Companion gardening for organic food from the garden

Companion gardening improves plant health and controls disease. It also helps with weed control and optimized productivity. However, it is mostly presented as a simple listing of which plant to put with what - perhaps including some comments about the benefits associated with each combination. The aim on this page is to go a step or two further and explain how companion gardening works in a way that will let you work out your own combinations and get all the benefits - even with plants that are not on the lists you may have found elsewhere.

The reason for doing this this way is twofold. The first, there is little point in simply duplicating the information found on other sites. However, the more important reason is that Companion gardening sits at the very core of ecological gardening. It is not for nothing that plants and animals within an ecosystem are referred to as 'Communities' - there inter-relationships are complex, subtle and often far reaching.

We often think of companion gardening only in terms of associations that provide benefits in terms of pest and disease control, but it goes much further than this. Any plant with a symbiotic fungus growing around its roots could be regarded as a companion planting. So too could vegetables and herbs that have better quality characteristics (especially where their so-called 'functional food' qualities are concerned) when grown with other plants is also a good companionship arrangement.

To illustrate this, one of the most amazing examples I encountered some years ago came from research on Oak trees in England. Oaks are beset by a range of pests that all await the opening of buds in Spring. The trees delay this as long as possible in an attempt to reduce the pest population through starvation. However, the buds do open and there are always some bugs waiting to pounce. But as they fed on the new leaves, researchers found the trees downwind suffered less. It turns out that the damaged leaves released a chemical signal to 'warn' others of the approaching infestation. The other trees responded by increasing tannin levels in their leave, so making them inedible to the pests. The trees were being companions to each other.

The structure of Companionship

The relationships we make use of in companion gardening are not accidental or random, they exist between plant groups that have been evolving in company with each other for a very long time.

There are two types of companionships - general and specific. An example of the general kind is the effect Marigold roots have on nematodes in the soil. This occurs regardless of what other plants are present.

An example of the specific kind is that seen between asparagus and tomatoes. While not strictly symbiotic, each plant in this partnership benefits from the presence of the other.

One interesting point about this combination is that these two plants come from different parts of the World. Tomatoes are from South America and Asparagus originated from countries around the Mediterranean. So how can it be that these two get along so well. The answer is that the relationship exists not just between these two species, but between their families.

Knowing this, we could expect Chillies, Capsicum and Eggplant may also do well with Asparagus. Even Potatoes may be included here - except differences in cultural requirements may keep them apart (digging spuds from under asparagus would probably not be practical.

On the other hand, onions and garlic also benefit from being grown close to tomatoes and we could reasonably speculate that the Saffron crocus could (theoretically) be another.

Another plant that does well with tomatoes is Basil. Since this plant belongs to the Mint family, other herbs in this group (mint, oregano, thyme, etc.) could might be expected to behave in a similar way.

Plants that tend to be good general companions tend to come from the inherently poisonous plant families - daisies (eg. Marigold, Pyrethrum), that produce root exudates and they grow; and crucifers (Broccoli, Turnips, etc.), that produce bio-fumigants in the soil as crop residues decompose.

Some groups have members in both camps and so are especially useful for companion gardening. The nightshade family (potatoes, tomatoes) and Legumes (Peas and Beans) are - generally speaking - the two most toxic plant families on Earth. Of course, our vegetables are exceptions to this rule. In each group, some are general companions and others specific. So chillies are better as general companions and tomatoes more specific. Likewise, broad beans (which also make a good green manure crop) are good general companions, but beans are more specific.

As a general rule, general companions are most useful in the regular crop rotations of a vegetable garden, while specific companions find a place in the herb and perennial gardens.

For more information, here is a good list of companion gardening plants.