Some plants are happier next to certain plants and others are not happy at all! When they are happy, the grow better and faster.
French marigolds (Tagetes patula) secrete an enzyme or a hormone into the soil that deters nematodes from infesting their roots, and it does seem that tomatoes or other nematode susceptible plants growing as neighbors will be protected.
Planting dill with your tomatoes will attract the tomato worm for you. Interplanting your tomatoes with basil is done because basil will help repel the tomato hornworm.
Nasturtiums are planted among cucumbers for protection against the cucumber beetle and the Mexican bean beetle. Planting tansy among your cucumbers will also send the cucumber beetles packing!
Many times, planting certain plants together is also for practical reasons. Planting lettuce next to corn means that the lettuces can be shaded during hot summers.
The common dandelion that some see as a scourge in the garden should think again. It is now known that dandelions attract pollinating insects. Furthermore, they also release ethylene which is a gas that encourages fruit setting and fruit ripening.
Daisies, dill, corriander and parsley are all good for attracting beneficial insects into the garden. The pollen they provide make them wonderful bee plants, but in addition they also attract parasitic wasps that prey on insect pests. These plants should be planted throughout the garden at regular intervals as many of these wasps are tiny and fly only over short distances.
For a companion chart to see what vegetables grow best together, please see the COMPANION CHART.