Companion planting

In their uncultivated state, plants must continually depend on their adaptability for survival. In addition to accommodating local variations in their environment, they need to compete against rival plants for their share of soil nutrients and other resources like water, sunlight, and physical space. While this competition can frequently be harsh and unforgiving, there are also situations where plants benefit from establishing mutually co-operative relationships with non related species. By closely observing nature over many years, I have learnt to appreciate and accept these complex patterns of ecological rivalry and cooperation. In turn, this provides me with a useful foundation for establishing companion planting programs within my own organic garden.

A process of discovery

To organic beginners, the concepts of companion planting may well appear inconsistent and shrouded in obscurity. I still recall examining the topic and becoming entirely frustrated by its lack of unity and scientific explanation. That was nearly fifty years ago. In the meantime I’ve mellowed considerably and am now prepared to exchange solid explanations for the flexibility of approximation and practical efficiency. I no longer feel pressured to understand the technical aspects of every situation. When a companion planting strategy works, my current approach is to relax and enjoy the success. Often there’s a scientific or technical rationale which becomes evident at a later date. I might discover this purposefully or by accident: through any combination of fortunate observation, applied research, casual reading, or conversations with other gardening enthusiasts. That’s the excitement of discovery. It can be wonderful, and deeply satisfying on occasions. Alternatively, I’m left unable to explain why a particular planting has succeeded or failed, either to myself or my friends and associates. That’s okay too. Perhaps I will figure it out sometime later.

Here are some examples of companion plantings which have succeeded in my own gardens on various occasions throughout my career. I’ll begin with plantings which can be rationalised to an extent then conclude with those which work for unknown reasons.

Combinations which promote an efficient distribution of resources

There are numerous planting combinations based upon the efficient distribution and utilisation of garden resources. It’s lovely to reap the rewards which arise from devising combinations which facilitate a mutual cooperation between distinct plant species.

The umbrella arrangement involves interspersing tall or climbing plants with several rows of shorter plants which can benefit from the physical protection and partial shade. One of my favourite examples is to plant several rows of English spinach between broadbean or sweetpea. In exposed areas, spinach requires substantial protection from drying winds and harsh sunlight. I particularly recommend this planting to warm climate gardeners who might have become frustrated by the tendency of English spinach to run to seed before harvesting.  

The most popular plantings within this category involve the positioning of shallow rooted species alongside those with deeper root systems. This enables water and nutrients to be shared while soil structure is improved by  the root foraging which occurs at separate levels. There are numerous examples which can be widely applied although their ultimate success depends upon local climate and soil conditions. Beginners might attempt a small plot combining lettuce with carrots or beetroot while experienced growers can often accommodate larger fields with productive combinations like potatoes and cabbage, or turnips with nasturtium.

Heavy feeders like sweetcorn and squashes are frequently planted alongside nitrogen fixing legumes such as beans or peas. This offers substantial protection to a soil’s nutrient profile, particularly its ongoing ability to break down and absorb organic soil additives such as dried manure, seaweed, and peastraw.

Combinations to discourage and control garden pests

A disadvantage of growing one species on a large scale (monoculture) is the devastation caused by sudden infestations of plant destroying insect. One theory suggests that monocultures produce concentrated biological signals, thereby attracting pests in significantly greater numbers, and from beyond their normal range. The introduction of carefully selected companion species weakens the attraction signals and discourages the arrival of many destructive insect species.

When cabbages, broccoli and related plants are interspersed with rows of onion, they are less prone to caterpillar and other larval infestation. This is principally due to the onion’s stronger scent and its capacity to confuse or deter cabbage moths.

Various culinary and medicinal herbs have gained reputations for their insect repellent properties. The complex aromatic oils of garlic, cloves, basil, lavender and sage will discourage aphid, fruitfly and other insects from attacking vulnerable species like citrus, rose, lettuce, and stonefruit. When planted in close proximity to seeding beds, tansy is an effective deterrent against cutworm, cabbage worms, and many types of caterpillar.

Astute organic growers frequently plant companion species to discourage pests from eating their primary crops. Redcurrants are used as a sacrificial plant to distract birds and other species which otherwise feed upon strawberries, raspberries, and blackcurrants.

Traditional planting combinations

There are traditional planting combinations which seem to work yet cannot be explained in any conventional sense. Chamomile (chamaemelum nobile) is frequently grown alongside culinary herbs where it has the effect of encouraging increased production of their essential oils and flavour characteristics. I first experienced this phenomenon on a friend’s property located near the Welsh border. He specialises in aromatic herbs like oregano, sage, and thyme. After lunch, we were invited to compare the difference between his plants raised alongside chamomile and those gathered from separate fields with a mixture of native grasses and bracken fern. I was astounded by the difference and found myself planting chamomile throughout my own herb gardens. Personal experience with this planting suggests an effective advantage which is restricted to the coarser Mediterranean and African varieties. There appears to be minimal improvement with the English, French, and Asian herbs.

Despite the conservative nature of their industry, viticulturalists are well known for the obscurity of their companion planting practices. My favourite example is the decision to grow ornamental basil between established and old growth vines. I have observed this practice on an exclusive Bordeaux vineyard renowned for the superior quality of its Grenache and burgundy varietals. In Argentina and parts of rural Spain, high quality organic grapes are occasionally interspersed between wild geranium bushes and mulberry trees. Ripening mulberries appear to attract local birds preventing them from interfering with the grapes. There is however more to these pairings than that which can be immediately appreciated by an outsider with limited conversational Spanish. I’m always fascinated by the fact that immature vines are abandoned season after season, until some combination of factors make them appealing and acceptable to the seasoned expert.

Experienced gardeners will commit themselves to few rules and even fewer certainties with companion planting. Most of us need to experiment with several approaches and will usually discover that success depends upon location, soil condition, and other factors in addition to the combinations themselves. Bearing this in mind, it is good practice to keep detailed records of each attempt so that similar conditions can be repeated whenever a successful outcome is achieved.