Even the most knowledgeable and dedicated gardeners can have perennial problems with gastropods. Snails and slugs are plant-munching pests and can seem just to be part of the landscape, because they are. Even if you work night and day to keep your garden snail- and slug-free, they can wash into your plot during rainstorms. Their eggs can hitch a ride in potting soil and nursery transplants. Unfinished compost is heaven for slugs. If you spread compost on your garden before it has generated heat, you perpetuate your slug infestation.
Much as you can’t stop birds from landing on your head but you can stop them from building nests in your hair, you can’t easily exclude all snails and slugs from your property but you can recognize the conditions in which you will need to make special efforts to control them.
Moisture is essential to both snails and garden slugs. A garden snail loses about 8% of its body weight every hour that it crawls on the slimy trail of mucus it secretes. It will usually crawl into its shell and shut down its metabolism to survive, but it will be especially vulnerable to predators (and to gardeners gathering them by hand). Snails almost always die after a week or two of exposure temperatures above 28 degrees C (82 degrees F) without moisture. Going just a few hours without water or damp food is fatal to a slug. They have to burrow into damp ground or find water to survive. If you deprive the most common pest varieties of snails and slugs of water, they will die in a few hours, for slugs, to a few weeks, for snails.
Snails survive hot spells by climbing plants. Even a meter above the ground is several degrees cooler than the temperature at ground level. If they are deprived of plants to climb, they succumb to heat more quickly.
Snails and slugs are eaten by many other animals. In Britain, the main predators of snails and slugs are the Song Thrush and hedgehogs. In the US, snails and slugs are a favorite of “lightning bugs,” roadrunners, and field mice. If there aren’t any birds in your garden, and you have blocked entry to small mammals, you are providing a haven for slugs and snails.
Any plant that is tasty to humans is also tasty to snails and slugs. Plants have chemical defenses that deter their being eaten. Vegetables have been bred not to have those defenses. Brown snails and field slugs will eat your veggies at any stage of development, although they prefer them as tasty young seedlings. It is especially important to protect lettuce, cabbage, and peas from slugs and snails.
The beneficial bacteria and fungi that make your soil healthy also make snails and slugs healthy. Snails and slugs depend on probiotic bacteria to help them digest woody plant stems, dead leaves, and other decaying matter. Healthy soil produces healthy slugs. You need to watch out for slugs around the compost pile.
Snails only eat living plants when decaying plant matter is not available. The “yeastier” and moister the rotting leaves, stems, or bark, the more attractive it is to snails. If you gather dead plant matter from beneath your perennials and return it only as finished compost, your garden will attract fewer snails.
Snails are attracted to well-limed soils. Acidic soils do not provide the calcium they need for their shells. Slugs, which do not depend on an external shell (they may have a vestigial shell inside their bodies), are better adapted to acidic soils than snails.
The slugs you do not see, those that attack bulbs and seedlings below ground level, are eaten by beetles. Overuse of insecticides can lead to problems with slugs.
Improperly prepared barrier defenses sometimes encourage slugs and snails to enter rather than keeping them out. Finely crushed eggshells, for example, serve as a scratchy surface that will cause slugs to turn around. Larger pieces of eggshell will provide a sunshade for slugs that encourages the babies to crawl into your planting beds.
A large part of a snail or slug’s metabolism is devoted to producing the slime on which it travels. The moister the environment, the less slime it needs to make. You will find snails and slugs in greatest abundance after rains and after you water your plants.
Snails and slugs need protection from winter cold. Although most of the slugs and snails you will find in your garden can survive a few degrees of frost, extreme winter cold will kill them. Soil that lies undisturbed during winter provides them with shelter.
Another strong attractant for snails and slugs is the presence of other snails and slugs. Even though most land-living snails and slugs are hermaphrodites, containing both sexes in a single individual, they take on the roles of male and female during their mating season. Mating with another individual prevents the accumulation of mutant genes that make most of the eggs of self-fertilized snails and slugs sterile.
Snails mostly mate in the late spring and early summer. If you have snails in your garden this time of year, they will attract snails from your neighbor’s place. Slugs mostly mate just before the end of autumn. Any slug problem you don’t have under control by the end of the growing season is likely to be worse the next spring, unless you expose the eggs to winter cold.
Giant African snails, which are exclusively a problem in places where winter temperatures don’t fall below about 8 degrees C (46 degrees F) mate all the time. The presence of giant African snails in your garden is a situation that requires immediate attention no matter what the time of year.
Simon Mann is a "handy man" to have around the house. Although he was a trained carpenter he went on to become a VP of a construction company. Any pest or DIY problem you may have, he always seems to come up with the right solution.