Reading my how to grow organic crops book at the moment. Just came across the term companion planting. It is based around the idea that certain plants can benefit others when planted next to, or close to one another. The theory is that companion planting benefit certain plants by giving them pest control, naturally without the need to use chemicals.
I have decided then to plant some french marigolds which should keep whitefly at bay around my budding tomatoes. I have already planted my leeks but if I had planted them beside the carrots, they may keep the carrot fly away. Perceived wisdom is to plant nasturtiums with cabbages, as they are a magnet for caterpillars and there is plenty more advice in the book and indeed online, for me to follow.