Firstly, you must understand that this line of horticultural experimentation is not foolproof. Who in their right mind decided that it was a good idea to plant nasturtium next to broad bean in the vain hope that the winged pest that is 'Blackfly' (Aphis fabae) would somehow prefer to live on the former more than the latter? Notwithstanding I persevere to understand the concept that planting one plant betwixt others somehow enriches the growth or eliminates pests and, I must admit, I have had some success.
For example, did you know planting onion or leek in alternate rows with carrot, not only deters the dreaded Carrot Root Fly, but also the Onion Root Fly. This process of using scent is a common theme, planting rosemary next to roses apparently deters aphid or the planting of garlic in flower beds offers some protection from insect attack.
Another tried and tested companion plant relationship is Brassicas next to a bean. This may at first seem odd, but there is scientific method. Beans hold nitrogen in the soil, brassicas need nitrogen to grow nice healthy leaves. Thus a match made in heaven and you should include in this list spinach and chard. Similarly, the native Americans in their wisdom, used to plant climbing beans with maize. The maize prospered due to the increased nitrogen, the bean gained a free trellis.
Finally, chemical warfare is a useful ally in the garden. By this I do not mean expensive and ecologically damaging pesticides. I am of course referring to naturally released oils and hormones. Tagetes, otherwise known as the African marigold, has a useful chemical which has been scientifically proved chemical to ward of soil born insects such as wireworm, so plant them liberally amongst potatoes and other root crop for a bumper harvest. They also provide a food source for beneficial insects such as hoverfly.
So next time you look at your planting plan, have a thought for what goes with what and how you can help your garden look after itself.
-- Guy Deakins