Nematodes are generally microscopic roundworms although some may be seen with the help of a hand lens of 10 power. There are many different types of nematodes but the ones important to the landscape gardener fall into three categories.

1. Beneficial Nematodes - Although we gardeners usually have negative things to say about nematodes, the fact is that there are some that are beneficial to our garden. Some of them are parasites of such critters as cutworms, corn earworm and slugs. They can help to keep these and certain other pests in line.
    

2. Root Nematodes - Some species of nematodes attack the roots of plants and, perhaps, the most common of these is the root knot nematode. It causes growths (galls) on the root systems which cause the loss of vigor of the plant.
    

3. Foliar Nematodes - A few nematodes feed in the leaves of plants. Perhaps the most important current pest of this type is the foliar nematode that attacks hosta plants.

Nematicides are pesticides made specifically to kill the nematode pests of plants. Many of them are broad spectrum poisons and often have to be quite volatile so that they will move down into the soil where the nematodes live. This trait can make them potentially very dangerous to use so very few are available to the home gardener. Generally, you need a license or special training to purchase and use many of the nematicides.

Work is being done on finding less dangerous but effective "natural" based miticides. Some of these are based on garlic, neem cake, marigolds and others.