onions at regular intervals works much better than sparsely planting onions between other crops I the same row.
By planting a solid row of onions, you are creating a much more potent spot. The interplanted rows don’t seem to have a high enough concentration of allyl-epropyl-disulfide in one spot. In trials, one row of onion for every four rows of other crops showed twice the pest reduction of interplanted rows.
You could alternate a row of onions then another crop across your entire garden, but then you’re probably going to be overrun with onions. If you are using a standard garden spacing of 18-14 inches between rows, every four rows look like a good place to start.
One row of onions has a good deterrent effect on 3-4 feet in both directions. If you have an 8-foot garden bed, a single row of onions in the middle ought to be good for you. For larger gardens, start with a row of onions, then plant another row of onions, garlic, or chives, every 8 feet.
Since they have a good effect 4 feet in both directions, planting a row every 8 feet will cover your entire garden. if you want to try a little more, go with a row every 6 feet or so. That should show you some very positive results in your garden.
You don’t have to cover your entire garden. Most of the time there are just a couple of plants that really get attacked hard. Most notably, Brassica’s (i.e. cabbage, broccoli) and Cucurbita’s (squashes, zucchini). If you are growing a crop that is more prone to pests, you might only want to put your onions or garlic next to that.
If you plant your Allium in close spacing or wide rows, it will work better. Stronger scented onions and garlic will also have a stronger deterring effect. Personally, I settled with Walking Onions because they are stronger than most, and they come up super early in the year, before Transplanting other crops.
What Pests do Onions Repel Best?
One of the most notorious garden pests that are kept in check by allyl-epropyl-disulfide is Aphids. Aphids are a bane, particularly to anyone who grows Brassicas. If you’ve ever grown turnips or brussel sprouts, I bet you had aphids on them.
The infamous Cutworm is another one that seems to be efficiently repelled by onions or garlic. Other soft-bodied grubs and bugs seem to be adversely affected by it too. The data I’ve seen leads me to believe that it would keep the Colorado Potato Beetles from plaguing my potatoes next year.
It doesn’t seem to affect mature beetles as much as caterpillars and larvae, but it still keeps them away for the most part and discourages them from laying eggs and establishing themselves as a garden plague.