Companion planting is about utilizing the beneficial properties that certain plants afford to one another. Some plants, such as basil with tomatoes, actually improve the flavor of the tomato fruit while others, such as sage mingling with carrots and cabbage, repels a number of pests including slugs and flea beetles. No matter if your goal is a natural pest repellent or cross-pollination, you can use the principles of companion planting in any size garden plot.
Tomatoes and basil not only taste great in a dish but the two plants grown together provide a number of benefits for each other. Basil repels certain insects but the most important benefit is the cross-pollination from bees, which actually improves the taste of your tomato yield.
For an added benefit, start a whole tomato sauce or pizza-themed garden plot by adding chives, onion and parsley. Garlic is also known to repel red-spider mites. To add color, plant marigolds throughout to protect tomatoes from nematodes, microscopic worms that can decimate plants from below.
Tomatoes should always avoid potato and fennel plants---they should not even be planted in the same section the following year---as disease can be spread between the two easily.
The most classic example of companion planting is an ancient Native American planting style known as the "Three Sisters." Corn, beans and squash planted together in the same mounded space grow better because of each other. Corn stalks provide a support structure on which the beans can grow. Squash provides a weed barrier, and beans, being a legume, creates its own nitrogen by collecting unused nitrogen from the air, thus feeding nutrients to the other two.
Most insects find their food by their sense of smell, so using strong, more pungent scents to mask the smell of their usual targets can protect your crops. Flowers such as marigolds and nasturtiums are naturals in the vegetable plots because of their ability to repel most insect pests. Many herbs such as chives, sage, parsley, mints, wormwood and tansy do the same.
On the flip side, other strong scents such as dill, parsley, carrot, coriander, catnip and parsnips attract beneficial creatures such as praying mantis and lady bugs that make a diet out of most garden insect pests.