Slugs And Snails And A Stout Cup Of Coffee

Slugs In Trees

This week, I’ve been pruning overgrown shrubs, thinning out the oldest trunks and removing crowded branches to allow more light and air to reach to core of the plant. I was amazed to see that where the foliage was most dense, the trunks and branches were thickly clustered with snails. As I merrily tossed them onto the nearby driveway, I felt a twinge of guilt. For many years now, I have let the garden find its own balance, using neither pesticides nor fertilizers (except for food crops). Still, the sight (and feel) of those slimed and goo-dripping branches just rankled. Surely the plants would be healthier with better air circulation and fewer non-native mollusks!

The Way The West Was Won

Now, the Northwest is rich in native slugs but most of our native snails are water dwellers. Our soils are mostly too calcium-poor to support the shell building that snails require. The proliferation of snails in recent years can largely be traced to plants imported from California. Many years ago, European snails were cultivated in San Francisco to provide high class treats for successful gold miners with more money than sense. The captives did what snails do, proliferating wildly and making their slow but steady way throughout the state. Like many pests, they hitchhiked past tough territory, hidden in market loads of produce and plants. As the years rolled by, snails spread throughout the West.

We are used to seeing lots of slugs and snails in spring, but our typically dry summers drive them into dormancy by now. However, the wetter than usual summer kept these slimy pests going strong. In summer, we can make the garden less attractive to slugs and snails by limiting water use. Watering less frequently but more deeply and allowing the soil to dry out between waterings is good for soil and plants alike.

Black Coffee

Since they travel on homemade slime trails, slugs and snails need a regular supply of water in order to get around the garden. The lubricating slime contains lots of water, which means slugs can’t cope as well in dry gardens. If slugs treat your garden like a salad bar, toss leftover morning java on beds and borders. Even mild dilutions of coffee can help keep slugs from destroying plants without harming other critters.

How? A few years back, Hawaiian scientists were seeking safe, nontoxic ways to keep plant-harming pests away from nursery crops of potted plants. They tried many things, but most harmed frogs as well as the pests. When they sprayed plants with concentrated caffeine, the local frogs were not bothered, but slugs and snails died.

Waiter Waiter Percolator

The researchers found that as little as 0.01 percent (1 part per ten thousand) caffeine kept slugs off foliage plants. Higher concentrations of 1 to 2 percent (1-2 parts per hundred) killed  slugs and snails outright. Since an ordinary cup of instant coffee typically contains about 0.05 percent caffeine, getting the concentration all the way up to 1 or 2 percent is more like showering your garden with a double espresso than using the rinse water from your coffee pot.

Exactly how does coffee affect slugs? Most likely, caffeine affects mollusks’ nervous systems. Coffee jitters that make humans uncomfortable might prove fatal for slugs and snails. Mollusks may be especially vulnerable to contact poisoning from caffeine because it is very water soluble. Slugs and snails move through the garden by creating a smooth layer of slime or mucus, residual trails of which often mark garden plants. This locomotive mucus contains lots of water, which makes it a perfect way to introduce water soluble substances to slugs. When they crawl over a caffeine-sprayed area, the chemical is absorbed directly into the mollusks’ soft bodies.

Mulch With Coffee Grounds

The research clearly indicates that frogs are not susceptible to sprays of caffeine. That’s very good news, since even environmentally benign slug baits such as Sluggo that are based on iron phosphate are not considered safe for use near water.  This is because frog and fish eggs are especially vulnerable to pesticide and herbicide damage. Even the nitrogen from grass clippings tossed carelessly into streams, pools, ponds or open ocean water can kill susceptible eggs. A few inches of fresh grass clippings generate enough heat to damage or kill eggs as well.

For now, mulching hostas and lilies with coffee grounds will keeps slugs at bay while providing nitrogen to your plants. Worms also love coffee grounds, which means you’ll be attracting tiny tillers into the garden to do the earth moving for you. Slug slaughtering coffee products could lead to a boom in the recycling of commercial coffee grounds.  A safe, eco-cool slug deterrent that could be used in damper sites would be a boon indeed for those gardening near water.