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If you do a search for "companion planting", you get millions of hits. However, it's hard to separate out the real legitimate information from the muck. There's a lot of misinformation based on pseudoscientific methods of divining which plants "like" each other.

The academic community seems to prefer the term "intercropping" to distinguish the real science from the hocus-pocus. Unfortunately, most of the legitimate research I've stumbled across seems geared toward disproving what doesn't work rather than finding what does work. So there's far less information available from academia.

Does anyone know of some credible sources that are actively researching this field?

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@KennethCochran What exactly do you mean by "credible sources that are actively researching this field", are you after academic, scientific institutions working in that field of horticultural? –  Mike Perry Sep 9 '11 at 22:11
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@KennethCochran Have you looked through the companion-planting tag here on SE? Rightly or wrongly I like this Companion Planting resource... –  Mike Perry Sep 9 '11 at 22:15
    
@Mike Perry: That's the one I always wind up relying on. –  baka Sep 11 '11 at 12:24
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2 Answers

I take it you're looking for credible information about companion planting that is relevant to the needs of home/ amateur gardeners, like ourselves, as well as to those of the farming community. As you point out, there are many theories, but it's hard to find one that is scientifically proven.

One 'legitimate' instance of intercropping (interesting and based on sound research, but unfortunately not one that will prove useful to most members of this forum) is a habitat-management strategy devised by the Rothamsted Research Centre, UK, to protect maize and sorghum plants from stemborers:

The approach relies on a carefully selected combination of companion crops planted around and among maize or sorghum plants. Both domestic and wild grasses can help to protect the crops by attracting and trapping the stemborers. The grasses are planted in the border around the maize and sorghum fields where invading adult moths become attracted to chemicals emitted by the grasses themselves instead of landing on the maize or sorghum plants. These grasses provide the "pull" in the "push-pull" strategy.

They also serve as a refuge for the borers' natural enemies. Good trap crops include well-known grasses such as Napier grass (Pennisetum purpureum) and Sudan grass (Sorghum vulgare sudanense). Napier grass has a particularly clever way of defending itself against the pest onslaught: once attacked by a borer larva, it secrets sticky substance that physically traps the pest and effectively limits its damage.

The "push" in the intercropping scheme is provided by the plants that emit chemicals which repel stemborer moths and drive them away from the main crop (maize or sorghum). The best candidates discovered so far with the repellent properties are members of leguminous genus Desmodium spp. which is planted in between the rows of maize or sorghum. Being low-growing plant it does not interfere with the crops' growth and furthermore has the advantage of maintaining soil stability and improving soil fertility through nitrogen-fixation. It also serves as a highly nutritious animal feed and effectively suppresses Striga. Another plant showing good repellent properties is molasses grass (Melinis minutiflora), a nutritious animal feed with tick-repelling and stemborer larval parasitoid attractive properties.

Push-pull Habitat Manipulation

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You can filter out a lot of the chaff by restricting your search to the .edu domain, e.g. "intercropping site:.edu" gives:

Both of these papers provide references to dead-tree publications. Access to some of these may require a visit to a university or other large library.

Unfortunately, neither paper is really aimed at home gardeners -- most are geared towards large scale agriculture. And, given that this is where research dollars come from, I suspect that much of the research is going to be targeted to this area. You might, however, be able to find research on vegetable intercropping for large market growers that transfers well to the home garden.

Another possible source of information, and a synonym to search on, is plant "guilds". This is a related, but somewhat deeper and richer concept than interplanting. Permaculture uses the concept of plant guilds to build "stacks" of plants that work well together, from the root zone to the tree canopy. You are likely to find more practical experience than scientific research. Some of it will be very localized -- since the specific plant communities that work well together will be different on different sites. In my experience, this research has less junk science than the biodynamic nonsense that's out there. However, since it is less formal and not coming from a rigorous scientific community, there are fewer references so you can't quickly build a long bibliography of articles to read. A good starting point with lots of tables and plant lists is "Gaia's Garden" by Toby Hemenway.

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