Companion Planting

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I have always wondered about companion planting. I have tried it over the years, sometimes it worked and sometimes it didn’t. Years ago we had a peach tree with leaf curl, so I planted garlic around the base of the tree and the next season the leaf curl was gone. I variably plant basil and tomatoes together sometimes and separately sometimes but have not noticed any difference between the two planting regimes.

With some of the books which have been written and innumerable internet companion planting charts it seems to be: plant this this this = good; plant this with that = bad. This sort of approach seems to me to have some problems attached to it.

The Problems

1. The information is quite often imported from overseas, particularly from the Northern hemisphere and even if it works over there, there is no guarantee that the advice, when transplanted into new continent, climate and ecosystems, will still be effective.

2. The information may be anecdotal in nature, with no confirmation by scientific trials. The assumption tends to be – “it worked here for me… will work for anybody!”

3. There is variable agreement between systems and sources of information. A quick review of some internet charts will show discrepancies. For example on one chart I saw, beans and beets are good companions, but on another chart they were bad companions. Both charts can’t be right.

4. Lack of information – Even if the “plant this with this” were to work there seems to be an enormous lack of information on how this pairing should be used to advantage. Why should they be paired together, who benefits and what do they get? Is a certain pest or disease deterred? Is there an improvement in flavour? How much of each one relative to the other needs to be grown to get the effect? Or is the answer some nebulous “they grow better together”?

In general terms, while it is possible to accept the claims of some of these systems on faith, I would rather see some sort of research to back up the claims. Companion planting is not all “woo” however, there are some plants which have a demonstrated beneficial effect on those around them, and to a lesser extent some have a negative impact.

Good Companions

Good companions are good companions because they exhibit one or more of the following beneficial effects, they –

1.  Disguise plants from pests
2. Encourage pest eating predators
3. Choke out weeds
4. Improve fertility
5. Act as trap crops

1. Disguising plants from pests

This effect usually works in one of two ways – the companion plant makes the target plant look different, or smell different.


Think “camouflage”, the idea is to break up the outline of the target plant so that the pest does not recognise it as it scans for food or a place to lay eggs. This can be achieved by interplanting the target with taller crops to break up the outline, rather than a discrete line or block of the target crop. It may also require planting different confuser crops around the target one.

Example: when planting cabbages (subject to cabbage moth and cabbage white butterfly) interplant with celery and silver beet which will grow higher than the cabbage and break up its outline.


Many pests pick out their mark by smell so that by interplanting your vegetable plots with herbs and other aromatic plants or using them as a border, the pest will be confused by the strong odours of the aromatic plants. Again, you may need to plant lots of confuser aromatic plants around a smaller amount of crop. These strongly smelling crops can actually repel pests as well as confusing them.

Examples – basil, chamomile, coriander, curry bush, dill, fennel, garlic, horehound,  yarrow

2. Encourage pest eating predators

There are two kinds of predators to consider here – insects and birds.

Plants which attract beneficial insects to predate the pest insects in your garden generally provide nectar which appeals predators. They will then either consume the insects as well, or lay their eggs in the pests and allow their larvae to consume them. Although it sounds disgusting, it is the way things work.

Examples: alyssum, calendula, heartsease, Queen Anne’s lace and zinnias

It is good to plant spring flowering natives to attract the types of birds which will help control the pests. The pests start breeding up at a lower temperature than the predators do and with the right natives in place the birds will be attracted and keep the pests in check until the predators have a chance to catch up.

Examples: grevillea, banksia, melaleuca and correas (research which varieties of these natives are spring flowing in your area).

3. Choke out weeds

To get the most out of these plants you need to plant them thickly along the edge you want to defend, eg the edge where the lawn meets the veggie patch or the fruit tree drip line. The edge needs to be planted with the weed chokers in rows three to five plants deep, but some may not be a permanent solution if the issue you are wrestling with is kikuyu grass.

Examples are – comfrey, garlic chives and lemon grass

4. Improving fertility

There are three effects to take into account here – nitrogen fixers, dynamic accumulators and green manures.

Nitrogen fixers (legumes) have a symbiotic relationship with bacteria in nodules on their roots which turn atmospheric gaseous nitrogen into soluble nitrates which plants can use. By growing legumes in the rotation their roots will release the soluble nitrogen after they die or by cutting back the foliage above ground there will be a corresponding die back of the roots. This will result in nitrogen becoming available for surrounding plants.

Examples: peas, beans, clover, vetch and lupins.

Dynamic accumulators are plants which put down a deep taproot allowing them to harvest nutrients below the root zone of the usual crops. To make use of these extra nutrients the leaves of the dynamic accumulator are pruned back and then left on the soil around the plants to be fertilised so that the leaves form a mulch, then break down to release these stored nutrients. Another way is to harvest the leaves and then compost them, before feeding to the plants.

Examples: dandelion, comfrey, stinging nettle and dock

Comfrey is an effective dynamic accumulator

Green manures are grown around plants to form a groundcover, then slashed down before they flower so that the plants will break down and release their nutrients for use by the current or future crops. Green manures are generally a mix of nitrogen fixers and other leafy plants appropriate to the time of year they are to be planted.

Examples: buckwheat, linseed, mustard, oats.

5. Act a trap crops

Trap crops are planted in and around the target crop to attract pests to them and away from the target crop. Of course it is possible that it might backfire and attract pests to your crop if that it your only pest control method operating so it needs to be carefully managed. When pests do cluster around the trap crop, this concentration of pests is highly likely to attract insect or bird predators in the area. Alternatively you could remove the trap plants, complete with bugs, for destruction elsewhere.

The use of trap crops is the most research intensive method of companion planting because the trap crop will be very specific, depending on your target crop and the pest you wish to deal with. So you will need to make sure what pest you ae trying to deal with before setting out to use this method.

In addition, trap crops can be used in association with highly aromatic plants used to disguise the target crop. Quite often these aromatic plants will act to repel pests and by having the appropriate trap crop in the area the pests will be repelled from the target crop but steered toward the trap crop. This is the so-called push-pull pest control strategy.

Examples: Chinese cabbage or mustard, allowed to go to seed to attract aphids, nasturtiums to attract whitefly.

Trap crops can encourage predators to come to your garden

Bad Companions

These are plants which will work to prevent your target crops from flourishing and so should be isolated from the main crop area, or put actions in place to deal with them. They are bad companions because –

1.  They will compete with your target crop for resources (water, nutrients, light), or

2. They produce and release into the air and/or soil chemicals which retard the growth of competing plants (ie your target crop) or reduce seed germination in the area, called allelopathy.

or both.

1. Competition

There are a number of ways where this may become an issue. Any large tree will be a bad companion for growing edible crops, vegetables in particular. At the very least they may shade the area where you want to grow your veggies, this will reduce the varieties you might want to grow to ones which are shade tolerant. Large trees also have extensive root systems and even if they are in a neighbour’s yard they can send roots out beyond the tree drip line to harvest all that water and fertility you work so hard to provide for your veggie patch. Some large trees, especially eucalyptus may also demonstrate an allelopathic effect on surrounding vegetation.

Bananas are heavy competitors for water and nutrients

Vegetables are also classified as light, medium or heavy feeders, so if you plant a whole stack of different veggies together which are also heavy feeders, they will compete with eachother for resources which in the end will result in reduced yields. It is better to interplant the heavy feeders with lighter feeding plants or else put more work into preparation of the beds before planting so that there is enough fertility for all. Interplanting with nitrogen fixers is one way of reducing problem due to this type of competition.

Examples of heavy feeders: corn, tomatoes and zucchinis.

2. Allelopathy

As mentioned above plants which exhibit allelopathy get and unfair advantage competing with other plants by releasing chemicals into the soil and air which retard the growth of other species, reduce germination of seeds or both. Sometimes the effect is even more pronounced when the plant dies and is decomposing such as rye (the grain) or marigolds. This can be put to use by harvesting and laying down these plants as cover in areas where plants are not wanted such as pathways.

The poster child for allelopathy is the black walnut, due to the production of a compound called juglone which is washed down from the leaves onto the surrounding earth, killing or stunting the growth of plants in the area. The black walnut is not alone, however and common vegetables such as corn show allelopathic effects when young and fennel and even broccoli and cabbage show allelopathy towards a wide variety of other vegetables.

A list of plants useful in the various aspects of companion planting may be downloaded here.

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