Companion Planting Mistakes Everyone Makes & How To Avoid Them

Companion planting, the practice of growing two or more plants side-by-side for symbiotic benefits, has been popular for generations. While some champion its effectiveness in pest control, increased yields, weed suppression, and soil enrichment, skeptics dismiss these benefits as coincidental. The truth, well, it probably lies somewhere in the middle. Companion planting lacks rigorous research and its lab-based results often don't reliably translate in real-life garden conditions. Consequently, gardeners have had to defer to folk wisdom and anecdotal stories, only to be disappointed with the outcomes. 

But just because companion planting isn't a magic bullet — and a single bush of basil isn't likely to banish pests from your yard — you shouldn't quit playing matchmaker for your flowers, herbs, and vegetables. Trial and error is how we learn what works for our plants and it makes gardening enjoyable, too. Set realistic expectations without buying into urban (or shall we say, garden) legend, or anecdotal regurgitation that sounds too good to be true. Instead, draw on common sense to figure out what planting combos are the most advantageous — such as a mix of early and late season vegetables in one bed, which ensures the ground is never bare, thereby suppressing weeds, or using leguminous crops to help add nitrogen into the ground. 

To help you successfully dabble with companion planting and maximise the potential benefits, we've compiled a list of common companion planting mistakes that a lot of people make, and how to avoid them. Remember, besides matching the plants to their cultural requirements, you must account for size, root systems, allelopathy, feeding habits, and pest susceptibility.

Perhaps you've heard that planting some arugula can keep flea beetles from destroying your tomato, potato, eggplant, or pepper plants. But relying on one companion plant might not cut it. If you're looking to confuse and lure away pests, having more than one companion plant is a much better recipe for success. One of the reasons for this seems to lie in the way insects scout out which plants to lay their eggs on. It's said that insects "taste" the area multiple times to ensure they're landing on a good host plant. For the companion plant to work its magic, the pests must land on it (rather than their intended target) and dismiss the location as unfit for egg laying. The more companion plants there are to land on, the more this can disrupt the decision to lay eggs. 

Considering this, grow at least three to four plants (or more, if the space allows) of different species to repel pests. Essentially, create a thriving mini-ecosystem in your garden that encourages biological action. For instance, when dealing with flea beetles, sprinkle in stands of mustard and rapeseed along with arugula. Similarly, to help discourage cabbage loopers, you can try growing nasturtium, thyme, and onion alongside your main crop. A mixed environment may also confuse the bugs that rely on visual cues (like foliage color or form) to zero in on their target. Additionally, a mix of companions may provide a better habitat for beneficial insects who will help finish off the pestering bugs.

To make the most of limited acreage in your home garden, you might be tempted to grow compact plants as understories to their taller companions. However, such intensive gardening may not always yield the best crop. There are multiple reasons for this, including shading. For instance, while there are lots plants and veggies that will thrive alongside green beans in the garden, a combo like bush beans and tomatoes probably won't succeed. This is because tomatoes are taller and will shade out bush beans, reducing their yield. Even if you took immense pains to plant your companions on the main crop's south side for greater sun exposure, the combination may not work if they have different habits. Case in point: tomatillos. As these plants fan out, they form an arching habit and any sun-loving plants placed underneath (like peppers) will be overshadowed.

Another reason understories may suffer is humidity. Apart from shade, taller plants may leave the underside area cool and damp, which might not bode well for your shorter plants, if they like dry conditions. A classic example? Lavender. Although herb-flower companion combinations are popular, small lavender starter plants may not survive next to nasturtium or calendula seedlings planted at the same time, as these annuals will grow far quicker, covering the lavender and creating a humid micro-climate.

So, you must only grow shade-loving companions with tall plants. Similarly, look for plants that won't be bothered by the greater humidity in cooler environments. Simply put, instead of bush beans, pair tomatoes with lettuce to prevent bolting in hot summers. In a similar vein, shade-tolerant radishes can be coupled with eggplant or corn. You'll also want to think about the rates of growth. For instance, another reason that makes radish and lettuce suitable understory companions for crops like eggplant and corn is that they'll mature faster than their taller neighbors, and be ready to pick before the corn or eggplant bushes have reached maximum size and entirely overshadowed or outcompeted them.

Checking your vegetables for their family name might not be high on your to-do list, but you must know them for the sake of your yields. The reason is that plants belonging to the same family group usually attract the same set of pests and diseases. For instance, tomatoes and potatoes both belong to the nightshade family. As such, they're often afflicted by early blight that can lead to premature fruit drop and reduced crops. They also have Colorado potato beetle and southern potato wireworm in common. Similarly, cole crops like cabbage, broccoli, kale, and cauliflower attract cabbage aphids, cabbageworms, and cabbage loopers. In short, growing plant family cousins side-by-side is a huge mistake, as it makes them vulnerable to easy pickings from pests.

That being said, if you'd love to grow vegetable cousins simultaneously (after all, no eager gardener wants to pick between juicy sun-ripened tomatoes and delectable baby potatoes), make sure to maximize the distance between them. This simple step may limit pest infestation (if there is any) and keep it from spreading further. Better yet, grow different plant species, so the damage remains limited. To get you started, our gardening expert tells us the top three companion plants for healthy cucumbers are nasturtiums, dill, and radishes. Avoid melons, since they sit in cucumbers' family tree. Likewise, look to the cabbage family as companion plants for the potatoes in your garden.

Organic farming enthusiasts often lean toward growing legumes in their vegetable beds as a replacement for nitrogen fertilizer. But that's a misconception. While legumes certainly convert airborne nitrogen into plant-usable forms and make it available in the soil, it doesn't happen overnight. If anything, while leguminous plants are actively growing, they're the ones using up the nitrogen, while their companions may or may not  benefit from it (depending on how favorable the conditions are). What happens during the growth season is that legumes improve the soil microbiome, aka the nitrogen-fixing bacteria attached to soil modules. In other words, your main crop will receive very little direct extra nitrogen. 

That's why nitrogen-fixing plants like peas and beans are usually grown a season before heavy feeders like grains and corn. That way, you can cut back their foliage to the ground after the harvesting season is over and work it into the soil. As the foliage and roots decompose, they will release nitrogen into the soil. This will come in handy for the following crop. Another surprising fact? You don't have to limit yourself to leguminous vegetables, as perennials like lupins also supplement the soil with nitrogen and can make good companions.

While there is a lot of talk about "certain" companions competing with and outwitting the main crop for resources, the truth is: All plants naturally compete with each other. In fact, the more intensively you garden an area, the quicker its resources will deplete. Worse, if you grow plants whose roots settle at a similar depth, you force them to mine nutrients from the same level. This won't bode well for either's health, productivity, and yield.

A better strategy is to couple plants whose root systems differ from each other. This will allow them to uptake water and other resources from different levels in the soil profile. For example, vegetables with long taproots and tubers, such as carrots, radishes, and potatoes, can be paired with shallow-rooted companions, including corn and onions. Combinations like tomatoes and lettuce, and potatoes and beans, work on a similar principle. As long as you keep to a fertilization schedule, their differing feeding habits won't create any significant hiccups. Another bonus? Plants with various root depths can loosen the soil, aerating it.

Intuitively, it seems sensible not to have cut flowers in your vegetable garden, but that's a big mistake. Solanaceous vegetables, including tomatoes and peppers, rely on buzz pollinators to aid fertilization. So, when your veggie bed has a generous presence of bees, they experience improved production and fruit quality. Cucurbits, like pumpkins and squash, benefit similarly. Large flowers are great to attract pollinators, and peppering your beds with blooming annuals like zinnias, borage, marigolds, and sunflowers is also advantageous to help control pests. They can provide good bugs like syrphid flies with crucial habitat. If syrphid flies lay eggs in your garden, the larvae that hatch will feed on any aphids. 

Put simply, there are lots of beautiful flowers you should plant in your vegetable garden for superior yields and effective pest management. This is sort of how you would create a potager — a French concept that combines edibles and ornamentals. It's also the secret to designing and planting the perfect cottage garden. Limit yourself to annuals, as digging around perennials can get cumbersome when you're switching out seasonal vegetables. Sweet peas (which are also nitrogen-fixing), snapdragons, and pansies are suited to cool-season produce, while celosia and zinnias are great for warm-season crops. Sweet alyssum and lettuce make good companions. You can also combine salad greens with cosmos and black-eyed Susans.

Ever had a situation where your peas looked dead, but the abutting sunflowers appeared happy as a button? Blame allelopathy, a phenomenon where certain flowers and veggies release growth-inhibiting chemicals into the soil. Allelopathic chemicals can be useful when you're trying to suppress weeds, as they retard seed germination and growth of small seedlings. However, such propensities aren't as welcome in vegetable beds, particularly one where you're going to sow seed. Transplanting your main crops instead of direct sowing works out better in this case. You must also cut back allelopathic plants before they go to seed.

What further complicates things is that allelopathy doesn't work equally against all plants. For instance, if you're growing sunflowers to attract pollinators, you should avoid planting them in the same bed as potatoes, peas, or pole beans because they'll be negatively affected. However, sunflowers probably won't be as problematic around squash, cucumbers, corn, and fava beans. If you don't want to deal with any uncertainty around the allelopathic properties of sunflowers in your bed, you must dig out the whole plant and place it in the compost where the growth-inhibiting chemicals should decompose. Another combination to steer clear of involves beans and onions, along with their cousins, such as garlic and chives. Essentially, onions release a gaseous chemical called "allicin," which kills the nitrogen-fixing bacteria in beans, stunting their growth. Finally, avoid growing sunn hemp and alfalfa in your garden beds (even as cover crops).

Many gardeners rely on trap cropping to better manage pest infestations. Essentially, one plant (the companion) is chosen as a sacrifice in the hope that the troublesome pests will find it attractive and crawl to it, leaving the main crop alone. Once the bad bugs collect on the trap plant, they're exterminated or removed, ending their legacy for good. Organic approaches include vacuuming them, squashing them, or dislodging them into soapy water. However, for this system to work, you must avoid one crucial mistake: planting your trap and main crop at the same time, especially if they mature at the same rates. Ideally, you want the trap crop to already be attracting any unwanted bugs by the time the main vegetable could become a target for them.

For instance, if you're growing cucurbits and want to keep squash bugs and beetles at bay, you could flank them with rows of blue Hubbard squash plants placed around 3 to 8 feet away. However, you must transplant squash seedlings two or three weeks in advance, so hungry pests find them munch-worthy, getting eliminated by the time your main crop lands in its ordained spot. Aim to plant about 10% to 20% of the garden bed with trap companions. Alternatively, you can site your trap crops in places where these pests usually overwinter, or around the edge of the garden, to catch them early. For less mobile nuisances, like aphids and thrips, you might have to position the companions closer in alternate rows. Tomatoes, cowpeas, and cabbage-collard greens are other groupings worth trying.

When you buy starters in their cute pots from nurseries to begin your veggie beds, it's easy to lose sight of their mature size and growth habit. Unfortunately, such an oversight can prove disastrous in some instances. If even one plant is a spreader, it can eventually grow over any short plants in its path. As a result, these oppressed plants could lose their access to sunlight, be smothered, stunted, and become susceptible to fungal diseases. 

You're most likely to experience such problems with vining crops like pumpkins, squash, sweet potatoes, cucumber, indeterminate tomato varieties, and melons. So, be careful about pairing them with shorter plants, ensuring their vines won't eventually overshadow their less rambunctious companions.

This might come as a shock to some, as most companion planting recommendations wax poetic about aromatic plants, especially mint. But unless you've planted mint because its aroma pleases you, this particular plant probably won't add much to your garden, as it's unlikely to repel pests. Although plenty of lab-based research shows aromatic plants release volatile organic compounds (VOCs) that repulse critters, these results don't appear to hold up in home settings. And the reasons are actually quite intuitive.

Unlike labs or field tunnels with their closed systems, gardens are open areas. So, the VOCs aren't concentrated enough to ward off insects. Moreover, the farther you plant your minty companions, the quicker their smells will disperse, affording little to no masking protection to your crops. Before you think, "Well, I can plant them closer," let's not forget that mint is an aggressive spreader that can quickly take over your yard. That being said, keeping mint contained in pots might be an option to look into, though its deterrence effects continue to remain uncertain.

Marigolds are great! They add a wash of sunny hues, attract pollinators, and repel pests. However, their last attribute may be a tad exaggerated. While many permaculture enthusiasts maintain that marigolds send common pests like aphids, spider mites, and invasive insects like Japanese beetles packing, there isn't much scientific evidence to back these claims. Although this doesn't preclude you from growing marigolds — their gorgeous flowers are certainly a sight to behold — don't plant them as a pest control panacea.

That said, marigolds do hold utility where root nematodes are a recurring problem. By emitting a toxic chemical called alpha-terthienyl, marigolds prevent nematode larvae from exiting their egg sacs. To gain this benefit, plant them directly in the site where you plan on growing vegetables two months in advance, or there won't be any significant advantages. Also, not every marigold variety has this effect against nematodes (Tagetes signata is one example), so that's another factor to consider. Given these facts, are marigolds a good companion plant? They can be if you're trying to draw the attention of beneficial bugs like hoverflies and ladybugs and send nematodes packing, but not as an all-round pest repellent (and yes, that includes Colorado potato beetles).