Well, most of the spring planting has been finished and we’re starting to harvest a lot of produce. Some of the very early, cool-weather crops are finished and being pulled up. This leaves us with open ground and the problem of what to do with it. That got me thinking about what goes with what, so I decided to write about companion plants today. So what comes to mind when you hear the words, “Companion Plants”? If you’re like most people, including me, you think of plant x improving the harvest or reducing the pest load of plant z. But how does that happen? Companion plants assist in the growth of others by providing nitrogen, and other nutrients, attracting beneficial insects, repelling pests, providing shade and/or support, or by maintaining moisture in the soil. Many plants we call weeds may also function as companion plants. Some examples of these are Russian thistle, chickweed, and dandelion.
How do you learn what plant actually helps what other plant? Let’s go to the internet and see what we can find. What I found were lists -- lots of lists: lists that are inconsistent, lists that disagree totally. Let’s try to get to the bottom of these lists and figure out what really helps what. While exploring the idea of companion plants I learned that our ancestors had some strange ideas when it came to plants. Some of the plant combinations in these lists actually come from medieval writings when plants were believed to be governed by planets. After all, there is only one letter different between ‘plant’ and ‘planet.’ [Smile] Later lists were made by writers who believed in the “Doctrine of Signatures” These people believed that God made plants in a way that signified their uses. A heart shaped leaf, of course, meant that the plant was good for the heart. Yellow wood or juice meant that the plant could be used to treat jaundice. Get it? Was it right? Well, we don’t believe it any more, do we? Yet some of these beliefs linger on in these lists. I think that the most common way that a plant gets on a list as a ‘good’ companion plant goes like this. Joe the Writer usually plants his tomatoes near the carrots. This year, for some unknown reason, he plants the tomatoes next to the zucchini – lo and behold – he gets a bumper crop of tomatoes. Why? Who knows? But our gardening friend, Bob the Writer, comes to the conclusion that it’s the result of planting them near the zucchini, and another companion plant is born. Can we decide that one plant makes a good companion for another plant based on one growing season in one locale? No. Emphatically, NO! To be valid, a study of this sort should be large, multi-season, uniform in measurements, the tester(s) should be known, and the results – positive or negative - should be published or otherwise distributed. Are there any studies like this? Yes, there are. Not nearly enough, in my opinion, but there are some. Unfortunately, there are a whole lot more done by Bob the Writer and his pals.
Let’s look at a classic example of companion plants, the Three Sisters of the Indian tribes of the American Southwest – corn, beans, and squash. The farmer would start by planting the corn, then when the corn plants were about 6 inches high, would plant the beans around the corn plants. Later when the beans were up, he would plant the squash seeds among the other two. Not only did the corn provide a trellis for the beans to climb, but the beans helped to stabilize the corn stalks against wind damage. Corn has short, shallow roots and is easily knocked over by strong winds. As we know, the beans fixed nitrogen, thus providing an important nutrient for itself and the other two plants, and the squash acted as living mulch, preventing water from evaporating. In addition, the living mulch kept the soil cooler, the spines on the squash kept certain pests away, and the large leaves and tall stalks added organic matter when turned in at the end of the season to improve the soil for the next crop. The companionship continued into the plants’ interactions when providing nutrition for the humans who planted and ate the crops. The corn and beans combined to provide a complete array of amino acids, thus creating complete protein. The squash and beans provided carbohydrates completing the nutrient profile of the meal.
Some plants have intrinsic systems that repel pests. These systems can be used to our advantage by planting the pest-repelling plant near other, pest-sensitive plants. A commonly cited example of this is marigold. A good scientific study was performed testing the ability of marigold and calendula to reduce the population of aphids and flea beetles when intercropped (companion planted) with cabbage. The study was titled, “Effect of intercropping white cabbage with French Marigold (Tagetes patula nana L.) and Pot Marigold (Calendula officinalis L.) on the colonization of plants by pest insects”. It was published in Folia Horticulturae, Ann. 21/1, 2009,95-103. The study was large, took place over three years, the test plants were randomly chosen, and the pests were carefully counted including eggs, larvae, pupae and adults. The results showed that calendula was actually better at reducing the infestation of aphids then marigold was, but marigold was better at reducing the infestation of flea beetles. Both did adequately reduce the numbers of insect pests.
Let’s start by thinking about what makes a good companion plant. First, the two plants should not be in direct competition for all the same environmental conditions or nutrients. For example, do you think that two plants, one with a short, spreading habit and the other with an upright habit, both of which like full sun and are heavy feeders for nitrogen would make good companions? Probably not. One would shade the other and they would compete for the available nitrogen. So we can start with simple things like sun/shade, heavy feeder/light feeder, or heavy nitrogen feeder/nitrogen fixer, spreading root system/deep taproot to start making decisions about what to plant near what. If you want a ‘companion’ that reduces pests, choose one with a strong smell, like marigold or garlic. Good old tomatoes are a great companion crop for asparagus because they produce solanine, a glycoalkaloid poison. This compound is harmless to humans, but deadly to the asparagus beetle. Sometimes the benefit is mutual as with bush beans and potatoes. The potato protects the beans from the Mexican bean beetle, while the bean protects the potato from the extremely destructive Colorado potato beetle. Flea beetles are attracted to eggplants, radishes, tomatoes and turnips and chew small holes in the leaves. Interplanting with lettuce or mint will control the pest. Marigolds exude thiopene into the soil which reduces the population of soil nematodes, making marigolds a great companion for tomatoes which are known to be bothered by nematodes. Salvias are also said to reduce the nematode population. Perhaps it would be beneficial to plant a stand of asparagus edged with tomatoes and marigolds. The beautiful nasturtium is an effective trap crop for red spider mites, aphids, and white fly.
Often a pair of plants may provide help for each other, like carrots that are bothered by carrot flies and leeks that are bothered by leek moths and onion flies. By interplanting carrots and leeks both plants are protected. Apparently, each plants gives off distinctive odors that attract its particular pest(s), but the pests either cannot tolerate the scent of the other plant, or the combination of odors confuses the pest, preventing it from finding its preferred crop. When carrot or leeks are planted alone (monocropped), the insect pest detects a smorgasbord and invites all its friends. To summarize, companion planting works in a variety of ways. Some of these are: As trap crops, where one plant is chosen to attract pests from a nearby crop. For example, collards will attract the diamond back moth away from cabbage. By biochemical suppression of pests, in which chemicals are exuded from the roots and stems of one plant and kill or suppress a pest of another plant. By nitrogen fixation, where legumes take nitrogen from the atmosphere and ‘fix’ it in the soil in a form that can be taken up by themselves and other plants. By physical spatial interaction, in which a tall sun-lover grows happily next to a low-growing plant that prefers some shade. By establishing a habitat to attract beneficial insects for the benefit of a second plant. By nurse cropping, where taller plants with thick foliage protect other plants by shading or shielding them while they are young. This is similar to physical spatial interaction, but is used to protect certain crops, like oats sown with alfalfa so that the alfalfa can become established in the protective shade of the oats, and lastly, By ensuring diversity, a more general approach that can confuse insect pests and helps to insure that at least some of the crops will produce a yield.
The Problem with Lists List 1 says Plant Beans with Carrot, cabbage, cauliflower, cucumber, celery, corn, marigold. List 2 says: Beets, cabbage, carrots, catnip, cauliflower, corn, cucumbers, marigolds, potatoes, savory, strawberries. List 3 says: Beet (to bush beans only), cabbage family, carrot, celery, chard, corn, cucumber, eggplant, pea, potatoes, radish, strawberry. They all agree on carrot, cabbage, cucumber and corn, but while we know that squash works in the Three Sisters, squash is not on any of these lists. Two of the three lists cite beets, but only one mentions that beets are only beneficial to bush beans. Why??? So while lists may give you a starting point, I really think that it is better to search out scientific studies showing real data. In the absence of such data, think about how the plants grow and what they need. Pair tall sun lovers with lower-growing plants that prefer cooler growing conditions and partial shade. Pair deep-rooted plants like daikon radish with shallow-rooted plants like corn, broccoli, or blueberry. Use a lowgrowing, large-leaved plant like squash or comfrey in combination with a taller-growing plant, or even a small tree. Experiment for yourself and above all, KEEP RECORDS so that you can see your results year to year, season to season. In time, maybe you can publish your own list. Until then, Good gardening
How do you learn what plant actually helps what other plant? Let’s go to the internet and see what we can find. What I found were lists -- lots of lists: lists that are inconsistent, lists that disagree totally. Let’s try to get to the bottom of these lists and figure out what really helps what. While exploring the idea of companion plants I learned that our ancestors had some strange ideas when it came to plants. Some of the plant combinations in these lists actually come from medieval writings when plants were believed to be governed by planets. After all, there is only one letter different between ‘plant’ and ‘planet.’ [Smile] Later lists were made by writers who believed in the “Doctrine of Signatures” These people believed that God made plants in a way that signified their uses. A heart shaped leaf, of course, meant that the plant was good for the heart. Yellow wood or juice meant that the plant could be used to treat jaundice. Get it? Was it right? Well, we don’t believe it any more, do we? Yet some of these beliefs linger on in these lists. I think that the most common way that a plant gets on a list as a ‘good’ companion plant goes like this. Joe the Writer usually plants his tomatoes near the carrots. This year, for some unknown reason, he plants the tomatoes next to the zucchini – lo and behold – he gets a bumper crop of tomatoes. Why? Who knows? But our gardening friend, Bob the Writer, comes to the conclusion that it’s the result of planting them near the zucchini, and another companion plant is born. Can we decide that one plant makes a good companion for another plant based on one growing season in one locale? No. Emphatically, NO! To be valid, a study of this sort should be large, multi-season, uniform in measurements, the tester(s) should be known, and the results – positive or negative - should be published or otherwise distributed. Are there any studies like this? Yes, there are. Not nearly enough, in my opinion, but there are some. Unfortunately, there are a whole lot more done by Bob the Writer and his pals.
Let’s look at a classic example of companion plants, the Three Sisters of the Indian tribes of the American Southwest – corn, beans, and squash. The farmer would start by planting the corn, then when the corn plants were about 6 inches high, would plant the beans around the corn plants. Later when the beans were up, he would plant the squash seeds among the other two. Not only did the corn provide a trellis for the beans to climb, but the beans helped to stabilize the corn stalks against wind damage. Corn has short, shallow roots and is easily knocked over by strong winds. As we know, the beans fixed nitrogen, thus providing an important nutrient for itself and the other two plants, and the squash acted as living mulch, preventing water from evaporating. In addition, the living mulch kept the soil cooler, the spines on the squash kept certain pests away, and the large leaves and tall stalks added organic matter when turned in at the end of the season to improve the soil for the next crop. The companionship continued into the plants’ interactions when providing nutrition for the humans who planted and ate the crops. The corn and beans combined to provide a complete array of amino acids, thus creating complete protein. The squash and beans provided carbohydrates completing the nutrient profile of the meal.
Some plants have intrinsic systems that repel pests. These systems can be used to our advantage by planting the pest-repelling plant near other, pest-sensitive plants. A commonly cited example of this is marigold. A good scientific study was performed testing the ability of marigold and calendula to reduce the population of aphids and flea beetles when intercropped (companion planted) with cabbage. The study was titled, “Effect of intercropping white cabbage with French Marigold (Tagetes patula nana L.) and Pot Marigold (Calendula officinalis L.) on the colonization of plants by pest insects”. It was published in Folia Horticulturae, Ann. 21/1, 2009,95-103. The study was large, took place over three years, the test plants were randomly chosen, and the pests were carefully counted including eggs, larvae, pupae and adults. The results showed that calendula was actually better at reducing the infestation of aphids then marigold was, but marigold was better at reducing the infestation of flea beetles. Both did adequately reduce the numbers of insect pests.
Let’s start by thinking about what makes a good companion plant. First, the two plants should not be in direct competition for all the same environmental conditions or nutrients. For example, do you think that two plants, one with a short, spreading habit and the other with an upright habit, both of which like full sun and are heavy feeders for nitrogen would make good companions? Probably not. One would shade the other and they would compete for the available nitrogen. So we can start with simple things like sun/shade, heavy feeder/light feeder, or heavy nitrogen feeder/nitrogen fixer, spreading root system/deep taproot to start making decisions about what to plant near what. If you want a ‘companion’ that reduces pests, choose one with a strong smell, like marigold or garlic. Good old tomatoes are a great companion crop for asparagus because they produce solanine, a glycoalkaloid poison. This compound is harmless to humans, but deadly to the asparagus beetle. Sometimes the benefit is mutual as with bush beans and potatoes. The potato protects the beans from the Mexican bean beetle, while the bean protects the potato from the extremely destructive Colorado potato beetle. Flea beetles are attracted to eggplants, radishes, tomatoes and turnips and chew small holes in the leaves. Interplanting with lettuce or mint will control the pest. Marigolds exude thiopene into the soil which reduces the population of soil nematodes, making marigolds a great companion for tomatoes which are known to be bothered by nematodes. Salvias are also said to reduce the nematode population. Perhaps it would be beneficial to plant a stand of asparagus edged with tomatoes and marigolds. The beautiful nasturtium is an effective trap crop for red spider mites, aphids, and white fly.
Often a pair of plants may provide help for each other, like carrots that are bothered by carrot flies and leeks that are bothered by leek moths and onion flies. By interplanting carrots and leeks both plants are protected. Apparently, each plants gives off distinctive odors that attract its particular pest(s), but the pests either cannot tolerate the scent of the other plant, or the combination of odors confuses the pest, preventing it from finding its preferred crop. When carrot or leeks are planted alone (monocropped), the insect pest detects a smorgasbord and invites all its friends. To summarize, companion planting works in a variety of ways. Some of these are: As trap crops, where one plant is chosen to attract pests from a nearby crop. For example, collards will attract the diamond back moth away from cabbage. By biochemical suppression of pests, in which chemicals are exuded from the roots and stems of one plant and kill or suppress a pest of another plant. By nitrogen fixation, where legumes take nitrogen from the atmosphere and ‘fix’ it in the soil in a form that can be taken up by themselves and other plants. By physical spatial interaction, in which a tall sun-lover grows happily next to a low-growing plant that prefers some shade. By establishing a habitat to attract beneficial insects for the benefit of a second plant. By nurse cropping, where taller plants with thick foliage protect other plants by shading or shielding them while they are young. This is similar to physical spatial interaction, but is used to protect certain crops, like oats sown with alfalfa so that the alfalfa can become established in the protective shade of the oats, and lastly, By ensuring diversity, a more general approach that can confuse insect pests and helps to insure that at least some of the crops will produce a yield.
The Problem with Lists List 1 says Plant Beans with Carrot, cabbage, cauliflower, cucumber, celery, corn, marigold. List 2 says: Beets, cabbage, carrots, catnip, cauliflower, corn, cucumbers, marigolds, potatoes, savory, strawberries. List 3 says: Beet (to bush beans only), cabbage family, carrot, celery, chard, corn, cucumber, eggplant, pea, potatoes, radish, strawberry. They all agree on carrot, cabbage, cucumber and corn, but while we know that squash works in the Three Sisters, squash is not on any of these lists. Two of the three lists cite beets, but only one mentions that beets are only beneficial to bush beans. Why??? So while lists may give you a starting point, I really think that it is better to search out scientific studies showing real data. In the absence of such data, think about how the plants grow and what they need. Pair tall sun lovers with lower-growing plants that prefer cooler growing conditions and partial shade. Pair deep-rooted plants like daikon radish with shallow-rooted plants like corn, broccoli, or blueberry. Use a lowgrowing, large-leaved plant like squash or comfrey in combination with a taller-growing plant, or even a small tree. Experiment for yourself and above all, KEEP RECORDS so that you can see your results year to year, season to season. In time, maybe you can publish your own list. Until then, Good gardening
MaryAnn Armbruster, Ph.D.
MaryAnn Armbruster, Ph.D. is a certified Advanced Master Gardener and member of the Lafayette Parish Master Gardeners.
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