Q I have many slugs in a garden area that is wet and shaded. I have added organic matter to the soil to improve drainage. Is there any long-term solution to reducing the number of slugs?
A Slugs can be major garden pests, especially in shaded gardens where they find ideal living conditions. These shell-less mollusks feed on decaying matter as well as growing leaves and fruits. As many a frustrated gardener has discovered, slugs can devour tender young plants overnight. Older plants may have many holes chewed in their leaves. Slugs thrive in moist, cool areas, hiding out during the day and emerging at night to feed. They move on a slimy trail of mucus, the shiny remains of which can often be seen the next day.
Most slugs are hermaphroditic, having both male and female organs, so they can all lay eggs. The pearly white eggs are about 1/8 inch in diameter and are layed underground in clusters of 25 to 100. A single slug can lay as many as 500 eggs in a year, so you can see how populations can explode in a garden. If you come across slug eggs while digging in the garden, remove and smash them. Make a point to check for eggs in late spring; by destroying eggs early in the season, you can reduce the summer population of slugs. Shallow hoeing or cultivating may help expose clusters of slug eggs, but don't hoe so deeply that you chop up the roots of perennials and other plants.
While total control of slugs is difficult, there are some things you can try to reduce their damage. Although adding organic matter to your soil can improve soil drainage, it is unlikely to discourage slugs. In fact, they may appreciate the organic matter as food and for the fact that it makes the soil easier for them to burrow into. A large percentage of the slug population is lurking underground, making eradication even more difficult.
Keeping the soil surface as dry as possible helps, though this can be quite difficult in shade gardens. Always try to water in the morning, because this allows the soil surface to dry slightly before nightfall. Keeping a foot-wide border of roughed-up bare soil (scuffle the soil surface with a hoe or rake frequently) around the garden may help keep slugs out.
A coarse or rough mulch may discourage slugs, because they prefer smooth surfaces to move across. Coarse wood chips may help, though they are less effective as they decay. You also may try surrounding especially vulnerable plants with a ring of sharp material such as diatomaceous earth (which is the silica-rich outer cell walls of a type of algae), sharp sand or crushed eggshells. Roofing shingles or sand paper also can be placed around plants.
Some gardeners have had success surrounding their gardens or pots with strips of flat copper tape. Slugs supposedly avoid crossing this tape because it gives them a slight electric shock. Copper tape is available at some garden centers and through gardening supply catalogs. The strips need to be at least 2 inches wide, so you may have to place a double row of copper tape to get this width. One drawback to this method is that any slugs that are already in the garden are then trapped within the tape border.
To reduce slug populations you will have to actively kill them. Start an aggressive control program in the early spring; this should slow the population growth during the summer. You can try traps, such as shallow containers set in the soil partly filled with stale beer or other yeasty liquids. Commercial slug traps are sold in gardening catalogs, though I don't really know if these are more effective. Clean out the traps and replace the liquid every couple of days. You also can place a number of flat boards around the garden, then pick them up each morning and collect and destroy slugs that have crawled under them.
Really dedicated slug hunters have been known to patrol the garden at night, headlamp strapped to forehead, to hand pick the slimy pests from prized hostas. While slugs will shrivel up if you sprinkle salt on them, salt can easily damage plants, so don't use it in the garden.
As a last resort, poison slug baits can be used in the garden. Baits require that the slugs ingest the product, not just come in contact with it, so they don't provide 100 percent control. The chemical poison metaldehyde is sold in pellet or granular form as slug bait and can be sprinkled on the ground. However, this poison is also extremely toxic to birds, wildlife and pets so use caution when applying it. There are several organic slug baits available with iron phosphate as the main ingredient: Sluggo and Escar-go (hey, I didnt come up with the names!) are two of them.
Birds, snakes, toads and some predatory insects such as ground beetles eat slugs and slug eggs, so it's worth encouraging their presence by limiting pesticide use and growing a broad range of plants. And, if it's any consolation, we can be grateful that we don't have the banana slugs of the Pacific Northwest -- they grow to 6 inches or more in length!
-- Nancy Rose is a research horticulturist at the Minnesota Landscape Arboretum. She spends her spare time gardening, inside or outside, depending on the weather. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.