Many curious gardeners will often find themselves wondering, “Is it OK to plant carrots with turnips, or some of these next to some of those?” Just receiving a simple yes or no answer doesn’t really tell a gardener much, so with this article, I want to dive deeper and explain the science behind companion planting.

Growing two or more different plants in the same garden or container raises a few concerns. Will one or more of the plants interfere or compete with another? Will one plant provide benefits to the adjacent plants? Before making these determinations, you need to look into things like mutual climate co-operation, nurse cropping, trap cropping, symbiotic nitrogen fixation, biochemical pest suppression, attraction of beneficial insects and biodiversity. Phew! Let’s look at how these work individually.

Mutual Climate Co-operation

Mutual climate co-operation refers to planting crops that, when they become full size, complement the needs of plants growing around them. A good example of this is planting tall, sun-loving plants alongside short, shade-tolerant plants. These combinations will help both plant types produce better yields, and also provides more pest control benefits overall.

Nurse Cropping

A variation of mutual climate co-operation, nurse cropping refers to the use of annuals to protect perennials. Annuals are planted to help establish sun-tender perennials until they can handle the greater sun radiation or wind on their own. This same principle is applied when an annual is planted to reduce soil erosion, protecting the perennial’s root system.

Trap Cropping

Trap cropping refers to the planting of a specific crop—a decoy plant—that will attract certain pests to itself, away from more valuable crops. For example, collards draw certain moths away from cabbage. This is different from creating a beneficial habitat in that one crop is essentially sacrificed to protect another. Another example of trap cropping is planting sunflowers along the perimeter of an outdoor garden to protect the smaller plants growing beneath them.

Symbiotic Nitrogen Fixation

Plants cannot survive without nitrogen, yet they cannot use it in the gas form abundant in air. Instead, living organisms use the ammonia form of nitrogen to manufacture the proteins and other nitrogen-containing nutrients needed to survive. Peas, beans and other legumes have the ability to fix atmospheric nitrogen using small growths on their roots called nodules.

Within these nodules, nitrogen fixation is done by bacteria, and the ammonia form of nitrogen produced is absorbed by the plants, which can benefit neighboring plants not capable of doing this. Plants that get nitrogen through symbiotic nitrogen fixation will require less nitrogen fertilizer.

Biochemical Pest Suppression

Some plants exude chemicals that can protect neighboring plants. For example, marigolds release thiopene, an aerial pest suppression that repels nematodes. In addition, some plants emit a pheromone that confuses male insects, causing them not to mate. Artemisia is great at repelling rabbits and other animals that are a nuisance to your garden. Make a border of it to discourage their presence.

Another form of biochemical suppression is a phenomenon called allelopathy. Allelopathy is a term used to describe the release of chemicals from the root system to discourage competition, and in some cases, insects. These chemicals can have a positive or negative influence on surrounding plants.

Those that have a negative influence are an important part of other plants’ defense, reproduction and growth systems. Lantana, for example, puts out chemicals that will reduce the spread and growth of various weeds. Broccoli is also allelopathic and deters the growth of other cruciferous crops around it, like cabbage, cauliflower and Brussels sprouts.

Attracting Beneficial Insects

Also known as habitat influence, this refers to neighboring plants that attract various beneficial insects, predators and parasites that consume or destroy insects that might otherwise damage the protected crop. This strategy is used when the neighboring plant lacks the ability to do so on its own.

To determine what type of beneficial insects you want to attract, get an idea of what pest insects you are likely to encounter. Yarrow and dill attract ladybugs, and just about everyone knows how helpful these little creatures can be. Learn more about what these and other beneficial insects do to help in the garden, then do things that attract and keep them around.

Biodiversity

Many insects and disease organisms prefer specific plant species. For example, the leaf-footed beetle sticks primarily to pomegranate trees, and horn worms adore tomatoes so much they are commonly called tomato worms. By mixing up your crop types, you are reducing the likelihood of a massive infestation that might come after a single dominant crop. If you want to know what crops are related to others, learn the botanical names of your plants.

Knowing these Latin names will help you become more aware of which plants are similar, helping you increase the diversity in your garden. Attempt to use plants with different generas. For example, every kind of apple tree has the genus Malus. Their common names may seem diverse, but the plants are quite similar.

Companion Planting in Hydroponics. Is it Possible?

When growing hydroponically, or with soilless media like clay pebbles, coco coir or stonewool, each plant tends to have its own container, unlike plants grown in a raised bed of soil. In this case, curious gardeners might wonder if companion planting is relevant. The answer is yes.

For instance, if one plant’s roots are putting out allelochemicals, these chemicals are likely to be getting into the recirculation tank, where they will later be exposed to other plants feeding from that same tank. Keep this in mind if you are recirculating your water.

There are many other influences from nearby plants besides allelopathy. Is your greenhouse or growroom completely free of insects? In most cases this is difficult or impractical to achieve and maintain. Even though you may be growing in a reasonably closed environment, you will probably find yourself dealing with insects.

If you want your veggies to flower and set fruit, you will probably be wanting pollinating insects or you will be doing all the pollinating that needs to be done by yourself. So, here too, soilless gardeners will ultimately find that companion plants can provide many benefits in a controlled environment.

Additional Companion Plants to Consider

If you intend to grow tomatoes, consider basil a good neighbor. It will help deter flies, hornworms and mosquitoes. If you are growing carrots, use chives to chase away aphids, mites and nematodes. Marigolds are a great companion for most any plant as they discourage a number of damaging insects. Growing cabbage? Plant some mint nearby and keep the moths, aphids and beetles away. Also recall the traditional Three Sisters combination of corn, beans and squash. These three crops grow in perfect harmony.

Plant Combinations to Avoid

There are some plants that won’t get along, no matter how hard you try. If you’re growing legumes, avoid planting onions or garlic in the same rooting area. Also, a rooting conflict will be likely if you plant sunflowers with potatoes. Corn and tomatoes are not going to get along well, either. Though the list of non-companions is rather short, learn more about plants that are allelopathic and apply that knowledge in your garden to avoid creating bad garden marriages.

Diversity is a great way to ensure your garden is healthy without having to do too much work or use too many pesticides. Understanding how one plant affects another can help you conduct more detailed research about the crops you intend to grow next. Agriculture is a science and you will find your future garden is much more successful when you continue to learn and take all of these things into consideration.