Companion planting pt1 the many interactions

Fifteen years before this I had just finished writing my first book on Companion Planting. Back then even 'Organic' was widely misunderstood and not considered scientific by some. So I could almost understand the hostility of the supposedly 'science based' gardeners to such another piece of 'muck and magic'. Of course even back then I found it strange to say the least that allegedly scientific people would dismiss companion planting effects out of hand. It was not only the thousands of years of observed interactions they were ignoring though that was a sin against science in the first place. Nor was it any lack of evidence as I then shew in my book. No; it was, and too often still is, their lack of the truly scientific principles of impartiality and investigation.

Science proceeds with the best hypothesis to fit the facts, with the weighing of evidence to support other hypothesis put forward, and then experiment to confirm any predicted outcomes. Many jumped to a disparaging conclusion without any examination of even the probability of any effect occurring at all! That is not good science!

Look! How many families, genus and species are we growing in our gardens deliberately and inadvertently? (To say little of the extreme range and number of individual cultivars, each of which may or may not retain any original companion effect; just as randomly as was scent when flower colour was selected in sweet peas, freesias and roses.) I estimate we probably have tens of thousands of different plants in our gardens in total though maybe only a few dozens or up to a thousand or so in individual plots. (Don't forget the weeds, wild plants, hedges, trees and so on all around you as well as the plants you actually cultivate.)

Would it not be very odd if out of all these thousands of plants that may end up next to each other that not one combination had an influence, more benign, beneficial or hostile in place of another? Indeed it would seem pretty obvious with so many possible combinations that at least one or two plant combinations may result in some stronger or weaker growth depending on the neighbouring plants! And not just down to straight competition for resources either.

Ironically, the Bible of the soil scientists, Russell's Soil Conditions and Plant Growth, actually has several pages devoted to Mixed Cropping. There, this other expression for companion planting, shows increased yields of about a fifth resulting from the mixing of two or more crops in the same area as compared to each being grown separately on parts. And this result is not confined to where legumes are included, or green manures such as clover, but even to mixtures of cereals.

Indeed trials have shown that mixed crops are also less prone to disease and pests than monocultures. (As anyone who has cultivated in the Tropics will soon confirm!) Once popular in the Middle Ages (Maslin, Dredge and other mixed crops were commonplace especially for fodder) Mixed cropping is now becoming more economically interesting as the modern combine harvester can separate, say peas and wheat, and modern cultivars can be had which will ripen at the same time facilitating the process.

I myself have long found that three beds, each with a mixture of sweet corn, peas and potatoes, will far outcrop the same three beds each individually planted with the same three crop plants. Each finds more of the same more competitive than plants of other species with differing requirements, and the increased spacing with other plants coming between the similar plants handicaps the pests and diseases. And of course we all know how those who are legumes create a surplus of nitrates which are available to other plants- but what they don't tell us is that it is not only legumes who have nitrogen fixing nodules on their roots but half a dozen other families of plants as well! (true most of these are not native species to the UK). But more on that later.

In theory companion planting offers tremendous potential but in practice it is held back by the lack of systematic knowledge of the various plant combinations and their effects. It may be because these have so rarely been studied -except in the case of weeds some of which are known to be particularly deleterious because of their enhanced ability to rob the soil of specific nutrients, their ability to choke out others, and, indeed their direct interactions such as with dandelions suppressing germination and altering the growth and ripening of other plants about them with ethylene emissions (just as a ripe banana or most well ripened fruits, will ripen others in their proximity.)-which of course could never be described as companion effects!

So although our knowledge is somewhat sketchy we may still deduce the many various ways plants may interact and then find examples typifying each case even if this is by no means comprehensive. Some of them are straight forward, some circuitous and indeed some are negative rather than positive enhancements. Nature is not into altruistic assistance UNLESS there is also advantage to be gained, BUT we can turn such effects to our own use -just as by putting a ripe banana, or a dandelion, by a near ripe tomato we can accelerate it's readiness.

The simplest assistance one plant can gain from another is Support. Not particularly applicable to most of the kitchen garden however the vast majority of climbing plants are rather dependent on more rigid taller plants for their well being. Not so much assistance or effect as plain necessity dictates that climbers must be grown in association with the other plants or a substitute. In nature orchids, bromeliads and countless minor plants inhabit trees and never come near soil or the ground as such but we have little use for most of these. However sweet corn or sunflowers can support climbing beans or sweet peas while the cucurbits have long been observed to enjoy the dappled shade at their feet occasionally becoming lax climbers themselves. (The combination of corn, beans and squashes was known as the Three Sisters by native S. American tribes who grew them together for hundreds if not thousands of years.)

Shade is not so obvious but many plants scorch and burn without it. True they can also exist by cliff bases but generally they depend on other plants -though as with the climbers not any particular ones specifically though. We benefit our saladings and the more lush leafy crops such as spinach by growing them in the shade next to taller (and leguminous) peas and beans. However, in the UK, for sweetness, and speed, we generally grow most of our kitchen garden crops in full sun. If climate warming proceeds then one day we may end up needing to grow shade trees as is often done in hotter climes.

Shelter is even less obvious but the benefits of a hedge are amazing. A hedge raises the temperature of the ground it surrounds as well as reducing the buffeting of the wind. Big plants such as kale can shelter smaller more tender crops such as sweet peppers on their sunnier side. On an even smaller scale even the stumps of last year's crop, a stand of weeds or other more advanced crops can shelter small seedlings as they emerge. I find broad beans sown in the same holes as main crop potatoes emerge before them and give them wind and some frost protection. This latter is greatly enhanced as any sheet draped over them will rest on the beans' haulm and be clear of the potato's thus keeping their foliage better protected from frost. The beans are cropped and gone in early summer leaving the potatoes which grow on into autumn.

Obviously a tree or shrub will usually benefit from the leaf litter that accumulates underneath -even if it is not it's own, which the wind usually ensures. Later this becomes fertility but initially it may be an effective mulch retaining soil warmth and moisture and suppressing seedling weeds from emerging. Harder to spot though potentially as valuable may be similar benefits derived from the falls of flower litter (the petals under my cherries carpet the ground), pollen (very rich in many nutrients, too small to observe, yet in vast amounts per acre), seeds, rotting fruits and shells (just look at the ground under old hazels to see the last in quantity.) (I've not yet worked out for sure but I suspect that hazels may well do better if their ground contains bluebells, -and I've not yet had a damned truffle from my, very very expensive, inoculated bushes!)

Indeed do we create or find companionships? It can be no coincidence most of our fruit cage fruits endure some shade, they are plants of the woodland's edge naturally. While grapevines are the climbers who get on top of everything else in the wood so when we put them in the cage we must keep them up in the sun and then prune them unbelievably hard to keep them from choking everything else. And strawberries naturally inhabit the dappled shade of the floor underneath the others -though are sweeter in full sun.

Probably the most useful companion effect is with those plants that bring into the garden and their proximity beneficial creatures in greater numbers than would be there otherwise- pollinators, predators, parasites that then also control pests. The extremely useful Limnanthes douglassi Poached egg plant, Phacelia, Convolvolus tricolor, Pot marigolds and just about every other flower; in some way they all bring in insects. I love hollyhocks and mulleins as they produce masses of pollen for hungry insects. Even ghastly double flowers that have no nectar or pollen may still bring insects towards them-who are then obliged to look elsewhere for lunch! Most beneficial insect adults thrive on nectar and or pollen along with any meat they may eat, and it is often their larvae who eat the most pests. More of them = less pests! Ladybirds living on aphids on vetches produced more eggs than average indicating these (also leguminous) plants are particularly useful about the garden.

Discouraging pests by camouflage both visually and by scent is another immediately obvious benefit. It's been proved mixing French beans and brassicas reduces pest attacks, that onions with carrots do help hide each other from their respective flies, that French marigolds reduce new infestations of whitefly in greenhouses. Most pests home in on their lunch by smell from a distance, so can be confused by powerful similar smells elsewhere or other stronger smells nearby. Those that visually navigate may be confused by similarly leafed and coloured plants but these pests are few in number as most rely on smell! Thus the benefits of pungent herbs and Alliums planted as thick borders around the plot and under fruit.

Some plants may help by discouraging pests physically. Few slugs or snails will want to crawl over the powdery and pointed debris under holly or rosemary so a hedge of this will deter them from the salads by heading them off somewhere else before they get in 'sight'. Lavender, heather and pines can also be of similar service. Even Box hedges, renowned for snails living in them help discourage slugs on the ground -and you may observe the snails are themselves mostly consuming the algal coating on the leaves of the box. ( Could we confine snails on the outside of polytunnel roofs to keep them clean I wonder?)

Sacrificials are plants we provide so our chosen ones survive. If you want to keep lettuces free of slugs and snails then plant a row of Chinese cabbage either side and ruthlessly cull and replace them. Raspberries reduce the bird damage on strawberries underneath, blackberries for grapes. It may hurt but it works. . Other plants can be grown as a living palisade to suffer pest attacks just to protect others so a barrier of tastier nasturtiums is munched by caterpillars reducing the attacks on the cabbages within.

Unseen to us are the gaseous interactions- I mentioned the ethylene that comes off dandelions affecting other plants nearby. It's been shown tomatoes attacked by pests give off substances (methyl jasmonate amongst them) that 'warns' other plants who then adjust their defences. Sounds like science fiction but more and more such interactions are being discovered.

The most important gas plants give off is oxygen, but that's to us and is a by-product they don't want -and only during the day from the leaves. At night the leaves, and the plants' roots all the time, give off the most essential gas for their fertility- carbon dioxide. This can then be absorbed by the soil solution or dew on the leaves to drop, become combined in the soil and then returned to the same or another plant. Even the amount of dew can be increased by mixing clover (another legume) with your orchard grass sward. And of course the dew with it's load of carbon dioxide may be absorbed by the plants directly through their leaves. As can nutrients in honeydew, guttation drops (water exuded by leaves) and even dust and pollen, all of which are altered in quantity and quality by other plants in the vicinity.

Some such as Nicotiana sylvestris collect inumerable insects on their sticky leaves which once fallen add to soil nutrition. Sundews and other insectivorous plants deliberately trap insects to aid their nutrition directly and although not known to return any surplus into their soil may. If you can't see that being much use; I admit it is dubious, but sundews may just be The companion for brilliant crops of Blueberries and Cranberries! And that is the interest- we just do not know. Plants are so strongly affected by minute amounts of hormones and other substances that it seems likely that they may all influence each other far more than we appreciate. It is also likely that the vast majority of these interactions, as yet unmentionned, must lie in the soil which I will deal with next month.