Vegetables need friends! Give your carrots an onion and some leeks to snuggle up to and Carrot Fly will be put off by the smell of the onions, and Onion Fly will be confused by the presence of the carrots, so no one gets attacked and you get more vegetables from your plot.
Companion planting is a big topic at the moment. Pesticides are not the answer, as they kill off beneficial insects too and merely breed resistance into the surviving pest insects making them even more difficult to kill in the future. Instead, work with nature not against it. Look at what kills the pest species and then develop the conditions necessary for the predator species to thrive. Ladybirds, hoverflies and lacewings eat aphids, one of our most damaging pests. If you plant the types of flowers that will attract the beneficial insects, and provide nesting sites for them to overwinter and breed then the chances of them keeping your aphid problem under control for you are significantly increased.
Vegetables also need food! They are designed to manufacture sugars by photosynthesis and to take up other nutrients and minerals through their roots, food which is provided for them by the truly incredible number of fungi, nematodes, protozoa, worms and beetles living in the earth structure around their roots. This structure is rich and complex, balancing the right amount of nitrogen, potassium and phosphorus (plus magnesium, selenium, boron, and so on), and can become unbalanced if too much of one or other mineral is introduced. Rather than just giving them fertiliser out of a box (most of which will be flushed straight through the soil and wasted), why not encourage the structure to build itself. Implement a no-dig policy (your back will thank you!); give the soil a rich layer of composted material and lay it on the top rather than digging it in. The worms will do the digging for you. If you don’t have compost, then use a mixture of grass clippings (not too much), leaves, chopped up stalks of spent plants, etc. This, after all, is what happens in nature – decaying matter falls to the ground, worms drag it underground, it rots and the nutrients it contains are released to feed the newly emerging crop.
One of the major food requirements for vegetables is nitrogen; how much nitrogen do they need? Well, the plant knows how much it needs. The leguminous group of plants – peas, beans, gorse – have the ability to take nitrogen out of the air and fix it in their root nodules, releasing some of it into the soil around them while they are alive (and all of it if stopped from flowering and chopped off at ground level). So, if vegetables need friends and also need nitrogen, why not give them both. Plant a vegetable, such as sweetcorn with peas sown in between the sweetcorn (for the nitrogen fixing) intertwined with courgettes or squash to attract the insects. The peas get something to climb up (the sweetcorn); the beneficial insects are attracted by the huge courgette flowers, and the courgettes cover the ground suppressing weeds. This type of planting is called, in permaculture design, a Guild. A guild consists of a plant that needs nitrogen, a plant that fixes nitrogen, and a plant that attracts beneficial insects or deters non-beneficial ones.
Herbs are great companions for fruit and vegetables. Grow Borage amongst your strawberries; the borage has beautiful star-shaped flowers that are edible in a salad (remove the green calyx first), and very attractive to bees; borage dynamically accumulates minerals such as Potassium and Silicon and when chopped down is a great addition to your compost bin. Comfrey is another dynamic accumulator of Potassium, as well as Magnesium, Calcium, and Iron, and makes excellent (if smelly) liquid plant food; chop down leaves and stalks and immerse in a tub of water for two weeks, then strain and dilute before watering plants with the mixture. It can also reduce scab on potatoes by planting a couple of wilted comfrey leaves with the seed potato. Other dynamic accumulator herbs are Horseradish (Potassium, Calcium and Sulphur) can be made into a tea and sprayed on apple trees to control brown rust, Tansy (Potassium) is a good companion to fruit berries and grapes and is especially good in orchards, Valerian (Valeriana officinalis rather than false valerian accumulates Phosphorus) stimulates composting and when made into a tea encourages flowering in fruit trees, Wild Chamomile (Potassium, Calcium and Sulphur) is a host plant for hoverflies and wasps which will eat your aphids and also stimulates composting, and finally Yarrow (which accumulates virtually everything!) is also a host plant for hoverflies, ladybirds and predatory wasps.
The poached egg plant (Limnanthes douglasii) is a self-seeding annual that provides valuable food for bees during the hungry gap, when fruit bushes have finished flowering in the spring, but other vegetables have not yet flowered. Scatter them around the fruit bushes and they will return year after year; indeed they can be rather invasive but are easily removed if they get out of hand. A mass of poached eggs in the fruit patch lift the spirits on dull days.