We have all heard of companion plantings. “Roses love garlic.” Right? So what is the scientific basis of that? Do plants really complement each other? I thought it might be interesting this summer to explore a little of this lore. As a science education organization, well, we just really ought to know if roses really love garlic!
I’m sorry to tell you that apparently it isn’t as simple as roses loving garlic and, in fact, there is very little scientific evidence to support the “parings” of plants in many of the companionship charts we all see. But that does not mean that there is no scientific evidence at all to support the concept of companion planting. It’s just not as simple as roses loving garlic. Deeper thinkers on the subject of companion interactions usually break the idea down into three component interactions: physical interactions, biological interactions and chemical interactions.
Physical interactions speak to the idea of such things as root depth and canopy on plants. Plants with different root depths can often do well as companions to each other since they are not competing for the same root space. Similarly, the spread of one plant’s leaves and branches may provide needed shade to a companion or conversely may interfere with the companion’s need for sunlight by shading them out. So, do roses love garlic because garlic tends to have a narrow, shallow root system while the rose, being a bigger plant, has a wider root system? At least there is some intuitive logic to that.
Biological companion planting is probably what you think of when you hear the term companion planting and most of it has to do with insects. Haven’t we all heard that you should plant marigolds in your garden to “trap” bad bugs before they get to your plants? And then we all know that you should plant umbels (which are plants that have flat tops) to attract good bugs to eat the bad bugs. And then there is the idea that you should plant aromatic herbs around your garden to serve as natural insect repellants. I’m sorry, folks, but the latest research doesn’t support any of that. No test to date has been able to demonstrate that planting one plant near or around another has any influence on insect control at all. In fact, it really appears that maybe those marigolds, aromatic herbs and umbels actually do more to attract you than anything else and you may well be the best companion of all for your plants!
Chemical interactions may be the single area where there is some science to back up the idea of companion plants. Chemical interactions include such things as the benefits that plants get from nitrogen fixing neighbors in the garden. We all know that peas, beans, clover and alfalfa fix nitrogen and this is very beneficial to the plants that grow around them and in the same spot after them.
Of course, we know from our soil studies that most plants have relationships with microbes that assist them in such activities as mining chemicals from the soil and air and that the activities of these plants benefit other plants in the immediate areas. And marigolds do produce chemical thiophenes that deter certain soil nematodes from attacking the marigolds. Perhaps a plant growing near a marigold could benefit from that although I haven’t found a study to support the idea.
I’m sorry to tell you that apparently it isn’t as simple as roses loving garlic and, in fact, there is very little scientific evidence to support the “parings” of plants in many of the companionship charts we all see. But that does not mean that there is no scientific evidence at all to support the concept of companion planting. It’s just not as simple as roses loving garlic. Deeper thinkers on the subject of companion interactions usually break the idea down into three component interactions: physical interactions, biological interactions and chemical interactions.
Physical interactions speak to the idea of such things as root depth and canopy on plants. Plants with different root depths can often do well as companions to each other since they are not competing for the same root space. Similarly, the spread of one plant’s leaves and branches may provide needed shade to a companion or conversely may interfere with the companion’s need for sunlight by shading them out. So, do roses love garlic because garlic tends to have a narrow, shallow root system while the rose, being a bigger plant, has a wider root system? At least there is some intuitive logic to that.
Biological companion planting is probably what you think of when you hear the term companion planting and most of it has to do with insects. Haven’t we all heard that you should plant marigolds in your garden to “trap” bad bugs before they get to your plants? And then we all know that you should plant umbels (which are plants that have flat tops) to attract good bugs to eat the bad bugs. And then there is the idea that you should plant aromatic herbs around your garden to serve as natural insect repellants. I’m sorry, folks, but the latest research doesn’t support any of that. No test to date has been able to demonstrate that planting one plant near or around another has any influence on insect control at all. In fact, it really appears that maybe those marigolds, aromatic herbs and umbels actually do more to attract you than anything else and you may well be the best companion of all for your plants!
Chemical interactions may be the single area where there is some science to back up the idea of companion plants. Chemical interactions include such things as the benefits that plants get from nitrogen fixing neighbors in the garden. We all know that peas, beans, clover and alfalfa fix nitrogen and this is very beneficial to the plants that grow around them and in the same spot after them.
Of course, we know from our soil studies that most plants have relationships with microbes that assist them in such activities as mining chemicals from the soil and air and that the activities of these plants benefit other plants in the immediate areas. And marigolds do produce chemical thiophenes that deter certain soil nematodes from attacking the marigolds. Perhaps a plant growing near a marigold could benefit from that although I haven’t found a study to support the idea.