Everywhere you look on the internet, you’ll find gardeners who consult lists of plants that are good or bad ‘companions’ as they plan their vegetable garden every year. The idea is that these simple lists can ensure the success of your veggies by laying out which ones grow well together, and which ones don’t.
The problem is, as with so many things in life, it’s not quite that simple. Over the decades in which agriculture has marched, almost without deviation, in the direction of monoculture, plant science has begun to uncover the numerous and almost imperceptible ways in which plants interact with each other, and with their environment.
Every plant that grows, even if it’s in a pot, is part of a community of organisms with which it interacts. And those interactions can be mutually beneficial, or they can be competitive. Some plants actively try and poison other species around them (walnuts being a prime example) – a tactic called allelopathy. Others do grow harmoniously together. A Native American example is the Three Sisters, a combination of sweetcorn, beans and squash. The sweetcorn provides support for the beans, which fix nitrogen into the soil whilst the squash keeps the root zone nice and cool. In the right climate, with the right timing and the right varieties of plants, the Three Sisters works really well. Elsewhere in the world, it’s trickier to get it right.
So it seems as though companion planters are on the right track; however, an assertion along the lines of ‘carrots love tomatoes’ is simplistic. Tomatoes and carrots grow in lots of different environments around the world, in differing climates and soils, surrounded by different sorts of vegetation and other organisms. There are too many variables involved for us to say that growing carrots and tomatoes together is always beneficial.
What we do know is that there are problems with monocultures, and that it is far healthier for plants to grow in vibrant, living ecosystems. So, rather than consulting prescriptive lists, I prefer to think of companion planting in terms of principles that can be used to inform your garden planning.
Some plants are bigger than others, and whilst some have very thick canopies of leaves others have a slimmer profile. Whether or not a plant is casting useful shade or overshadowing its neighbours depends on the neighbours you’ve chosen.
The sage advice for vegetable gardeners is that taller crops go on the north side of the garden, where they won’t cast shade on shorter ones. That works for everything that enjoys full sun; you can use it in reverse to create a shady spot for your lettuce in midsummer, or for anything else that wilts in the heat.
Windbreaks are another example of companion planting to create microclimates, and the protection they give makes a real difference to plants in their lee.
Weeds are generally considered to be negative companions – they compete with your crop plants for sunlight, water and nutrients and can quickly overcome an allotment while the gardener’s back is turned. They’re just doing what comes naturally, and the important thing to remember is that not all weeds are created equal. Whilst you may wage endless war on the brambles or the bindweed or the couch grass (or your own personal nemesis, whatever the species), there may be other weeds that aren’t causing you or your plants any problems and may be bringing benefits.
Dandelions flower early and often, for example, and although this causes some problems it also provides a valuable source of nectar for hungry bees. Deep-rooted species can act as ‘dynamic accumulators’, bringing up nutrients from the subsoil where the roots of polite plants don’t normally reach. Properly killed and composted, they can add fertility to your garden. Weeds also protect bare soil from weather damage, so don’t be too quick to entirely clear an area you won’t be using for a while.
On the other hand, companion planting can help you deal with your weed problems. You can choose plants that quickly develop a thick canopy of leaves that prevent light from reaching the soil – and weed seeds – lower down. A low lying green manure crop, such as trefoil, makes a weed-suppressing non-competitive mulch for your main crop.
Pests and diseases
One of the main issues with monoculture is that, by planting blocks of a single species, you’re ensuring every pest of that crop within a given radius can find it, snack on it, and set up home there to raise a new family. Some pests find their host plants visually, some via smell and some (including the pesky cabbage whites, which have taste organs in their feet) by tasting plants. Whichever way they do it, polyculture – planting different species of plants together – makes it much harder for pests to move in en masse. They can’t do a quick hop from one plant to the next if the next plant over is a species they don’t like.
An area where companion planting can be of real benefit is in encouraging beneficial insects into the garden. You don’t need to interplant each row of crops with a flowering plant, you can have them in dedicated areas if you prefer, but there’s no denying that flowers and flowering herbs bring in bees, butterflies and all of the creepy crawlies that we struggle to identify but which enable a self-sustaining ecosystem to develop. A lot of our crops depend on bees and insects for pollination, so there’s a good, solid reason to have more flowers. And who doesn’t love the sight of bees swarming around a lavender bush?
Fixing nitrogen with friends
Some plants, notably those in the legume (pea and bean) family have the ability to take nitrogen gas from the air and turn it into nitrogen fertilizer in the soil. They don’t do it on their own, though, they have help from soil bacteria. And they don’t necessarily benefit the plants around them immediately; the current theory is that peas and beans use their nitrogen themselves and there’s not much left in the soil when the plants are pulled up. Adding their remains to the compost heap can help add nitrogen to the soil next year, though.
But if you look at things on a larger scale, and read about forest gardens, then these rely on adding nitrogen-fixing species into the mix rather than adding artificial fertilizers. And we know that trees and other plants can exchange nutrients with fungi in the soil. Plants usually offer up the sugars they make via photosynthesis in return for minerals the fungi find with their more extensive network of hyphae (the fungal equivalent of roots).
There are plants that just can’t live without their fungal partner, but as work continues to uncover what’s going on underground, it’s becoming increasingly obvious that most – if not all – plants could be connected to this fungal network, and would be healthier if they were.
So, here’s the kicker. Rather than pouring over matchmaking lists for our plants, our time is probably better spent ensuring our soil is a healthy as we can make it – so plants can connect to the natural underground dating app, and sort these things out for themselves.
Emma Cooper has been gardening, and blogging, since the dawn of the new millennium. She’s utterly smitten with edible and useful plants, and is never happier than when she’s in the garden, up to her elbows in compost. She’s in the process of building a new garden, and you can follow her progress on her gardening blog, The Unconventional Gardener.