Nematodes can be a real nuisance for the vegetable garden especially. They are microscopic wormlike creature that works its way into the roots of plants, causing them to swell and distort which eventually leads to the death of the plant.
They are especially fond of tomatoes and if your soil rife with nematodes you often will not get any tomatoes.
One garden writer that I saw said he sowed the whole of his vegetable garden with French marigolds. He then dug them in, they rotted and it worked – and the nematodes were controlled. The main problem being though that he lost the whole seasons worth of crops.
One alternative that can be tried with marigolds is to plant them amongst the crops. I think this will depend on the type of marigold as some of the newer forms do not act as deterrents to pests and nematodes. Where I live, it is not possible to tell what marigolds are what or where they are from. So going by the garden writers use of French marigolds I suspect they are the ones to plant if you can find any.
I have read also that Mexican marigolds are said to offend a host of destructive insects and wild rabbits as well.
They must be the scented type of marigolds if you are to plant them and use them as a repellent. One side-effect of marigolds I have noticed is that they do attract snails though and possibly spiders.
Here is a list of some plants that can be companion planted to help repel pests.
SUNFLOWERS – these can draw aphids away from other plants. Sunflowers are tough enough that they suffered no damage.
NASTURTIUMS – these can be planted with your tomatoes and cucumbers as a way to fight off woolly aphids, whiteflies, squash bugs, and cucumber beetles. The flowers, especially the yellow blooming variety, act as a trap for aphids. Of course, as you know, the flowers can also be eaten.
GARLIC – apart from being a great addition to any cooking and general health benefits especially to fight colds, garlic planted near roses repels aphids. It also helps to deter codling moth, Japanese beetles, root maggots, snails and carrot root fly.
DAHLIAS – these old-fashioned favourite cottage flowers repel nematodes and the blooms are great or adding bold splashes of color.
DILL – these are best planted with cucumbers and onions. Dill repels aphids and spider mites. Dill leaves sprinkled on your squash plants will help repel squash bugs. Tomato hornworms are attracted to deal so if you place it at a distance away from your tomato plants it will help draw destructive insects away.
CHIVES – not only a great herb for cooking planted they can repel Japanese beetles and carrot rust flies.
CHRYSANTHEMUMS – you can make an organic insecticide from these flowers called pyrethrum. Sprayed on your plants helps control things like roaches, silverfish, lice, fleas, bedbugs, and also to control ants in your garden.
There are many more plants that are claimed to help control pests and diseases in your garden. There may be some doubts as to their effectiveness, but if nothing else they will add some color and possibly herbs for your cooking if nothing else.
See this list from Wikipedia for a comprehensive list of companion plants – click here