Photo: Bordeaux barrier on a lemon tree
Several types of barriers will keep snails and slugs out of planting beds or greenhouse benches. The easiest to maintain are those made with copper flashing and screen. It is believed that copper barriers are effective because the copper reacts with the slime that snails and slugs secrete, causing a disruption in their nervous system similar to an electric shock. When erecting vertical copper screens, it is best to use ones that are at least 4 inches tall, so you can bury a portion of it a few inches below the soil to prevent slugs from crawling beneath the barrier.
Copper foil (e.g. Snail-Barr) wrapped around planting boxes, headers, or trunks will repel snails for several years. When banding trunks, wrap the copper foil around the trunk, tab side down, and cut it to allow an 8-inch overlap. Attach one end or the middle of the band to the trunk with one staple oriented parallel to the trunk. Overlap and fasten the ends with one or two large paper clips to allow the copper band to slide as the trunk grows. Bend the tabs out at a 90° angle from the trunk. If the bands tarnish, you can clean them with a vinegar solution.
When using copper bands on benches, be sure the pots and bench surface are free of snails and slugs before applying them. The best method is to wrap the bench or the legs with the copper tape before placing any plants on the bench.
Instead of copper bands, Bordeaux mixture (a copper sulfate and hydrated lime mixture) or copper sulfate alone brushed onto trunks will repel snails. One treatment should last about a year. Adding a commercial spreader or white latex paint can help the Bordeaux mixture remain effective for two seasons.
The information in this section has been modified from the University of California IPM website (http://www.ipm.ucanr.edu/).