Companion Planting: What Works and What’s Folklore
Garden books are full of plant “friends” and “enemies.” Some of those pairings are backed by real research, and some are just pleasant tradition passed from one book to the next. Here’s how to tell the difference, which pairings earn their space in a windy, dry Lethbridge garden, and how simple crop rotation keeps the soil healthy year after year.
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What companion planting really means
Companion planting is simply growing different plants close together so that one helps the other. The help can come in a few ways, and it’s worth knowing which, because some are well understood and some are mostly hope:
- Feeding: one plant adds something to the soil the other can use, as peas and beans do with nitrogen.
- Shelter: a tall plant blocks wind or afternoon sun for a smaller one.
- Space and time: a fast crop and a slow crop share the same row without getting in each other’s way.
- Bugs: a plant lures pests away from your crop, or feeds the insects that eat those pests.
- Chemistry: one plant’s scent or root chemicals are said to repel pests or boost a neighbour’s growth or flavour.
The first four have been studied by universities and extension services, and they hold up reasonably well. The last one is where most of the folklore lives. Plants really do make chemicals that affect insects and other plants, but most of the specific claims you’ll read (“basil makes tomatoes taste better,” “beans hate onions”) have never been tested in a garden, or were tested and didn’t hold up.
None of this means companion planting is a waste of time. A garden with a mix of vegetables, herbs and flowers is generally healthier than a block of one crop. It just means you can plan with your eyes open and spend your limited space on pairings that pull their weight.
Pairings with good evidence
Legumes and nitrogen
Peas, beans and clover host bacteria in little nodules on their roots that pull nitrogen out of the air. The catch: most of that nitrogen stays in the legume during the season. Neighbours get the real benefit later, when the roots and leftover plants break down. Pull the vines in fall but leave the roots in the soil, or dig the plants in, and next year’s crop in that spot gets the boost.
The Three Sisters
Corn, beans and squash grown together is one of the best-studied plant partnerships in the world. Corn is the pole, beans climb and feed the soil, squash leaves shade out weeds. Research has measured more food from a Three Sisters plot than from the same crops grown separately. More on this below.
Flowers that feed the pest-eaters
Hoverflies, tiny parasitic wasps and lacewings eat aphids and caterpillars as larvae, but the adults need nectar and pollen. Small, open flowers feed them: sweet alyssum, dill, cilantro left to bloom, calendula, yarrow. University of California trials in lettuce fields found fewer aphids where sweet alyssum was planted among the crop.
Trap crops
A trap crop is a plant the pest likes better than your crop. Nasturtiums draw black aphids; radishes and mustards draw flea beetles. It works only if you then deal with the trap: blast the aphids off with the hose or pull the infested plants and bin them. A neglected trap crop just becomes a pest nursery.
Windbreaks and shade
This one matters more in Lethbridge than almost anywhere. A row of corn, sunflowers, pole beans or peas on the west side of a bed cuts the wind for lettuce, spinach and seedlings behind it. Tall plants on the south or west can also give cool crops a bit of afternoon shade in July, which slows bolting.
Fast and slow in the same row
Radishes come up in four or five days and are ready in about four weeks. Carrots take two to three weeks to sprout and months to size up. Sown together, the radishes mark the row, break the crust that forms on our clay, and are pulled just as the carrots need the room. Lettuce between young tomatoes or squash works the same way.
What it means for you: the companions that really work are mostly about structure, not magic. Shelter from wind, shade from the hottest sun, plants that use space at different times, and enough flowers to keep beneficial insects around all summer. Those are worth planning for.
Lethbridge averages about 116 days a year with wind gusts of 40 km/h or more. A living windbreak of corn or sunflowers on the windward (west and southwest) side of the garden is companion planting that pays off on almost every one of those days. See our growing conditions for more on the wind.
The Three Sisters, up close
The Three Sisters, corn, beans and squash planted together, come from Indigenous farming in eastern North America. The Haudenosaunee (Iroquois) Confederacy, whose homelands span what is now New York State, southern Ontario and southern Quebec, grew them together for centuries, and many other First Nations grew versions of the same system.
How it works
- Corn grows tall and strong, and gives the beans something to climb.
- Beans climb the corn, and their root nodules add nitrogen to the soil over time. Their vines also help brace the corn stalks against wind.
- Squash sprawls across the ground. Its big leaves shade the soil, hold in moisture and smother weeds, and the prickly vines discourage some animals.
Agronomist Jane Mt.Pleasant, who is Haudenosaunee, grew traditional Three Sisters plots and published the results in 2016. The interplanted plots produced more food energy and more protein from the same area than corn, beans or squash grown on their own. In other words, this isn’t just a nice story; it’s a highly productive farming system.
Growing it in Lethbridge
Our season is short and windy, so a few adjustments help:
- Use short-season varieties. Choose an early sweet corn or a short-season flint corn, a pole bean that matures quickly, and a smaller squash such as an early zucchini, acorn or buttercup. Many traditional Haudenosaunee varieties need a longer season than we have.
- Plant the corn first. Sow corn in late May or early June in low, wide mounds about 30 cm across, four or five seeds per mound, mounds about 1 to 1.5 m apart. Corn needs warm soil (at least 10°C).
- Add the beans when the corn is 10 to 15 cm tall, three or four seeds around each mound. If the beans go in at the same time, they can climb and pull down the young corn.
- Add squash at the same time as the beans, one or two plants in the spaces between mounds.
- Plant a block, not a single row. Corn is wind-pollinated and needs neighbours. A block of at least four mounds by four mounds pollinates far better than a line, and the block shelters itself from the wind.
Be realistic about yields. In a small Lethbridge yard, a Three Sisters patch usually produces a bit less of each crop than a bed of that crop alone, because the plants share light and water. The payoff is in total food per square metre, less weeding, and a living lesson in how people farmed this continent long before tractors.
The story of the Three Sisters. For the Haudenosaunee people, corn, beans and squash are sisters who grow best together and look after each other. In one telling of their creation story, a woman from the Sky World had a daughter, and when the daughter died, the first corn, beans and squash grew from the earth where she was buried. That is why the three plants are honoured together as a gift, and are sometimes called the “sustainers,” the foods that keep people alive. Stories are told a little differently by different nations and families, and they belong to the people who tell them.
Wonder: The land around Lethbridge is the home of the Blackfoot (Niitsitapi) people, who lived for thousands of years mostly on bison, saskatoon berries and wild plants rather than corn fields. Why do you think people in different places grew or gathered such different foods?
Try this: Plant one mound of Three Sisters with a grown-up. Measure the corn every Saturday and write down the day the first bean tendril grabs onto a corn stalk.
Nice tradition, little proof
These ideas are everywhere, and none of them will hurt your garden. They just shouldn’t be the reason you buy seed or give up space.
| The claim | What the evidence says |
|---|---|
| “Marigolds repel everything.” | The best-known real effect is underground: French marigolds grown thickly for a whole season and then dug in can reduce some root-feeding nematodes. That isn’t a common problem in prairie home gardens, and a few marigolds dotted along a row don’t do it. Studies on marigolds repelling flying insects are mixed and mostly small. They’re cheerful, easy and attract some beneficial insects. Grow them for that. |
| “Carrots love tomatoes.” | The phrase comes from a popular 1975 companion planting book of that name by Louise Riotte. Much of it is drawn from older folklore and the author’s own experience, not trials. Carrots and tomatoes can share a bed fine; tomatoes may even shade carrots in July heat. There’s no evidence they “love” each other. |
| “Basil makes tomatoes taste better.” | There’s no good evidence a plant growing next to a tomato changes the tomato’s flavour. Tomato flavour comes from variety, sun, heat and ripeness. Basil and tomatoes do like the same warm conditions, and they taste great together on the plate. That’s reason enough. |
| “Beans and onions are enemies.” | Onions and garlic contain sulphur compounds, and some books claim they stunt beans and peas. Garden trials showing real harm are hard to find. In a small bed, give each crop its own space for practical reasons (onions dislike shade and competition) and don’t worry about the feud. |
| “Strong herbs confuse pests.” | There’s some research showing that mixed plantings can make it harder for certain pests to find their host plant, and a few studies have found fewer cabbage pests near aromatic plants. Results vary a lot by pest, crop and year. Mixing things up doesn’t hurt, but it won’t replace a row cover. |
Watch out for “friends and enemies” charts. Many are copied from book to book and website to website, with no source behind them. If a chart can’t tell you why two plants help each other, treat it as tradition, not advice.
Practical pairings for Lethbridge
These combinations are chosen for our conditions: strong west wind, hot dry summers, a short season and alkaline clay. Each one works for a reason you can see.
| Pair | How it helps | Tips for here |
|---|---|---|
| Radishes with carrots or parsnips | Fast radishes mark the row and break crusted soil; pulled before carrots need space. | Mix a few radish seeds in with the carrot seed. Keep the row moist; our clay crusts hard after a hot day. |
| Peas or pole beans on the west side of lettuce and spinach | Living windbreak and afternoon shade; legume roots enrich the soil for next year. | Peas go in early (late April to early May) and are tall by the time lettuce needs shelter. |
| Sunflowers or corn along the windward edge | Windbreak for the whole bed. | Plant two or three rows deep, on the west or southwest side. Stake sunflowers in exposed yards. |
| Lettuce between young tomatoes, peppers or squash | Uses the empty space while the warm-season crops are small. | Harvest the lettuce by early July, when the tomatoes fill in and summer heat makes lettuce bolt. |
| Sweet alyssum along the edges of lettuce, cabbage and kale beds | Feeds hoverflies and parasitic wasps that eat aphids and caterpillars. | Alyssum is an annual that handles light frost. Sow or set out in May for flowers all summer. |
| Dill and cilantro left to flower near the cabbage family | Flat flower heads feed tiny beneficial wasps and hoverflies. | Both bolt fast in our heat anyway; let a few go to flower instead of pulling them. |
| Nasturtiums near beans or at the end of a row | Trap crop for black aphids; also attract pollinators. | Check them weekly. Hose off aphid clusters or pull badly infested plants. |
| Radishes or mustard greens at the edge of a cabbage-family bed | Trap crop for flea beetles. | Sow a week or two before transplanting cabbage or broccoli. Combine with a row cover over the main crop. |
| Calendula, borage or phacelia in the vegetable bed | Bring in bees and beneficial insects; useful for squash, cucumbers and beans. | All are easy annuals from seed. Calendula and borage often self-seed. |
| Corn, pole beans and squash | The Three Sisters: support, nitrogen over time, living mulch. | Short-season varieties, planted in a block. See above. |
Keep paths for water. Interplanting means more roots sharing the same soil. In a dry Lethbridge July, a crowded bed dries out faster. Mulch between plants and water deeply. See soil, compost and watering.
Crop rotation: the companion planting of time
Companion planting is about which plants grow side by side. Crop rotation is about which plants follow each other in the same spot. It’s less glamorous, but its benefits are much better proven. Plants in the same family tend to share the same pests and diseases and use the soil in similar ways. Grow them in the same spot every year and the problems build up. Move them around and many problems fade.
| Family | Includes | Notes |
|---|---|---|
| Cabbage family (brassicas) | Cabbage, broccoli, cauliflower, kale, Brussels sprouts, kohlrabi, radishes, turnips, rutabaga, mustard, arugula | Root maggots build up when they stay in one spot, and so does the soil disease clubroot where it has been brought in. Radishes count too. |
| Tomato family (nightshades) | Tomatoes, potatoes, peppers, eggplant | Share several soil-borne diseases. Don’t follow potatoes with tomatoes. |
| Legumes | Peas, beans, faba beans | Leave roots in the ground; good to grow before heavy feeders. |
| Squash family (cucurbits) | Zucchini, squash, pumpkins, cucumbers, melons | Heavy feeders; love a spot with fresh compost. |
| Onion family | Onions, garlic, leeks, shallots, chives | Onion maggot is the main pest that builds up. |
| Carrot family | Carrots, parsnips, celery, parsley, dill, cilantro | Dill and cilantro belong here too, so count them when you plan. |
| Beet family | Beets, Swiss chard, spinach | Fairly trouble-free; useful filler in a rotation. |
A simple four-bed rotation
If you have four beds or four sections, move each group one step each year: legumes → cabbage family → tomato and squash families → roots and onions → back to legumes. The cabbage family follows the legumes and gets their nitrogen; the heavy-feeding tomatoes and squash get the bed with fresh compost; roots finish the cycle in soil that isn’t too rich (too much nitrogen makes forked, hairy carrots).
In a small yard with only one or two beds, don’t let perfect be the enemy of good. Just avoid growing the same family in exactly the same spot two years running, and keep a rough sketch of where things were. A photo of the garden every June does the job.
Test it in your own garden
Curious whether a pairing works in your yard? Gardeners can do real experiments, and the results are often more useful than any book.
- Pick one question. For example: “Do my cabbages get fewer aphids with alyssum around them?”
- Plant two matching patches. Same variety, same planting day, same watering. One gets the companion, one doesn’t. Keep them a few metres apart.
- Count, don’t guess. Once a week, count pests or damaged leaves on five plants in each patch. Weigh or count the harvest.
- Write it down. A notebook or a phone note with the date, what you saw and the weather that week.
- Repeat next year. One season can be fooled by a hot spell, a hailstorm or a grasshopper year. Two or three seasons tell you something.
Share what you find. The Lethbridge & District Horticultural Society is full of gardeners who have been running quiet experiments like this for decades. Local experience, written down, beats a chart copied from a book written for somewhere else.
Sources
- Mt.Pleasant, J. (2016). “Food Yields and Nutrient Analyses of the Three Sisters: A Haudenosaunee Cropping System.” Ethnobiology Letters 7(1): 87–98.
- University of California Statewide IPM Program, “Del-Pozo Valdivia confirms the benefit of insectary plants in lettuce” (2019).
- University of Arizona Cooperative Extension, Yavapai County Master Gardeners, “The Art of Companion Planting.”
- Mississippi State University Extension, “Companion Planting: Myth or Truth?”
- Washington State University Extension, “Trap Cropping” (K. Wolfe, 2020).
- Riotte, L. (1975). Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening.
- Environment and Climate Change Canada, Canadian Climate Normals 1991–2020, Lethbridge Airport (wind gust days).