Companion planting is the term used when you use one plant to benefit another or when you grow more than one plant together for a mutual benefit to both plants; in essence, it’s when you plant different plants together in your garden rather than rows and rows of the same thing. When we grow large areas of just one crop it’s called a monoculture and problems can spread quickly when this method of growing is used. Imagine a large field of cabbages: when a problem like cabbage white caterpillar starts there is little you can do to stop them working their way through the entire field (apart from spray with pesticides), as all the plants are the same type and all touch each other.
Now imagine the same field with one or two rows of cabbages next to a row or two of onions, then cabbages and so on. By growing like this the caterpillars may affect some cabbages, but the rows of onions will both form a physical barrier so the caterpillars can’t move from plant to plant, but they will also add an onion smell to the field, along with the cabbage smell, meaning the butterflies find it harder to find the cabbages. There are actually commercial fields that are producing strips of crops like this to help cut back on the use of pesticides and to enable farmers to grow in a more manageable way, rather than turn to chemicals straight away.