Companion Planting: What Is It?:
The Roman agriculturist Varro noted that "walnut trees close by make the border
of the farm sterile." In doing so, he probably made the first reference to companion
Surprisingly, Varro was right - walnut trees are not the kind of thing you want
around the border of your farm. Their roots release a chemical called juglone
which inhibits the growth of anything else nearby. This kind of anti-social (allelopathic,
properly speaking) behaviour is behind such wisdom as "keep dill away from carrots"
and "pumpkins and potatoes don't mix." A similar piece of advice, "don't rotate
radishes and cabbage" is based on the fact that the cabbage root maggot is the
same bug as the radish borer - so your radishes would be destroyed.
So far, so negative. However, companion planting can have extremely positive effects
in several different ways. The most pleasing to the organic farmer must be symbiosis
- organisms working together for their mutual benefit. Among the roots of legumes,
such as beans, peas and clover, live Rhizobium bacteria. These consume some of
the legumes' excess sugar while ploughing otherwise unusable atmospheric nitrogen
into the soil - so that neighbouring plants (not just the hosts) can flourish.
Peas and carrots work well together for this reason.
Pest control is another important factor. Instead of spraying crops to get rid
of pests (which is at best over-reaction: less than 1% of all garden insects are
harmful), it is possible to dissuade or confuse harmful bugs. Nasturtiums, for
instance, repel the cucumber beetle; some marigolds can be used as a lure for
destructive aphids. A variation on this is planting strong-smelling, pollinating
herbs among (say) cabbage; such plants attract predatory insects which will eat
the pests but not the crop.
Finally, the physical interaction between some plants can be of great benefit.
Tall, sun-loving crops can shelter more sensitive, low-growing cultures; in some
instances the foliage can act as a deterrent for pests. Planting deep- and shallow-
rooted crops together is often a good plan - because they are not competing for
space, they grow unhindered, and can even inhibit undesirable weeds. Radishes
and carrots work together physically, because radishes germinate quickly and loosen
the soil for slower-growing carrots.
Of course, it is not always so simple. Insects repelled by one plant may well
just wander across the garden and find another to harm - the location and manner
of planting the repellant are as important as its beneficial properties. Orchard
brambles are another thorny (ahahaha) issue: they harbour useful predators, but
also cause disease in summer.
For all that, little scientific research has been done on companion planting -
most guides are based on traditional methods which seem to work. Some biodynamists
point to the sensitive crystallisation technique developed by Dr. Ehrenfried Pfeiffer,
which entails studying the patterns left as solutions of plant extracts and salts
react. This is often - legitimately - decried as bad science, although that's
another argument. Let's just say it can serve as a useful rule of thumb.
Such rules of thumb are the basis of companion planting, and often organic farming
in general. Who needs science when you have old wives' tales that work?!
County Garden. A good starting point for tips on companion planting, and some
Technology Transfer for Rural Areas. ATTRA's well-documented guide to the
science behind companion planting.
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