The basic idea behind companion planting is both simple and sensible: many plants grow better near some companions than they do near others or when alone. By itself it will not work miracles, but applied in a well-maintained garden, it can produce startling results. It can drastically improve the use of space, reduce the number of weeds and garden pests, and provide protection from heat, wind, and even the crushing weight of snow. In the vegetable garden, all this adds up to the best thing of all: increased yield.
Most people think of companion planting in connection with vegetable gardens, but it can also be used when flower gardening and in full-scale fields. Some of the most familiar examples come from farming, where it’s a long-standing practice to sow vetch or some other legume in the fall after the harvest. This cover crop provides erosion control through storms, and supplies both nitrogen and organic material to the soil when it is plowed under in spring. Most such crops themselves need a helper, known as a nurse crop, usually a grain that is sown along with the legume. The grain provides weed control while the legume gets established, and helps protect the legume from both wind and the weight of snow.
Native Americans in the north-east region of what is now the United States practiced another familiar but very different version of companion planting, known as the Three Sisters. Corn, beans and squash, planted in the same field, supported each other in several ways: the beans provided nitrogen for the nutrient-hungry corn, the corn provided a support for the vining bean, and the squash, a living mulch, suppressed weeds between rows.
What’s often referred to as “traditional” companion planting (that popularized by Louise Riott’s best-seller Carrots Love Tomatoes) involves yet another kind of support. Though the process used to determine plant compatibility is closer to alchemy than chemistry, the principle behind it is similar to the idea that some plants chemically enhance or inhibit each other’s growth and well-being. Such chemical interaction does indeed happen: one oft-cited example is the black walnut tree, whose bark, leaves, and roots contain juglone, a compound toxic to many other plants, including most vegetables. This is an example of alleopathy, where one plant secrets a substance harmful to others. Alleopathy can be helpful to farmers; a 1998 dissertation project published online by Weed Biology and Management suggested that it might be a factor in why squash plants so successfully suppress weeds in corn fields.
But this sort of chemical interaction is more rare than some sources suggest, and as the examples above show, it’s far from the only way that plants can help each other. Plants can protect each other from wind or weather, act as decoys for harmful insects, attract beneficial insects that eat pests, or provide nutrients, physical support, or shade for other plants. Early season, cool-weather crops keep weeds at bay before later crops can be set out or mature; vetches planted in fall protect soil from eroding and add nitrogen and organic matter to it when dug under in spring, improving both its nutrient content and its structure.
Spurred largely by Riott’s enormously popular book, interest in companion gardening in the United States rose sharply in the 1980s and 90s. A number of university sites and individual gardeners and scientists, irritated by the semi (some would say pseudo) scientific basis of the traditional method, have rejected companion gardening outright. And indeed, the crystal chromatography, discovered by Ehrenfried Pfeiffer in the 1930s, has not been found to have any basis in botany or any other science.
However, several recent studies by reputable universities have established that some companions, at least, actually do help each other. The dissertation mentioned above is one example. Another Ph.D. dissertation from 2004 concluded that intercropping basil with either tomatoes or Brussels sprouts improved plot yield (PDF format). Yet another study conducted in 2005 and 2006 in New Jersey found that dill, coriander, and buckwheat intercropped with bell peppers significantly increased the presence of beneficial insects, which lowered the damage done by European Corn Borers, a major predator. Furthermore, since the peppers did not have to be sprayed with pesticides, which kill off aphid predators, the usual problems with aphids did not arise (see Investigating the Effects of Companion Plantings - PDF).
These studies, along with generations, even centuries, of observation and experience, suggest that companion planting can indeed boost yield and help control pests in gardens. This approach, which draws on common sense, traditional practices, and recent scientific research, opens up a whole range of possibilities to both back-yard gardeners and large-scale farmers.
Companion Plants in the Garden
Since most companions must be planted very near each other in order to have any effect on each other, companion planting is especially well-adapted to small gardens where plants are grown in close proximity and space is at a premium. Gardens that use raised beds, wide rows, or intensive square foot gardening methods make natural candidates for companion planting. It is also a natural ally for organic gardeners, since much companion planting is designed to control pests.
Where a conventional vegetable garden creates a series of small monocultures (all the lettuce is grown together over here, all the tomatoes there, and never the twain shall meet), companion planting encourages a carefully planned and densely planted mix to take advantage of the many possible relationships mentioned above. The mix alone tends to repel many flying insect pests, which actually get confused and give up if they don’t find what they’re looking for soon enough
If the long lists of compatible vegetables at various sites online leave you dizzy, you can instead focus on a couple of basic principles and keep in mind a much shorter list of Don’ts. The principles follow quite directly from the discussion above:
- Plant short, shade-tolerant plants beneath taller, bushy plants.
- When you mix sun-loving plants, put tall ones at the north end of the plot and small ones at the south end, so all will get needed sun.
- Plant herbs throughout the garden, especially basil, mint, sage, and dill. EXCEPTION: Keep dill away from carrots.
- Plant cosmos and French or Mexican marigold here and there in and near the garden to repel pests and encourage beneficials that prey on them.
- Do the same with chives, garlic, or onions EXCEPT near or amongst beans.
- Exploit the different maturation rates of different crops: plant lettuce, cilantro, spinach, or chard early where you plan to set out squash and melons later, so that weeds don’t have a chance to move in, and you get two crops instead of just one.
Here are the Don’ts:
- Don’t mix dill with tomatoes or with carrots.
- Don’t plant garlic, onions, or chives with beans.
- Fennel does not mix well with most other plants; keep it in its own corner.
Matching Specific Goals with Specific Techniques
There are seven basic areas in which companions can help each other: insect, weed, and disease control are the first three, while nutrient sharing or provision, physical protection or support, and efficient use of space are the next three. Erosion control is an important issue on large fields and farms, but not for most small-scale gardeners, so it is not addressed in detail here.
Companion plants can help control pests in several different ways, the most obvious of which is by repelling them. Many flying pest insects are put off or confused by the smell of onions, garlic, and French or Chinese marigolds, so planting these here and there throughout the garden helps to control insect pests.
The other two methods make less sense at first glance, because both involve attracting insects. In the first of these, the gardener provides habitat for beneficial parasitic and predatory insects. Cosmos, black-eyed Susans or blanket flowers, asters, and many other flowers in the Compositae family attract ladybugs and others that prey on insect pests. Providing a continuous sequence of blooms helps keep beneficial garden insects around all season.
The most counter-intuitive method involves what’s known as trap-cropping, in which one plants a crop which the harmful insects love, so they’ll stay away from the crop you really care about. This technique works best when the trap crop completely surrounds the garden area, so that approaching insects will encounter the trap crop first, no matter what direction they approach from. For example, collards draw the diamond back moth away from cabbage, and a planting of Chinese mustard will help protect spinach, chard, and other vulnerable crops from the flea beetle by giving it something else delectable to eat. Those same leafy crops can be protected from leaf miners by radishes.
Weeds seem to sprout instantly from bare ground, so dense planting helps suppress them simply by covering every inch of available space and shading out competitors. Two techniques are key here: intercropping (planting two crops on the same ground at the same time) and sequential cropping (planting crops in sequence, so that ground is never left bare even after a harvest). For example, if ground is planted in an early crop while slow crops such as squash and tomatoes get established, spring weeds never have a chance to take hold, and the gardener gets an extra crop to boot.
Growing a wide diversity of plant species can ensure that if one crop takes a beating from pests or disease, there are still lots of others left. Monocultures have long been known to be especially vulnerable to both disease and insect invasion; companion cropping as practiced in most vegetable gardens creates such a diverse environment that most plant diseases can neither establish themselves nor spread easily.
Nutrient Provision, Nutrient Sharing
All living things need nitrogen, and there is a plentiful supply, as 70% of our atmosphere consists of this element. However, no animals and very few plants can make use of nitrogen as a gas. Peas, beans, vetches and clovers, all members of the legume family, are distinguished amongst plants by their ability to take nitrogen from the air, rather than from the soil. They do this with the help of certain soil bacteria: the legumes provide a hospitable environment for the bacteria, and the bacteria share with the legumes the nitrogen they “fix.” The legumes, in turn, share some of that nitrogen with other plants. This is why vetches and clovers are so widely used as cover crops; when dug or plowed under, they not only add the organic matter of their bodies — including all the nutrients they have absorbed from the soil — but they also add the nitrogen they have fixed from the air.
One can take advantage of this unique ability of legumes in several different ways. Growing bean and pea crops in different areas around the garden over successive years (rotating crops) will allow soil in different spots to benefit from the nitrogen they fix. Intercropping other crops with a particular planting of legumes will let that second crop take advantage of this year’s nitrogen. Planting legumes as a cover crop over winter and then digging it in come spring gives the soil both a nitrogen boost and plenty of green compost (see Cover Crops for Home Gardens - PDF).
Most plants cannot actually provide nutrients for each other, but those that do not compete for the same nutrients often make good companions. Bush beans and potatoes appear on many companion lists in part because the former, a flowering, fruit-producing plant, draws heavily on the phosphorus in the soil, while potatoes need a great deal of potassium. Inter-cropping tomatoes, lettuce, carrots, and basil makes use of the fact that only two of these plants — lettuce and basil — depend primarily on the same nutrient, nitrogen, which can easily be provided with a foliar spray.
Physical Protection and Support
Plants can provide physical protection in a number of ways. Tall, sun-loving plants such as peppers or tomatoes can protect short, shade-tolerant ones like lettuce from sun, and a row of sturdy bushes can protect young bush beans, easily snapped at ground level, from wind damage. A dense planting of tall plants can actually help support weaker ones. And of course, tall sturdy plants like corn can serve as climbing poles for beans.
Using Space Efficiently
The combination of intercropping and sequential cropping allows for a far more efficient use of space than can be achieved with a single monoculture. Intercropping takes advantage of such simple factors as the different heights at which plants grow. Sequential planting makes use of the fact that plants grow at different rates and have different seasonal needs. For example, squash and melons require large amounts of space, but only in summer. Warm-season crops, they cannot be set out early, and even after they sprout or are transplanted, they seem to take quite a while to get organized and grow in earnest. It’s usually several weeks before those huge leaves appear, and in the meantime, all that space around them is sitting there, a veritable weed invitation. Sequential cropping lets a gardener get an early spring crop such as spinach from that space.
One Example in Detail
Suppose you have a small, 4′ x 4′ bed, just big enough for four tomato plants. Companion planting will let you get not just tomatoes, but a season’s worth of carrots, lettuce, and basil from that plot, plus a few onions and perhaps herbs as a bonus.
The lettuce and carrot seeds (plus an onion and a French marigold or two) would go in first, in early spring. Four circles should be kept free of carrot seeds. The lettuce to the south and west of these circles should be used first, leaving these sunny areas for basil. If pulling lettuce threatens to disturb the carrots, cut it instead, leaving the roots to rot or rejuvenate. Just before it’s time to transplant the tomatoes, the lettuce in the circles can be harvested. By the time it’s warm enough to transplant basil and tomatoes (started indoors several weeks earlier), there should be room for them. The tomatoes will shade the remaining lettuce, extending its season. Meantime, the carrots, a light feeder, slowly mature underground.
The carrots, lettuce, and basil will make a nearly solid ground-cover under the tomatoes, leaving weeds very little space to get established and providing a living mulch. They’ll shade the ground, preventing loss of water through evaporation. Though the plot will certainly require more water than it would if it supported only tomatoes, the net water use for the four crops grown together will be far less than if the same amounts of each were grown in four monoculture plots.
In this example as in many others, companion planting exploits the differences between crops. Lettuce and basil require mostly nitrogen, but the others do not, and since lettuce is primarily an early crop and basil a later one, they will not be competing for this nutrient. Tomatoes are heavy feeders, carrots light feeders. Planting these crops in the same space is possible precisely because they are so different in height, nutrient need, growth pattern, and season.
Before starting any garden project, it’s helpful to check with local experts to find out what plants grow best in your area and what harmful insects or diseases you should be on the lookout for. Two excellent sources, available throughout the U.S., are the Master Gardeners program, run by university extension services, and Native Plant Societies. Local nurseries and garden shops can provide invaluable advice as well, but you may have to hunt around for the ones that have truly knowledgeable staff.
Companion planting is not a sure-fire single-item ticket to garden success. It must be accompanied by other good gardening practices, such as deep and timely watering and careful spacing of appropriate plants. Soil is particularly important in companion gardening, since intercropping and sequential cropping make more demands on the soil than does conventional gardening.
Finally, each garden is unique, and what works for one person often doesn’t for another. It’s important, therefore, to keep good notes and to experiment with different companions from year to year until you find the best recipe for your own success.
Lists of traditional companion plants are available on many websites. Those hosted by Gardens Ablaze stand out for several reasons: they’re more extensive than many, they include not just compatible but incompatible plants, they appear to be compiled by an actual gardener from actual experience (no name is given, so this isn’t absolutely clear), and there are several lists, not just one. You can choose to focus on controlling pests, attracting beneficial insects, or increasing yields.