By Cyndy Crist
As we plan our gardens, we consider an array of things. We likely start by deciding what we’d like to grow (with edibles, this obviously is based on what we want to eat), then consider what we can grow given available space, growing conditions, and the time we can devote to gardening. An additional factor we may wish to consider is what grows well together, an approach generally called companion planting.
Companion Planting 101
Many swear by the benefits of companion planting; others doubt that they exist. As a Master Gardener, my first responsibility is to provide advice based on research, and frankly on this topic, it’s pretty mixed.
For example, a research study conducted by Minnesota Master Gardeners several years ago found no benefits of growing several plant combinations, including French marigolds with tomatoes. However, the number of growers who completed the study was small and the only information reported was about yields, not insects.
As the interest in organic growing and strategies like Integrated Pest Management (IPM) grows, so does attention to the potential benefits of companion planting.
If you want to consider companion planting, here are some things you may wish to consider.
Nature and Dynamics of Companion Planting
(1) Plant combinations generally are put into one of three categories: beneficial, compatible, or incompatible (or, said another way, great, okay, and bad partners). For tomatoes, there are a few combinations repeatedly cited as beneficial for tomatoes (and/or for the companion), a few cited as pairings to be avoided, and many identified as generally compatible. We’ll get to those shortly.
(2) Some “companionship” needs to be sustained. For example, in Carrots Love Tomatoes: Secrets of Companion Planting for Successful Gardening author Louise Riotte notes that French marigolds need to be grown for at least a full season before they will control nematodes in the soil. In other words, sometimes it takes more than a short-term romance to realize the benefits of a partnership.
(3) Companion planting is particularly effective as part of intensive or small space gardening strategies. These approaches focus on placing plants close to one another and thus can maximize the benefits of growing them together. This also makes it a good strategy for urban gardeners who have limited space.
(4) Some combinations work especially well in terms of plant placement. For example, one reason why tomatoes and asparagus are great companions is that they grow at different strata, or levels, in the soil, so they don’t compete with each other for water and nutrients, Competition is also reduced because tomatoes don’t need to be planted until the asparagus spears are harvested, and cultivating the tomatoes through the later part of the growing season will help keep the space weed-free.
(5) Sometimes the benefit of a companion extends beyond actually growing together. One example is tomatoes and roses. There is some evidence that the solanine in tomato leaves may help prevent black spot on roses. Growing them close to one another may be beneficial, but Louise Riotte also suggests making a spray from tomato leaves to use on roses. This is an aspect of a beneficial relationship that seems at once more intimate and less companionable than growing in close quarters.
So, which plants love each other, which get along, and which need to be kept apart for their own health and vitality?
Beneficial, Compatible and Incompatible Plant Combinations
(1) Beneficial Companions. Some of the pairings with tomatoes most often cited as helping improve the vigor and/or flavor of tomatoes include bee balm, mint, basil, parsley, celery and borage. Combinations in which tomatoes are the “heroes” that help other plants include gooseberries and peppers (potential protection against insects) and roses (potential protection against black spot).
Asparagus and tomatoes seem to have a mutually beneficial relationship. In Good companions: A guide to gardening with plants that help each other, Bob Flowerdew notes that tomatoes provide protection against asparagus beetles while asparagus returns the favor by killing trichodorus, a nematode that attacks tomatoes.
One tomato companion especially worth highlighting is basil. Not only do these two make for a flavor match made in heaven, which I find reason enough to grow them together, but there is some evidence that basil improves the growth and flavor of tomatoes while protecting them against some insects.
Notably, basil flowers attract braconid wasps, whose eggs laid on the backs of tomato hornworms hatch into larvae that parasitize them. As long as I have enough full sun to keep them happy and healthy, basil and tomatoes will always be close neighbors in my garden.
(2) Compatible Combinations. Among the vegetables and herbs generally considered to be compatible with tomatoes are oregano, carrots, onions, radishes (discouraging two-spotted spider mites), garlic (protection against spider mites), amaranth, chives, stinging nettle, lavender, thyme, and lemon balm.
Flowers identified as workable companions for tomatoes include marigolds (several sources recommend French marigolds as the “work horse” of pest deterrents), geraniums, petunias, nasturtiums, pot marigolds (also known as calendula), and foxglove.
(3) Incompatible Pairings. A few combinations are repeatedly identified as needing to be avoided. Although there is some evidence that the use of tomato leaves can repel cabbage worms, tomatoes and anything in the brassica (cabbage) family have generally been found to be incompatible with each other.
Even though they are both in the nightshade family, tomatoes and potatoes don’t grow well together. Some believe that fennel has an inhibiting effect on tomato growth as well, and it is not recommended to grow tomatoes close to corn because of the similarity of pests that feed on each.
Also, tomatoes are among a fairly long list of plants that won’t grow close to black walnut trees, and they can inhibit the health of apricot trees. Other plants identified by some sources as poor companions with tomatoes are peas, beets, and rosemary.
Why and How Companion Planting Works
Why do many gardeners believe that companion planting works? One belief is that the mix of smells may confuse or even repel insects, many of which operate on the basis of scent. Another is that chemicals in the leaves and/or roots of some plants have a positive impact on others (such as the possible effect of solanine on black spot noted earlier).
A third is that mixed plantings generally “mimic” nature by offering a diversity of plants that attract some insects and organisms while repelling others. This benefit is described clearly in The Organic Gardener’s Handbook of Natural Insect and Disease Control: A Complete Problem-Solving Guide to Keeping Your Garden and Yard Healthy Without Chemicals, an invaluable Rodale publication for those interested in taking an organic approach to gardening.
So, scientifically speaking, the jury may still be out on the impact and value of companion planting, but many gardeners swear it has made a difference in their gardens.
In Designing the New Kitchen Garden: An American Potager Handbook, Jennifer R. Bartley reminds us that there are aesthetic as well as practical benefits to companion planting. This approach, she suggests, “doesn’t require a grand, complex plan” but simply offers a strategy for combining plants in ways that create beauty while attracting beneficial insects and birds that feed on the less friendly insects that chew leaves and damage roots and fruits. That’s reason enough for me.