Over the last two days, I presented two slug treatments of varying effectiveness. The eggshell barrier treatment does not work at all. The beer slug trap does work to a certain extent, but is not very effective. Here are other treatments against slugs along with an assessment of their effectiveness.
Treatment: Surround the plant with a circle of diatomaceous earth (a white powder derived from fossilized algae).
Effective: Slugs seems to hate diatomaceous earth and will avoid crossing a barrier made of it unless very hungry. The theory I keep hearing is the sharp particles of diatomaceous cut the slug’s delicate skin, leading to its death. This is nonsense, of course. First, slugs have a very tough epidermis, not easily scratched, and secondly, they give off a slimy mucus to allow them to travel over the roughest, sharpest surfaces, so ought to be able to travel readily over diatomaceous earth. The proof of the pudding is that, if they are forced to cross diatomaceous earth, slugs are in no way damaged by their passage. The real reason they avoid this product remains a mystery… but they do!
Downside: The barrier must be replaced each time it rains.
Treatment: Surround the plant with a circle of wood ashes.
Effective: Again, slugs have so far refused to answer questions as to why they avoid wood ashes, so it is not known if they avoid them because they are so alkaline (pH of 10 to 12!), whether they don’t like their texture, or if some other factor is involved. Whatever the reason, slugs don’t seem to appreciate them,
Downside: The barrier must be replaced after each rain. Also, if you apply too many ashes too often they can render the soil too alkaline for most plants.
Treatment: Surround the plant with a circle of coffee grounds.
Ineffective: Slugs will cross a barrier of coffee grounds barrier as it if weren’t there. Totally ineffective.
Treatment: Surround the plant with copper tape or netting. You can also fix copper tape or netting to the sides of pots or raised beds to prevent slugs from climbing up.
Effective: It is often said that slugs receive a small electric shock when they touch a copper barrier. Whether that’s true or if some other factor is the cause is not known, but it has been clearly demonstrated that slugs will generally avoid crossing a copper barrier.
Downsides: Copper in any form can be quite expensive. Also, when it rains, copper barriers placed on the ground are often partially covered in soil, in which case the slugs will not hesitate to cross them.
Slug Bait Containing Metaldehyde
Treatment: apply pellets containing metaldehyde to the ground or pour them into a trap with openings only large enough for slugs.
Effective: Slugs eat the poison and die.
Downsides: Product not recommended! This product is highly toxic to pets and wildlife and even (to a lesser extent) to children. Most suppliers add Bitrex to their pellets, a very bitter product that slugs don’t notice, but that mammals quickly spit out, thus reducing the risk of accidental poisoning. Birds, however, generally ignore Bitrex, so can still be poisoned. If this product is used, it’s best to place this type of bait in a trap or under a length of board, away from curious animals. Even though this product is still available, few experts now recommend it for domestic use. It has been widely replaced by slug bait containing iron phosphate (described immediately below). Other caveat, granules exposed to rain decompose and have to be replaced.
Slug Bait Containing Iron
Treatment: apply pellets containing iron phosphate or sodium ferric EDTA (iron in a sodium EDTA chelate) to the ground or pour them into a trap with openings only large enough for slugs.
Effective: When slugs eat iron, they stop eating immediately, then die of dehydration three to six days later. The two products mentioned as effective with slugs as metaldehyde baits, but are non-toxic to other animals (mammals, birds, fish, insects, microbes, etc.). Moreover, they also act as fertilizers to a certain degree. Both seem to be accepted in organic gardening.
Downsides: The granules decompose when exposed to rain. Suggestion: place them under a board or an inverted bowl to protect them from the elements. Also, if you apply iron-rich year after year in the same place, excess iron can build up in the soil and that will have negative effects on plant growth.
Watering in the Morning
Treatment: water plants in the morning rather than late afternoon or evening.
Effective: The damage by slugs is greatly reduced when plants are watered in the morning rather than late in the afternoon or in the evening: often by up to 80%! Why? When you water in the evening, the soil remains damp all night and slugs, who are most active at night, just love moist conditions. By watering in the morning though, when slugs are not active, your plants will receive the water they need, but the soil has time to dry out before nightfall and that discourages slugs. Note too that watering in the evening also tends to also increase plant diseases. Avoid watering plants in the evening and you’ll help solve both problems.
Treatment: Spray or water affected plants with a solution of 1 part household ammonia (NH₃) to 6 parts water. Repeat weekly as needed.
Effective: Ammonia is toxic to slugs, but little else in the garden, at least not when properly diluted (it’s very alkaline and, at full strength, may burn the foliage of delicate plants). It’s also a good source of nitrogen for plant growth. This treatment seems most effective on young slugs when they hatch in the spring.
Treatment: sprinkle ordinary table salt on slugs
Somewhat effective: Even a grain or two of salt causes slugs to start to dehydrate. Apply just a bit more and they will be killed. However…
Downside: Salt is hard to apply, as slugs are not always out in the open on a flat surface where you can reach them with a salt shaker. And you have to apply it directly to the pest, whereas most slugs are pretty much hidden from view. Finally and even more importantly, salt is toxic to plants too and it is almost impossible to apply so it only touches the slugs. Most of it ends up on the ground, contaminating the soil.
You’re allowed to wear gloves while hand picking.
Treatment: Harvest slugs in the morning or after a rain… even at night with a flashlight.
Effective to somewhat effective: To make slugs easier to find, leave a piece of board, a half grapefruit or an inverted clay pot on the ground: slugs readily hide in the shade near their favorite plants during the day, so you can just lift the “trap” and collect them. When they’re in hand, deposit them in soapy water or use any other method you desire to dispatch them.
Downside: The effectiveness of this method largely depends on your conditions. If slugs are very localized, hand-picking can seriously reduce the population. If there are slugs everywhere, you won’t likely make much of a dint in the population.
Treatment: release the ducks in the garden, especially early morning when slugs are still active.
Effective: ducks are very attracted to slugs, seeming to prefer them over almost anything else. In Germany you can actually rent ducks for slug control.
Downside: Ducks pull up and eat young seedlings and can also crush plants by accident. It is best to keep them out of the garden in the spring until the plants are well established. Or, release them at the end of winter, before planting, to clean up before planting starts.
Encourage Slug Predators
Ground beetles, often seen in our gardens, are major slug predators.
Treatment: Create an environment that is conducive to slug predators.
Effective: Provide an environment for wildlife, including hiding places, a bit of water, a wide range of plants, and varied environments to encourage slug predators such as snakes, frogs, toads, fireflies, beetles, birds and shrews. And above all, learn to tolerate the presence of these predators. You don’t have to hug a garter snake or kiss a frog: just learn to let them be.
Downside: Some slug predators are less desirable in the garden, among them raccoons and skunks.
Hostas with thick leaves repel slugs.
Treatment: Replace the plants subject to slug damage by species that slugs dislike.
Top tip! This truly solves the slug problem once and for all! Once you get rid of slug magnets like thin-leaved hostas and leaf lettuce, the slug population will start to drop. And since the few slugs that remain are no longer eating your plants (because they don’t like the plants you’re now growing), but instead actively help to decompose dead vegetation, which is theoretically their true role in nature, do you even care if they’re still there? After all, detritus-eating slugs are no longer your enemies, but now your friends!
This is the number one slug control method for truly laidback gardeners, the secret to gardening in harmony with a former enemy. Sometimes gardening is so simple!
Here is a list of slug-resistant plants… and also a list of plants that you should avoid: the ones that attract slugs to the garden.