15 Companion Planting Mistakes to Avoid This Season

Companion planting in your garden can be as much of an art as it is science. There are many considerations to take, especially when trying to plant for pest prevention, or other symbiotic benefits. In this article, organic gardening expert Logan Hailey walks through some of the top companion planting mistakes you'll want to avoid this season!

companion planting mistakes

Whether you’re a beginner or veteran gardener, no season goes by without its share of challenges. As climate pressures become increasingly intense and weather fluctuations become more drastic, our gardens have to be more resilient than ever before. What if there is a way to repel pests, reduce weed pressure, and improve your garden health without relying on more chemical inputs?

Companion planting does just that. Drawing on ancient agricultural knowledge, this practice creates symbiotic relationships in the garden. Certain flowers, herbs, and veggies boost the growth of their neighbors and help you maximize space, yields, and flavor in the process.

When things go right, companion plants can drastically improve your garden yields by reducing the amount of problems you may face with your crops. However, when improperly planned, companion planting can cause some major issues or even crop failures. Let’s dig into the most common mistakes with companion planting and how to avoid them.  

What is Companion Planting?

marigolds grow in the vegetable garden
Companion planting improves overall plant health and yields.

Companion planting (or interplanting) is a gardening technique that uses certain combinations of flowers, herbs, and vegetables to improve the overall health and yield of garden crops.

Companion plants can:

  • Repel pests
  • Attract beneficial predators
  • Provide helpful shade (to reduce bolting)
  • Improve soil fertility
  • Loosen the soil root zone
  • Act as a living trellis for vines
  • Maximize yields in a small space
  • Reduce incidences of soil-borne diseases
  • Attract pollinators and improve fruit set
  • Improve crop vigor and yields
  • Add biodiversity to your garden and plate

Instead of focusing on individual crops in isolation, many growers are turning to this more ecological approach to growing food. In other words, they are mimicking nature’s wild ecosystems by pairing certain plants together.  

But that doesn’t mean your garden has to look unruly and overgrown! Companion planting is all about strategically choosing and spacing beneficial plants to improve their growth. As a bonus, this can also make your garden appear more beautiful and lush.

While monoculture involves planting large amounts of the same species (for example, a large field of corn), companion planting incorporates multiple species into a planting to create a polyculture.

These neighborly interactions can be mutually beneficial (symbiotic), or neutral to one plant and helpful to the crop (commensal). Either way, incorporating companions into your garden has an abundance of potential benefits and very few drawbacks– as long as you avoid a few common mistakes.

Most Common Mistakes

If you want to become a pro interplanter, it all starts with research, planning, and diligence. Thankfully, gardeners have been doing this for centuries (if not millennia), and they’ve left a lot of their knowledge behind for us to learn from. In addition, modern science has confirmed the benefits (and drawbacks) of certain companion combos.

Avoid these common mistakes to ensure that your companions actually help your crops rather than accidentally setting back their growth:

Not Enough Spacing

marigold grows near tomatoes
When planting certain plants together, keep a distance between them so that they each have enough room to grow.

Nobody likes to be overcrowded, but beginner gardeners often transplant or seed plants too close together. It’s important to ensure that companion plants don’t accidentally invade your crop’s personal space as they grow.

But sometimes this can be hard to tell at the time of planting. For example, some beneficial flowers (like nasturtiums, borage, or African marigolds) start the season off at a reasonable size, then quickly overgrow the crops they’re supposed to help.

How to Avoid It

Always take into consideration the fully mature size of a plant (both width and height). Check the “spacing requirement” on the back of a seed packet or the tag of a nursery seedling to determine how far from the crop it needs to be to ensure plenty of airflow and space to grow. When in doubt, wider spacing tends to be best.

Companions Compete with Crop for Water

watering vegetable garden
Pair different plants that won’t steal water from each other.

Depending on your soil type, water holding capacity and irrigation may be an issue in your garden. When water is scarce, more vigorous and deep-rooted plants can “steal” water from your veggies. This can also happen when certain plants are sown too close to a crop. 

Shallow-rooted crops tend to have a harder time hanging onto water because they don’t have the deep taproots and root hairs that carrots, tomatoes, peppers, squash, and potatoes do.

When choosing companion plants, be sure that deep rooted veggies or herbs won’t accidentally steal moisture away from your shallow-rooted crops like spinach, lettuce, corn, radishes, or onions.

How to Avoid It

The easiest way to prevent this mistake is simply to keep the soil consistently moist. Mulches and soaker hoses or drip irrigation are great ways to ensure that all the plants in a bed get the water they need.

Companions Compete for Nutrients

Sugarcane growing with cabbage
Make sure your companions don’t compete for nutrients with each other.

Vigorous fruiting crops like tomatoes, squash, cucumbers, brassicas, and peppers need ample fertility to crank out juicy fruits all summer long. These heavy feeders can “rob” nutrients from less aggressive companions like spinach, scallions, or garden herbs. When plants compete for nutrients, one or both of their yields can be harmed. 

How to Avoid It

Try to pair crops with similar fertility needs next to each other. You can also choose light-feeding plants that don’t usually need added fertility (such as alyssum, marigolds, and legumes).

If growing in a poor soil, you want to be sure that the soil has enough minerals to supply your heavy feeding crops as well as neighboring companions. To do this, add more biologically-rich compost or slow-release fertilizer to remedy any nutritional deficiencies. You can also widen the spacing between plants.

Shading Out Your Crop

Organic kale growing in the garden
Make sure companion plants don’t start to shade other crops in the garden that need full sun.

Many of the awesome benefits of a symbiotic planting are counteracted if one plant overgrows and starts to shade out the other. Because sunshine is the fuel for photosynthesis and plant growth, this competition for light can really cause some issues for your crop.

For example, if you let your cucumbers vine along the ground (I recommend trellising them), tall companions like sunflowers or unruly nasturtiums can create a shady canopy above them. Another example is tomatoes and bush beans. The vigorous, tall growth of tomatoes will quickly shade out beans planted near their base.

However, remember that partial shade can also be beneficial for some crops. For example, head lettuce planted 6-8” from tomato plants may enjoy the slight shade from their canopy. This can also prevent bolting in the heat of summer.

How to Avoid It

Most major garden crops need full sunlight, therefore you should choose low-growing companions whenever possible. It’s best to trellis vining crops to save space and ensure that companions can still benefit them.

It also helps to pay attention to the solar aspect (how the sun hits your garden). You are less likely to have issues in a south-facing garden where plants are properly spaced and able to get the warm sunshine they need.

Planting Allelopathic Companions

Beans and onions
Avoid planting highly allelopathic plants as companion plants.

Just like people, some plants just aren’t meant to hang out together. Certain plants can actually inhibit the growth of others, resulting in a whole cascade of negative effects on your crop.

Allelopathic plants produce compounds in their root zone that can harm or even kill neighboring plants. They use these compounds to suppress the growth of their neighbors so they can succeed in nature (survival of the fittest, right?)

Highly allelopathic plants include perennials like black walnut, rhododendron, sumac, and elderberry. Garden vegetables that have mild allelopathic properties include brassicas and mustards, fennel, sunflowers, and buckwheat.

How to Avoid It

Check that your companion pairing doesn’t include allelopathic or incompatible plants. The most common incompatible combos include:

  • Mint or alliums with asparagus: Both mint and alliums produce volatile oils that can be very helpful or repelling pests, but can also reduce the growth of asparagus.
  • Beans and onions: These plants are known to inhibit each other’s growth, particularly during the seed germination phase.
  • Potatoes and sunflowers: Sunflowers release terpenes and phenolic compounds that can harm or reduce potato growth. They can also shade them out.
  • Fennel: Fennel releases soil compounds that limit the growth of most competition. It’s best kept in its own corner of the garden.

Different Soil Requirements

Growing vegetables in garden
It is important to comply with the requirements of plants for soil fertility.

When you’re planting two species in the same place, it’s difficult to change the soil composition between just a few square feet. For example, brassicas (broccoli, kale, cauliflower, cabbage, etc.) prefer well-drained, loamy, slightly alkaline soil.

Potatoes like a slightly acidic, sandy soil. If these crops were planted next to each other, one of them might end up unhappy because of the pH or soil texture.

How to Avoid It

Research the soil preferences (pH, drainage, organic matter, and texture) of each species ahead of time. Check this chart for common garden plant pH preferences.

Wrong Timing

Green onion, tomato plants and garden lettuce
Observe the planting time to get a quick harvest.

Like most things in life, timing is everything when interplanting. The best companions complement each other’s growth habits over time. For example, you can get a quick harvest of radishes or lettuce out of a tomato bed if they are transplanted at the same time as the tomatoes. By the time the tomatoes start taking off and shading the area, the radishes or lettuce will be ready to harvest.

Some plants also just need to be established before they can provide their companion benefits. For example, if you want to trellis cucumbers up corn stalks, the corn needs to be seeded at least 2 weeks before transplanting the cucumbers so they have enough time to get established.

A final timing consideration is the flowering stage of companion plants. Many of the best benefits of interplanting come from the flowers. While the foliage of alyssum, dill, marigolds, borage, or calendula may be somewhat useful for repelling pests, it is their flowers that really do the heavy lifting by attracting beneficial insects.

How to Avoid It

Compare the Days to Maturity (DTM) of each species before planting. Maturity for a vegetable typically refers to when you can begin your first harvest. Some crops (like tomatoes, peppers, cucumbers, and zucchini) will continue yielding all summer long.

For flowers, the DTM refers to the days from seeding until the first blossoms typically appear (remember that many flowers will also continue blooming until the first frost).

Try to sync up planting times to benefit all species involved. Don’t be afraid to experiment with staggered plantings to see what works best.

Aggressive Companions

vegetable garden bed
It is not recommended to plant the following aggressive crops as companions: clover, rosemary, and bamboo.

Some plants are simply too aggressive to grow right next to your veggie crops, such as:

  • Mint
  • Clover
  • Rosemary
  • Bamboo
  • Thyme
  • Bee balm
  • Blackberries
  • Morning glories
  • Jerusalem artichokes (sunchokes)

While beautiful and often delicious, these plants tend to “choke out” garden crops and overgrow them. Some of them (specifically bamboo, rhubarb, asparagus, and jerusalem artichokes) are even spread by underground rhizomes that can help them creep under barriers and show up in random parts of your garden.

How to Avoid It

Keep aggressively spreading plants out of your garden beds. If you still want to grow these eager herbs and veggies, plant them in their own separate beds or on the margins of your garden where they can be contained.

Perennials in Annual Beds

Lavender and Yarrow
Do not plant annuals and perennials in the same garden bed.

Perennials are a delight for any gardener who likes hands-off plants that are easy to maintain. However, planting perennials in annual garden beds can be disastrous due to the major life cycle differences between the two.

Annual vegetables are replanted every season (or multiple times per season) and harvested in short windows. On the other hand, perennial plants live for 3-10+ years and often become woody as they grow established in their place. You don’t want to mix these two together or you may wind

How to Avoid It

Plant perennials with other perennials and annuals with annuals. Your annual vegetables can still get all the benefits of perennial flowers and herbs if they are planted nearby (within 6-10 feet), so don’t waste your time and space planting them in the same bed.

There are many incredible perennial companion plants, including:

But all of these should be kept in their own beds, preferably in perennial border beds lining the margins of your garden. This reserves your precious raised beds or loamy areas specifically for annuals.

Shapeless Plantings

Tagetes flower and lettuce growing in the garden
Experienced farmers advise planting a vegetable garden in rows.

While having a circular or sporadically-shaped garden may seem more fun and natural, there is a reason farmers grow crops in rows. Rows are easier to tend and weed, plus they keep the garden more tidy so you can see what’s going on with each crop.

How to Avoid It

When possible, plant your companions in clean rows alongside your crop rows. Keeping straight lines will make weeding and maintenance so much easier. It also helps you to better gauge the spacing requirements for each plant.

Different Maintenance Needs

lettuce, onions, beans
It is necessary to take into account the fact that the care needs of your crops and companions are compatible.

By now you can tell that the most important secret for companion planting is keeping like with like. Birds of a feather flock together! And so do plants with similar needs. So when you try to combine species with wildly different maintenance requirements, things can inevitably go awry.

For example, potatoes need to be hilled to keep their tubers underground. If they were planted next to broccoli, cucumbers, or strawberries, those crops could end up buried or suffering from stem rots due to excess soil mounded up at their base. It would also be logistically difficult to get your shovel or hoe into the space to mound up the soil around the potato plants.

How to Avoid It

Make sure you are aware of the maintenance needs of your crop as well as its potential companions. If two plants are physically incompatible, pruning, hilling, mulching, and weeding can become more difficult than they need to be.

Wasting Space

Marigolds and capsicum plants
Try to fill the space between the beds to the maximum by adding fast-growing crops: radish, young greens, lettuce.

For small space gardeners, interplanting is an incredible way to maximize the yields from your garden. But if you don’t know how to properly time, space, and trellis (when applicable) your plantings, you can end up missing out on valuable opportunities for increasing your yields.

How to Avoid It

Notice every bit of bare soil in your garden and contemplate ways that it can be filled.

When you first plant young tomatoes at 12-24” apart, think about quick-growing crops (radishes, lettuce, or baby greens) that could be tucked into the empty area and harvested by the time the tomatoes start to get big.

If you’re transplanting slow-growing peppers into a bed, consider adding scallions or basil in between the rows to get the most out of a bed.

Attracting Similar Pests

Cabbage and Spring onions growing as companion plants
Avoid planting crops that attract the same pests.

Two plants that magnetize the same pests can be a recipe for disaster. In the world of entomology (the study of insects), researchers have found that pests are “more likely to find and more likely to stay” in large clumps of crops from the same plant family. This is because closely related crops may look similar or smell the same to pests.

For example, kale and cabbage (both brassicas) are notoriously popular with aphids. It could be helpful to plant sweet alyssum, onions, or calendula in between the rows to disrupt the aphids from bouncing between the two hosts. For this same reason, you may keep nasturtiums at a distance because they attract aphids as a “trap crop”.

How to Avoid It

When selecting companions, be aware of what kinds of pests you may be dealing with. Typically, it’s best to stagger plants from the same family that may attract the same pests.

Pay attention to separating different crop families when possible:

  • Brassicas: kale, cabbage, cauliflower, broccoli, mustards, radish
  • Solanaceae: tomatoes, peppers, eggplant, potatoes
  • Cucurbitaceae: cucumbers, melons, zucchini, summer squash, winter squash
  • Amaranthaceae/chenopodiaceae: chard, beets, spinach, quinoa

Alliums (onions, garlic, shallots, scallions, and chives) tend to be an exception because they are so effective at repelling pests.

Too Many Companions

Marigolds in Vegetable Garden
Don’t over-plant companion plants as this can harm your crop.

Sometimes beginner growers get a little overzealous with their interplantings. They want to toss 5 or even 10 different species in a single raised bed and accidentally end up with an overgrown mess. It is hard to tend this many species in one area and difficult to gauge which plants are actually serving their purpose.  

How to Avoid It

While a super diverse garden is awesome, try to keep things simple with your initial companion experiments. Stick to 1 or 2 combos per bed so you can understand which plants grow best together.

Forgetting Groundcover

Nasturtiums as a companion plant
Nasturtiums repel some pests and form a lush, green ground cover that prevents weeds.

Weed-suppressing ground cover plants are often the most underrated companion plants. Although they don’t always have the most dazzling flowers or strong aromas, these low-growing species are workhorses in the garden.

Groundcover companions can:

  • Suppress and out-compete weeds
  • Keep the soil cool during hot weather
  • Keep fruit (like strawberries or cucumbers) off the ground
  • Act as a living mulch
  • Add diversity to the soil root zone
  • Conserve water

How to Avoid It

Remember that not all companion plants are only for repelling pests or attracting pollinators. Experiment with beneficial groundcover species like creeping thyme, sweet alyssum, dichondra, microclover, or creeping phlox. Just be sure to plant them beneath tall crops like tomatoes, trellised peas, trellised cucumbers, berries, or brassicas.

Final Thoughts

Companion planting doesn’t have to be super complicated, but planning drastically increases your chances of success. Let’s recap the most important aspects of companion planting.

Before planting two species in the same bed, ask a few questions:

  • Did you provide enough space for both plants to grow to their full size at maturity?
  • Will the species compete for water and nutrients? Do they have similar fertility needs?
  • When fully grown, will one plant shade out the other or compete for sunlight?
  • Does one plant attract or repel pests? Are they susceptible to the same pests?
  • Can the companions interact badly with each other? Is one of them allelopathic?

If these considerations aren’t properly addressed, you might accidentally end up with two crop failures at once. But, rest assured, if you accidentally try out a companion combo that doesn’t work, it’s happened to the best of us! You can easily prune back the companion plant or pull it up and restart in a different part of the garden.

Interplanting undeniably takes some trial and error to get it just right. Use your observation skills to take note of how your plants interact and document each season’s experiments in a garden journal.

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