Question
In order to keep snails out of my fruit trees, can I mix copper sulfate with tree trunk paint and paint it on?  I know snails will not cross copper and I heard this technique works.

Answer
Several types of barriers will keep snails and slugs out of planting beds. The easiest to maintain are those made with copper flashing and screen. Copper barriers are effective because it is thought that the copper reacts with the slime that the snail or slug secretes, causing a flow of electricity. Vertical copper screens can be erected around planting beds. The screen should be 6 inches tall and buried several inches below the soil to prevent slugs from crawling through the soil beneath the barrier.

Instead of copper bands, Bordeaux mixture (a copper sulfate and hydrated lime mixture) or copper sulfate alone can be brushed on trunks to repel snails. One treatment should last about a year. Adding a commercial spreader or white latex paint may increase the persistence of Bordeaux mixture through two seasons. Barriers of dry ashes or diatomaceous earth, heaped in a band 1 inch high and 3 inches wide around the garden, have also been shown to be effective. However, these barriers lose their effectiveness after becoming damp and are therefore difficult to maintain and not very useful in most garden situations.