Companion Planting Gone Wrong: 21 Planting Combinations to Avoid
Not everyone is meant to be paired together. In gardens, just as in life, some companions simply don’t work out. Companion planting is a great way to improve yields, save space, and reduce pest pressure, but some planting combinations can be detrimental to the growth of one or both crops.
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Not everyone is meant to be paired together. In gardens, just as in life, some companions simply don’t work out. Companion planting is a great way to improve yields, save space, and reduce pest pressure, but there are some plant combinations to avoid because they can be detrimental to the growth of one or both crops.
Incompatibilities happen most often when you forget to consider a plant’s growth habit. For example, a big-leaved zucchini plant will quickly shade out a tiny carrot. Similarly, an overbearing kale plant will heavily compete with humble lettuce when planted too close together.
Moreover, some plants produce compounds that inhibit the growth of others. Companion planting mistakes with allelopathic crops can dramatically reduce your yields or even kill your plants.
Here are 21 plant combinations to avoid and how to prevent incompatibilities in the garden.
What Are Bad Companion Plants?
Companion planting is the science and art of growing two crops next to each other for mutual benefit. However, bad companion plants may end up harming each other or reducing yields due to incompatibility. The most common reasons for bad crop pairings include:
Improper Spacing
No companion pairing is exempt from spacing requirements. Even if two crops thrive in each other’s presence (like tomatoes and basil), they still require adequate spacing to grow to their full glory.
Spacing becomes especially problematic when you interplant a giant crop with a tiny one. For example, winter squash tends to ramble and vine all over the garden. It will easily trample root crops like radishes. The broad leaves and fast-growing squash vines are also prone to shading out anything planted too close to it.
Incompatible Soil and Moisture Needs
Species prone to root rot typically do not thrive alongside those that require consistently moist soil. This is why you don’t see swamp plants growing in the desert or vice versa. For example, lavender hates to have “wet feet” or soggy soil in its root zone and can quickly succumb to root rot. It actually prefers poor soil with a gravelly or sandy texture.
However, crops like carrots and spinach enjoy consistent moisture in a loamy, rich soil. Combining either of these two with lavender would be detrimental to everyone involved. One crop would suffer from too much water, and the other would suffer from a lack.
Incompatible Lifecycle
In the above example, we discussed why lavender and carrots would be an unreasonable combination due to differing soil moisture needs. The plants’ life cycles are another reason for this unsuitable pairing. Lavender is a perennial semi-woody crop that grows for 5-15+ years. Carrots are an annual crop that you grow in one season (unless you’re trying to grow it for seeds, at which point it flowers in its second year as a biennial).
Generally, it’s best to avoid planting perennials and annuals in the same bed because managing weeds, nutrient requirements, and harvests is more difficult. This is why many gardens have raised beds for annual vegetables and border, in-ground beds for companion perennials. However, this rule has a few exceptions, particularly with ornamental beds where you can mix herbaceous perennials and annual, self-seeding flowers.
Conflicting Nutrient Needs
Two heavy-feeding crops typically make poor companions because they aggressively compete for limited soil nutrients. However, two light-feeding crops can do well together. It is vital to pay attention to the fertility needs of your crops when determining which ones to plant in the same bed.
While nitrogen-fixing crops like beans and peas can enhance the fertility of the soil, heavy-feeding crops like brassicas and squash often hog all the nutrients for themselves. For example, cabbages are very hungry for fertilizer and may snatch away the fertility from less-competitive crops like beets.
Some plants also have different requirements at different phases of plant development, as well. For instance, tomatoes require a lot of nitrogen when they’re first planted for healthy vine development, but once they’ve sized up, it’s best to focus more on providing phosphorus for good flowering and fruiting. This may not be ideal for plants like spinach that require a constant supply of nitrogen for good, leafy development.
Conflicting Seasonality
We all know what happens when you plant a crop in the “wrong” season without proper preparation. Cilantro and lettuce almost always bolt in the heat of summer, unless you take extra precautions such as shade cloth or bolt-resistant varieties. Similarly, melons and corn planted too early or late in the season are prone to cold damage.
While companion planting can be used to extend your harvests (e.g. planting lettuce in the dappled shade of tomatoes), you must take the seasonal needs of both crops into consideration. A combo of kale and peppers is not ideal because kale prefers cool weather and becomes especially prone to aphids during the heat, while peppers love the heat and can’t handle temperatures below 50°F.
21 Plants That Should Not Be Planted Together
Not everyone mingles together well, and this remains true in the garden. Here are 21 plant combinations to avoid.
Mint and Vegetables
Despite its glorious fragrance and amazing pest-repellent properties, mint is an herb you never want to grow in your vegetable beds. Mint is a vigorous, often aggressive spreader that is best contained in a pot, raised bed, or ground cover.
To reap its companion planting benefits, you must ensure it is close but not too close to your crops. The fragrance easily travels, and the flowers still attract beneficial insects within 2-5 feet of a vegetable bed. If you have raised beds, you can grow mint in the pathways or along the margins of your garden where it can’t creep in. However, never plant this herb in a bed with annuals, or it might overgrow them.
Lettuce and Zucchini
Although lettuce likes a little shade from companions like tomatoes or peppers, zucchini can completely shade these greens and out-compete them. Zucchini is an incredibly fast-growing Cucurbit plant with big, broad leaves.
Those leaves are great at suppressing weeds because they don’t leave much space for light to filter through to the soil beneath. If you plant lettuce next to summer squash, you may be disappointed by the result. The little heads cannot properly compete with a giant, prickly zucchini plant.
Black Walnut Trees and Almost Anything Else
Although it produces delicious nuts, this unique tree also makes compounds that are toxic to other plants. Black walnut trees are allelopathic, meaning they secrete a natural herbicide in their roots, husks, and leaves to deter other plants from growing nearby.
Moreover, you don’t want to mulch your garden with black walnut wood chips or compost the hulls in your kitchen compost pile. While you can compost your black walnut leaves, hulls, and shells in a dedicated compost pile, it’s best to use that compost right back under the black walnut tree where it originated, just to avoid any residual effects.
If growing under a black walnut tree, consider container-growing and remove all debris the tree drops from the garden. Some plants may appreciate its shade, but they won’t be excited about the juglone in all plant parts from the black walnut.
Fennel and Most Vegetables
Like black walnut trees, fennel is an allelopathic plant. It produces chemicals in its root zone to prevent seeds from germinating in its vicinity. This is an evolutionary adaptation to help fennel’s ancestors remain super competitive in the wild. It helps suppress weeds that might compete with fennel plants in your garden. However, it can do more harm than good if you’re trying to grow other vegetables nearby.
Symptoms of crops grown too close to fennel include:
- Poor seed germination
- Stunted growth
- Premature bolting
- Yellowing leaves
- Plant death
The chemicals are most concentrated in the seeds, but isolating the plants is best. Keep fennel in its own garden area, separate from your crops, particularly nightshades like potatoes, tomatoes, and eggplants. It is often grown in a large container or planted alongside dill, which is in the same family.
Asparagus and Potatoes
Perennial vegetables like asparagus are bad companion plants for many vegetables because they take up so much space during their active growth stage. In the winter, it may seem like you can plant anything near a barren asparagus patch. When the spears emerge in spring, the plants still seem lowkey. But when the sprouts bolt upward and take off, you’ll find yourself with a modest asparagus forest. Not much can compete with asparagus fronds in their full glory.
Potatoes are a particularly bad companion because their tubers require the same underground space as asparagus rhizomes. You won’t be able to properly plant seed potatoes or dig them up without damaging them or the asparagus. Keep asparagus in a dedicated perennial bed on your garden margins, and grow potatoes in deeper soil or a raised bed with annuals.
Brassicas and Tomatoes
Generally, two heavy-feeding crops make bad companions. The competition for nutrients can harm both plants and reduce your yields. Planting brassica family members like cabbage, cauliflower, or broccoli with tomatoes can stunt your tomato plants because they suck up so many nutrients.
In addition, most brassicas are cold-weather plants; they like cooler conditions to produce well (especially ones like cauliflower or broccoli that will almost immediately bolt in the heat). In contrast, tomatoes and most other nightshades (with the exception of potatoes) love warm weather and tend to wimp out in cooler conditions. These two aren’t ideal from a seasonal standpoint!
Corn and Tomatoes
Tomatoes and corn are among the most sun-loving crops in the garden. They are both heavy feeders that require rich soil and plenty of nutrients. Planting these two vigorous growers together can spell trouble because they are both so demanding. The rapid growth of corn could shade out tomato seedlings, while the expansive habit of tomatoes could reduce corn vigor and yield.
The height of these two competitive plants also means that sunlight may be limited for one or both parties. Avoid this plant combination so you can enjoy higher yields of both crops. Plant tomatoes in their own raised bed with trellises and companion plants like basil or lettuce. Corn can grow in its own patch or cluster with beans or squash.
Carrots and Dill
Dill is a highly beneficial companion for many crops because the umbel-shaped flowers attract predatory wasps, hoverflies, ladybugs, and other beneficial insects. However, planting dill near carrots can have the opposite effect. These two crops are in the same family, meaning that both can attract carrot flies, spider mites, and aphids.
In the early stages, their leaves look almost identical. When dill bolts (goes to flower), it becomes a more beneficial companion to other crops but can hinder carrot growth by shading and out-competing the roots. Sow these two in separate parts of the garden intermingled with other plant families to confuse pests.
Cucumbers and Melons
Generally, crops in the same plant family benefit each other because they have similar environmental needs. Cucumbers and melons are both members of the Cucurbitaceae family and share the same vining habit. But when you pair these two together (especially without a trellis), it can quickly become a mess of tangled vines and cucumber beetle damage.
It’s best to separate these two with a few other plants in between (like phacelia, marigolds, or alyssum to attract pollinators!) You can install a cattle panel trellis for cucumbers and train them to vine upward. Unless you grow smaller melons, it’s best to let these ramble along a mulched bed away from cucumbers. Prune the suckers off both crops to ensure they channel their energy into fruit production rather than excessive vine growth.
Though you should avoid this plant combination, there are many better companion plants for melons to consider.
Beans and Onions
Alliums are generally great companions because they have a strong sulfurous smell that repels many pests. Similarly, beans are compatible with dozens of garden crops because they fix nitrogen in the soil and don’t usually out-compete their neighbors.
However, science shows that beans and onions inhibit each other’s growth, specifically during the seed germination phase. Avoid this plant combination to ensure you get plenty of green beans and onions for your harvest meals. It’s best to interplant each species with more welcoming combos, like pole beans and lettuce or onions and kale.
Potatoes and Sunflowers
Like black walnuts and fennel, sunflowers are secretly allelopathic to some plants. They may not harm cucurbits like summer squash or cucumbers, but they can greatly reduce the success of potatoes. Sunflowers release phenolic compounds that can harm or reduce potato growth. The massive flower heads and talk stalks can also shade out your spuds, leading to leggy stems and weak tuber development.
Still, sunflowers attract an array of bees and beneficial predators that aid your garden. I like planting them along fence lines on the north side of the garden so they don’t cast a big shadow. This is less of an issue if you grow dwarf sunflower varieties.
Blueberries and Vegetables
Adding perennial berry bushes to your garden is an exciting step toward food self-sufficiency. While strawberries can grow alongside vegetables, blueberries have very different soil needs. Blueberries are known for their acid-loving nature, but most vegetables prefer neutral soil pH.
Additionally, blueberries are long-lived perennials that won’t appreciate the yearly disturbance required for annual vegetables. As a good rule of thumb, avoid plant combinations with very different lifecycles.
Keep blueberries in separate beds where you can mulch and amend the soil with mulches or fertilizers designed for acid-loving plants. They make good companions with gardenias, azaleas, and rhododendrons because those perennial shrubs also enjoy acidity.
Rhubarb and Low-Growing Crops
Nothing says summer like strawberry rhubarb pie, but this pink-stalked perennial does not pair well with strawberries or any other low-growing crop. In the full glory of summer, rhubarb plants can span a tremendous 4 feet wide and tall. The massive, broad leaves will quickly shade out anything growing nearby. The large woody rhubarb rhizomes spread over time and don’t leave much soil space for anything else.
Plant your rhubarb patch in the back of your garden or in an isolated bed where it can dig in for the long haul. Low-growing plants should be grown in raised beds or several feet away from rhubarb plants to ensure they have the sunlight, water, and nutrients they need without competition from a robust rhubarb.
Kale and Cauliflower
Most brassica-family crops have the same needs: full sun, mildly alkaline loamy soil, excellent drainage (to prevent black rot), and consistent moisture. Kale and cauliflower are no exceptions. However, these two crops both prefer plenty of space and don’t make great companions unless you provide 18-24” between them.
Depending on the variety, these two can cause unnecessary stress and competition with each other. Kale tends to grow large and upward, while cauliflower grows up then out, forming a stout foundation to support a big cauliflower head.
They can also attract similar pests like flea beetles, cabbage loopers, and dreaded aphids. If you want to grow these brassicas in the same bed, plant them on opposite sides and add some white alyssum or marigolds in between.
Nasturtium and Carrots
Carrots are an easy companion to sneak in almost anywhere, and nasturtiums are delightful edible flowers that attract many beneficial insects and butterflies. The nasturtium plant is also related to watercress; thus, it has a peppery fragrance that can repel pests.
However, these flowering vines also grow insanely large. A single plant can trail dozens of feet in any direction, climb any fence, or grow into a massive mound 5+ feet in diameter. Moreover, nasturtiums are warm-weather flowers, and carrots tend to do best in the cooler seasons of spring and fall.
It’s best to avoid plant combinations that conflict in size and seasonality. Instead, keep nasturtiums on your garden borders or in containers with a trellis so they can vine upward. Carrot greens are too frilly to compete with the broad, lilypad-shaped nasturtium leaves. These roots are best sown in their own bed with access to full sun and less competition.
Beets and Pole-Habit Legumes
Bush beans are beneficial for beets because they provide extra nutrients and grow to a similar size. But pole beans or peas can stunt beet growth due to their vining nature and potential for shading out the beet greens.
As members of the Chenopodiaceae family with chard, beets prefer at least 6 hours of sunlight per day. When they don’t get enough sun, the roots grow small, and the foliage can turn pale. This combo only works if you have the vining legumes on the north side of the bed and plenty of space for the beetroots to form.
Tulsi Basil and Peppers
Holy basil, or tulsi, is a delightful herb that produces fragrant, delicious foliage and gorgeous flowers that magnetize bees and beneficial predators. But this herb can get remarkably large and bushy. Interplanting tulsi with a compact pepper plant may create too much competition for the peppers and reduce your yields.
Keep holy basil in your herb beds or at least 1-2 feet from nearby plants. It can still be a great companion for peppers as long as the pepper plants have space to bush out and reach the sun.
Lavender and Joe Pye Weed
The major issue with this ornamental combination is water. Lavender is a Mediterranean herb adapted to extremely well-drained soil and prolonged periods of drought. Too much moisture can cause root rot and lavender plant death. On the flip side, a moisture-loving perennial like Joe Pye weed enjoys lots of water and does not look very happy during times of drought.
Neither plant wants soggy soil, but lavender would suffer with the amount of water required by Joe Pye weed. Pay attention to the water needs of your plants to ensure they grow in the vicinity of those with similar moisture needs.
Butterfly Weed and Impatiens
These two flowers have vastly different needs, making them a terrible pair in the garden. Butterfly weed loves lots of sun and dry, well-drained soil. But Impatiens crave moisture and suffer during drought. Although these flowers may look nice together, one will suffer if the soil and water needs of the other plant are prioritized.
Bamboo and Vegetables
Most people don’t realize how aggressive bamboo can be. This grass-family ornamental is amazing for preventing erosion and colonizing moist clay soils. However, I would never, ever plant bamboo near a vegetable garden. The plants spread via underground rhizomes and can quickly take over a yard if you don’t try to contain them.
Some bamboo species will stay behind a border ditch or a rock-lined bed, but others expand anywhere there is moisture and sun. Bamboo stalks grow very dense and close together, forming thick colonies that are difficult to eradicate. Keep them far away from your vegetables, and choose a bamboo species that is not invasive in your area.
Clover and Vegetables
Like mint, clover is an excellent companion plant with nectar-rich flowers that are great for pollinators. The plants also fix nitrogen underground and provide lovely ground cover to keep the soil cooler and weed-free. But you should never plant clover in your vegetable beds while the vegetables are in place.
These legumes spread rapidly and easily overtake annual crops. Their roots form thick mats in the soil that are difficult to remove. I prefer to enjoy clover as a lawn replacement or pathway ground cover between raised beds. Growing at ground level ensures it cannot spread up into your beds.
When you’re not actively growing vegetables in the bed, consider planting some clover in that space! This leguminous plant is a nitrogen-fixer, and as its roots decay after it stops growing, it releases nitrogen into the soil that other plants can benefit from – including your vegetables. But simultaneously growing clover alongside your veggies isn’t ideal.
Final Thoughts
While companion planting can provide tremendous benefits and beauty to your garden, it is important to plan before you cluster several species together. Just like humans, every individual has its own needs and preferences. The most compatible species will share similar growth habits, seasonality, water and soil needs, and nutrient requirements.
If a plant is overly aggressive or demands opposite conditions, save yourself some trouble and keep them separate. Also, pay special attention to allelopathic plants that produce compounds that can suppress or kill their neighbors.
