If you prefer to garden without using chemicals then spending some time planning your vegetable plot and herbaceous borders to take advantage of the benefits of companion planting will certainly be time well spent. Many plants can help others to grow better either by enriching the soil or by repelling pests. Some also attract pollinators or other beneficial insects which are predators of pests. The slight fly in this ointment is that some plants which are beneficial as companions can also be a disaster when planted with certain vegetables, flowers or even trees so if you decide to use companion planting to enhance your garden you will find that there is a lot of research to be done first.
Many gardeners instinctively know, either by observation or by listening to old wives’ tales, that some plants help others to grow better. One of the best known is that marigolds are one of the best companion plants of all and this is why in many very old cottage gardens, they are found just about everywhere. It doesn’t even matter what kind of marigold you plant, from the old fashioned almost wild kind to the most elegant of tagetes, the effect is pretty much the same. Marigolds help most plants, but they are especially good for brassicas (cabbages and that kind of thing) where they prevent root damage, tomatoes and peppers. The secret of the marigold family is that they have a very strong pesticide in their roots which actually stays in the ground for years after the plant has gone. They also suppress weeds and marigolds are certainly a lot more attractive than mares tails and less invasive. The downside of marigolds of course is that they are hard to control as they seed very freely, so the principal of ‘after the plant has gone’ is a bit theoretical! Also, the effect can be very strong, so some delicate plants can’t take it, but planted among the basic vegetables found in most kitchen gardens they can only do good.
Companion plants can also act as traps for pests and two of the best known are geraniums and nasturtiums. Any gardener will know that nasturtiums are really badly affected most years by blackfly, but better blackflies on your nasturtiums than on your broad beans. If you grow flowers to use cut in the house, a few nasturtiums in the border will mean that the cut flowers will be cleaner to use; blackfly exude a sticky secretion and can make a real mess when brought inside. Petunias and sunflowers are also trap crops and are certainly a very attractive way of keeping aphids off other plants. Sunflowers are one of the earliest known examples of early peoples employing the benefits of companion planting, as they were in use before Europeans arrived in America, to keep aphids off the maize crops.
The benefits of companion planting can be quite technical, as you need to decide whether you want a quick fix of keeping pests at bay or whether you want to take the longer view in attracting pollinators or other pest predators. This usually means that the companion plant has a more attractive smell or appearance than the plant needing its help and benefits may be less dramatic than, say, that of the ubiquitous marigold. Even more subtle can be the effects of the nitrogen fixers, such as lupin. These plants improve the soil over time, as they have nitrogen fixing bacteria in nodules on their roots. This is also an example of what might help one plant could disadvantage another, as not all plants can tolerate raised nitrogen, so care is needed when planting some of the soil improvers.
It is intriguing once you start planning your garden to take advantage of the benefits of companion planting to see how many of the classic garden pairings are beneficial. To take an example which everyone knows, drifts of daffodils growing under apple trees are not only beautiful, but also doing the trees the world of good. The details of the benefits are not known but there is good evidence that apple trees do better when daffodils are grown in the orchard. Garlic and leeks also are good for apples, but let’s face it, Wordsworth probably wouldn’t have written such a good poem if orchards were planted with leeks!
To take advantage of the benefits of companion planting, appearance mustn’t be forgotten. Sometimes only a few of the beneficial plants are needed, especially in the case of attracting pest predators, and so there is no need to swamp a border or vegetable garden with other plants to reap the rewards. The dark side of companion planting also has to be taken into account and it is important that in helping one plant you don’t disadvantage another. A good example of this and one into which many gardeners accidentally fall by lack of space is onions – they will help you grow great tomatoes and potatoes but your beans and peas will be rubbish if onions are nearby. So, with companion planting as in so many other gardening practices, you pays your money and takes your choice!