Slugs are a common problem for most gardeners, organic and otherwise. These innocent-looking little heathens can strip a gorgeous stand of pepper plants down to a mournful stand of dying twigs in a single night. Where I live, they grow to be as big as a mans thumb. In other countries they get big enough that safaris are organized after one has come into a village.
Or, something like that.
For chemical gardeners, there are a variety of potions capable of killing slugs. I can’t recommend any of them. For organic gardeners, there are a number of ‘solutions’, some of which are based on mythology, and two that actually work well enough to consider the problem gone … at least for the current season.
Deterrence
But, before we consider killing anything outright, let’s give some thought to what we are doing. If we can live with lessened damage instead of being absolutists and dogmatic about the matter, we may not have to go to such extremes.
Cultural
First, slugs prefer moist, secluded areas. So begin by cleaning up around the garden. Get rid of scrap lumber and logs, overgrown grasses and so on. If your garden is anywhere near a fence, clear out the weeds at the bottom of the fence.
Slugs prefer to eat partially decayed vegetative material, so keep your garden free of yellowing leaves and fallen tomatoes. Also, look for ways to keep the rinds of melons off the soil. If the rind stays in contact with moist soil, it will remain soft and vulnerable in the area of contact. Then, too, look for tomatoes that have been bird pecked (set some water out for the birds … they’re thirsty) or mouse-eaten (larger damage on the underside of low-hanging fruit). Either use these fruits immediately or discard them immediately; but do not allow them to remain in the garden.
The slugs will see this for what it is … a ghetto bulldozing. They may not have anywhere to go, but they’ll go just the same. Well, most of them.
Barriers
A two inch wide band of copper foil or gutter flashing will set up a galvanic barrier that the slugs will not, I am told, cross. This particular barrier has the added advantage of actually looking good in the garden. I am leery of it, though, because of the formation of copper sulfate (CuSO4) on its surface. CuSO4 is not an environmentally benign substance and it accumulates in the brain and liver. I don’t want to be alarmist about that; but you need to take it into account before you decide whether or not to use this material. I think it is safe to use, but I am not the expert, the EPA is.
Fresh coffee grounds applied in a thick band a few inches wide will keep slugs from entering your garden by killing them outright. They apparently absorb the caffeine through their skin. This is listed as a deterrent here rather than as a slugicide because it will have no effect on the slugs already in your garden, munching away on it as you read these words. But it’s pretty potent stuff, so we’ll get back to it.
Diatomaceous earth is made from the shells of little sea creatures called diatoms. (duh!) These shells are actually glass and jagged. The slugs crawl across them and they embed in their bodies, causing death by dehydration a few days later. After they have wiped out your peppers, started a family and made inroads on your lettuce. It’s that ‘started a family’ part that bothers me, because the kids are inside the barrier. Munching, munching, munching, munching. As barriers go, it doesn’t work very well. More than that, it does a better job of killing beneficial insects than slugs.
While tin cans will keep a wireworm off a tender young plant, they will barely slow a slug down. If you have wireworms, make a collar for your plants. But don’t think you’ve done anything to deter the slugs. Some folks recommend cutting the bottoms off 2 liter soda bottles for young transplants. The problem with this is that the protected growth will not harden off thus will will continue to be slug food.
Baiting
While somewhat underhanded and devious … if not downright anti-social … it is sometimes possible to pour the flat beer from last nights party in your neighbors garden. Most of the little lushes will smell the booze, head over there and leave you alone. Kind of like a software convention. If your neighbor ever tumbles to who did that and why, you can expect relations to take a definite downward turn. If they don’t, it is because he has revenge in his mind and doesn’t want to tip his hand. Be afraid. Be very afraid.
Murder
Beer or yeast traps have the the dubious distinction of killing dozens of slugs while calling hordes more to replace them. Especially beer bait. Slugs love beer. Beer kills slugs. Sounds like a plan. Just leave out a saucer of beer where they can find it, put up a “Free Beer” sign and you’re good to go, right?
Nope. Other slugs, beckoned by the yeasty scent of the beer are headed your way from all the neighboring yards. Silent, rapacious hordes of them headed for your free beer, but stopping off at your lettuce, pepper, bean and tomato plants along the way. So yeah, you’ll probably kill 50 or 100 of them but, in the end, they’ll win. You are a Spartan at Thermopylae. They are the invading Persian hordes. You’ll win the first night. You might even win the second. By the third night, they’ll out flank and overwhelm you. Forget it … the more beer you set out, they more will join the party. BTDT … got mah butt kicked. User tip: drink the beer yourself. It will help you forget how much you hate slugs.
So, what works? What works is what kills the locals without calling in the reinforcements.
There is a commercial preparation that consists of ferrous phosphate pellets. I think it’s called “Sluggo” or some such. It works and has the happy circumstance of also being a fertilizer consisting of iron, phosphorus and a bait / organic binder. We can live with that. But too much iron in your soil is not a good thing, so this has to be used sparingly. Sluggo works until it disintegrates and then it becomes available to the plants. I water pretty frequently, so Sluggo has lasted as long as a week for me, but never longer. One of the nicer things about Sluggo is that it doesn’t seem to be an actual bait that draws the slugs. If you apply it as directed, you will get some relief … generally for a couple weeks or more. If that is enough to take your garden into the dryer season, you may need nothing more until the monsoons hit again.
Forget Sluggo. It has EDTA or EDDS added to up the ante on its killing prowess. Neither of these are good to have in your garden and will, in fact, disqualify the food grown there as organic while harming beneficial insects and earthworms.
Lastly, there is what I generally use: coffee grounds. No … you can’t go cheap here and use brewed grounds. Previously brewed grounds go in the compost pile or worm bed. To kill slugs, you will need to sprinkle fresh grounds liberally all over the beds that are infested (showing damage or near a bed showing damage) and also around the perimeter of the area. What you CAN economize on is to buy the cheapest ground coffee you can find. Slugs are not gourmets.
I stumbled on this remedy when I encountered a University of Hawaii study about the effects of caffeine on a species of frog the planters there are not fond of. It killed them. It entered through their skin and overwhelmed their nervous systems. “Hmm”, says I, “frogs absorbed it through their skin, I wonder if slugs would too?” Unable to simply paint slugs with various chemically pure solutions of caffeine, I bought a 5# can of the cheap stuff and went out into the garden for a little “mano a mullosc” confrontation.
You know what? Slugs can’t handle coffee.
Once damage has reached noticeable levels or you see their shiny hardened slime trails in the morning, sprinkle fresh coffee grounds so liberally that they cannot help but encounter them on their way to denude your precious garden. Don’t leave them a safe path.
And don’t worry about other species … earthworms, especially, love coffee grounds. By the time they encounter them, the grounds will have been wet for a while and not nearly so potent. Which leads me to note that you’ll want to re-apply the grounds after a rain IF you see continuing evidence of slugs. Otherwise, don’t bother.
Instead, make a cup of coffee and enjoy your garden again. 
