Companion planting is the planting of mutually benificial plants in close proximity to each other with the result of deterring pests or attracting predators for any pests that may occcur. As well as being pleasing to the eye, as opposed to boring pest-inducing monoculture, it brings a more balanced eco-system into your garden. In essence, letting Nature take over.
Carrot fly seems to be a big problem in Ireland. The usual remedy is to put a 3-4 ft high barrier around the carrots, the reason being that the carrot fly does’nt fly higher than 3 ft. A more asthetically pleasing solution is to plant French Marigolds amongst your Carrots. Marigolds also exude a substance that kills nematodes and deters whitefly. Don’t plant them near Beans as the Beans won’t do well.
Nasturtiums planted around fruit trees deter wooly aphids. They are said to improve the flavour and vigour of their neighbouring plants. They act as a deterrent for aphids and but also attract hoverflies who love to dine on greenfly. The also have the added bonuses of being attractive and edible. The leaves give a spiciness to salads and the flowers look amazing when strewn on top. Once you have them you’ll never have to plant again as they self-seed profusely. The seeds can also be pickled as a substitute for capers. I can’t vouch for this as I have’nt tried it myself.
Rhubarb is a good companion plant for all brassicas. A spray can be made from it’s leaves, they contain oxalic acid, to control blackspot on roses and as an aphid deterrent.