Companion Planting

Companion planting is touted as an easy way to make plants help each other to grow faster and better. The problem is, most of the claims made about companion planting are bogus.


Companion planting fallacies
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Companion planting enthusiasts claim that different plants “like” and “help” each other when grown together. While it feels nice to believe there is a magic relationship that occurs between these ‘companions’, nearly all of the lists you will find on the Internet are not based on scientific research. Most of this useless information is based on a 1930’s study by Dr. Ehrenfried Pfeiffer. He used chromatography* in something he called the “sensitive crystallization method” to determine which plants worked well together. Unfortunately, Dr. Pfeiffer’s results don’t have anything to do with how plants actually grow.  

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* You can conduct your own chromatography experiment by crushing various leaves or flowers and mixing them with water. Then suspend a white coffee filter over the solution, with just the bottom edge touching the liquid. Over time, the various components will be drawn upward into the paper. Since different compounds have different weights and colors, the results look really impressive. This is a great kids’ activity!

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History of companion planting

Over 5,000 years ago, the Iroquois and other native Americans sowed corn, pole beans and squash together using the Three Sisters Method. Corn grows tall, providing the nitrogen-fixing pole beans with a trellis. The squash shades the ground, reducing competitive weeds and retaining moisture. As an added benefit to gardeners, eating corn (a seed) with beans (a legume) provides a complete dietary protein! This is an example of science-based companion planting. More accurate terms for the concept are polyculture or intercropping.


Benefits of intercropping

Folklore and mysticism aside, intercropping improves pest control, maximizes production, increases pollination, and provides better habitat for more biodiversity. Here are the real benefits of putting the right specimens together:

1. Spatial interaction
2. Trap crops
3. Chemical warfare
4. Nitrogen fixation
5. Biodiversity
6. Nurse cropping
7. Security

Getting started with intercropping

As seed catalogs begin to arrive each spring, many gardeners are thinking about garden design and the placement of the next season’s crops. Rather than following corporate agriculture’s lead, with its heavy focus on monoculture and the resulting susceptibility to pests and diseases, knowledgable gardeners recognize the value of polyculture. Polyculture more closely follows nature’s tendency to combine many different species in a single space. Let’s take a look at the various factors that meet the scientific criteria to actually make a positive difference in the garden.


Spatial interaction

Some plants need shade, while others grow straight and tall. Combining shade-loving plants, or climbing varieties, with taller specimens can help you make the most of a growing area. As in the Three Sisters Method, corn and sunflowers not only provide fresh food, but their sturdy stalks make excellent supports for climbing plants, such as pole beans and cucumbers. You can further support those plants with ground-shading squash or fast-growing lettuces. It may help to think of an area in terms of layers: the uppermost canopy, climbers, low growers, and even root crops can often be grown in the same place, at the same time, increasing the output of your soil. 


Trap crops

I have an ornamental apricot tree that grows in my backyard. Since it does not produce fruit, it’s not my favorite. In fact, I nearly took it out. Then, I saw that it was covered with aphids. At first, that seemed like yet another reason for taking the tree out, but then I learned about trap crops. Trap crops distract pests away from food crops. By providing a rich food source for these pests, my other plants were more likely to be ignored. Also, this heavy infestation acted like a restaurant road sign for beneficial insects, such as lady beetles and lacewings, inviting them to come and stay a while, which they do. Commercial growers have learned to take advantage of trap crops to lure pests away from crops. These heavy concentrations of pests are then killed off with insecticides or even vacuumed off of plants!


Chemical warfare

Biochemicals are produced by some plants that either benefit or harm neighboring plants. This is called allelopathy. In some cases, such as many aromatic herbs, these secondary chemicals are offensive to insects pests or herbivores. This chemical warfare is also used by sorghum to reduce nearby weed populations. Common pea plants also use chemicals to inhibit the growth of nearby lettuce, wheat, cress, or sorghum, along with weeds. On the flip side of the same equation, allelopathy research has shown that growing garlic and eggplant together benefits both plants. 


Nitrogen fixation
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Legumes have evolved a unique relationship with certain bacteria that allows them to fix atmospheric nitrogen into a form usable by plants. Neighboring plants benefit from this relationship by gaining access to more nitrogen. This benefits remains in effect until the legumes begin going to seed. At that point, there is no excess nitrogen to be shared. You can improve seedling growth of many other crops by adding an occasional lima bean, fava bean, alfalfa, or other legume plant in the mix, or using these beneficial plants as a cover crop or green manure.


Biodiversity

Diverse habitats attracts beneficial insects, such as predators and pollinators. Monoculture, growing the same crop over an area, is prone to serious pest and disease problems (which is why chemicals are used so frequently in modern agriculture). The diversity created by growing many different plants in the same space can confuse pests into looking elsewhere for a host. Also, one plant may attract a pollinator, which is then given easy access to the plants in need of pollination.

Photo of mixed planting (Kate Russell)

Nurse cropping

Nurse cropping uses larger plants to provide protective shade and increased moisture for smaller or more sensitive plants. Tall, sturdy plants, such as corn, Jerusalem artichoke, and sunflower can be grown close together as a windbreak or to provide shade. In another form of nurse cropping, fast growing annuals can be planted alongside tender, more slow-growing perennials, to provide shelter, or to reduce erosion, until the woodier plants are able to protect themselves. Some plants, from arid climates, are able to accumulate salts from the soil, making them good ‘companions’ to more salt-sensitive plants.


Security

Installing a wide variety of plants in an area also provides security. If one crop doesn’t survive, the other might. While most gardeners do not look to their plantings as a critical food source, this is not always the case. When a garden provides the majority of a family’s food, that security can be critical.


Succession planting is similar to intercropping in that plants, such as lettuce and spinach, are installed successively to provide an ongoing crop.


The science behind which plants perform best together is still being researched. Bottom line: pay attention to your plants and do what works best for you, your soil, and the varieties in your garden. And ALWAYS verify information before using it or sharing it.