Wednesday, March 21, 2012

Companion planting

So as many of you know, the nemesis of the garden since it's very creation has been Anasa tristis, common name: the squash bug.
Cue dramatic music.

These insects have specialized mouth parts that can suck the juices out of all kinds of summer squash, cucumbers, melons and winter squash such as pumpkins. They also multiply rapidly, can travel fairly long distances, and can overwinter well. Overall, they are one of the nastiest pests for home gardens.

Last summer, due to some already weakened squash plants, these bugs decimated the garden. This summer, we are looking at many different options for opposing the squash bug offensive. First, we are going to plant many types of squash. This diversity should help us find out which squash are more resistant, and also increases our likelihood of getting some sort of harvest. Second, we are going to keep our squash as healthy as possible. That way the squash plants will be able to put up a much better fight. Third, we are going to keep an eye out for the bugs. Last year, the bugs snuck up on us (did I mention they reproduce quickly). Hopefully, if we act quickly this year, we can keep the squash bug population low enough that they do not kill our squash plants.
There is one more method that gardeners can use: companion planting. The theory behind companion planting is that certain plants can be planted together to help fix an issue. The most commonly stated example is the use of corn and beans together. Corn provides a pole for bean plants to climb up and beans fix nitrogen to provide fertility for the corn. Companion planting can also be used for a variety of other issues such as pest prevention, and providing shade (especially in our hot, dry summers).

What I was interested in was pest prevention. I wanted to see what could be planted with squash plants to prevent or at least slow down our eventual squash bug invasion. Fortunately for me, gardeners have embraced the internet as a tool for sharing advice and stories so simply google-ing "squash bug companion plants" provided me with more than enough stories. As it turns out, the main plants for driving away squash bugs are nasturtiums, radishes, and tansy. The good news is that we already have plenty of the first two around the garden.
The bad news is that I noticed a pattern in these online stories. The companion plants did not seem to work. Story after story, gardeners lamented the damage and eventual death of their squash plants despite their best planning. Strangely enough, their response was consistently to try again next year using companion planting. Maybe there were not enough radishes? Maybe they were too far away? Maybe I should try tansy instead?

Reading these stories left me with a crisis of faith. It seems as though companion planting might not help. In fact, crowding squash with other plants might actually be causing the squash stress because it has to compete with other plants. I therefore whipped out my trusty "google" and searched for "companion planting evidence." As is usual with the internet, I found writers insisting that companion planting is completely proven as useful by scientists and others admit that they use companion planting despite a lack of hard scientific evidence.

Therefore, I jumped to the next step: looking at the scientific evidence. I read various published peer-reviewed studies involving companion planting. One of the first publications was a harsh but accurate critique of companion planting. It is a little dry and a little opinion based, but I highly suggest browsing it to learn a little more about the science behind companion planting. I found that this article is supported by many of the studies I was reading in the scientific literature. Most studies found no interaction between plants, but a few found that inter-planting two or three crops helped keep pest levels lower. I began to get frustrated because I wanted to end this blog post with a decisive conclusion. I wanted evidence saying that "yes companion planting works" or "nope, just a myth." In the end, companion planting does, in theory, work but the way gardeners are currently using it is doomed to be ineffective.

There are a few reasons current companion plantings probably won't work:
BEWARE: I am about to commit natural garden heresy. Please don't judge me too harshly.

First, most of the "traditional" companion plant pairings don't work. Any anthropologist could tell you that traditions can carry on even when they no longer have any value. It's what makes our lives so darn interesting.

Second, strong smelling plants do not make superior companion plants. It is intuitive for a gardener to think that since a plant is spicy, bitter, or fragrant then it will "mask" the smell of a companion plant or be toxic for pests. However, the world looks, smells and tastes completely different to other organisms. Dogs can hear sounds that we can't and can smell many times better than any human. Birds can see colors that we can't and some can even sense magnetic north. Any characteristic that will effect a plant pest will most likely be undetectable by humans.

Third, there is a technique called trap cropping where a gardener plants crops that pests like more. For example, I could plant a squash variety that pulls in squash bugs so that one plant takes most of the damage. There are a few problems with this. The most notable is that traps, be definition, have a punishment for what gets trapped. To use a mousetrap analogy, these gardeners are placing out a piece of cheese and calling it a trap. Most likely, you are just delaying the issue and also making it worse in the future.

Phew, I do want to end this on on positive note. There is one way in which companion planting always works. Companion plants act as physical barriers for pests. Most damaging garden pests are specialists that can only eat one type of plant (example: squash bugs make terrible corn bugs). Therefore, the more time our squash bugs have to travel around to find a squash plants, the better off our garden will be. Therefore, intermixing plants in your garden is very helpful. Just don't spend too much time pouring over charts of companion plants.

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