I will be employing companion planting in my vegetable gardens, but it can also be used when flower gardening and in full-scale farm fields. In fact, 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.
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 others 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.
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. However, several recent studies by reputable universities have established that some companions, at least, actually do help each other. Research dating back to 2004, concluded that inter-cropping basil with either tomatoes or Brussels sprouts improved plot yield. Yet another study conducted in 2005 and 2006 found that dill, coriander, and buckwheat inter-cropped 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.
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.
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.
You can find a comprehensive list of companion plants here at GardeningAustin.com that you can download free of charge but if a long list of compatible vegetables leaves 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.
- Plant cosmos and French or Mexican marigold here and there in and near the garden to repel pests and encourage beneficial insects that prey on them.
- Do the same with chives, garlic, or onions.
- 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 from the same amount of space.
- 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.