As the weather warms this spring and gardeners sit outside, sipping their morning coffee, they may face a new temptation -- to share that caffeine with their plants.
USDA researchers in Hawaii have discovered caffeine solutions aren't just a wake-me-up for tired humans. Depending on strength, caffeine can repel or kill two major garden/greenhouse pests: snails and their slug kin.
"That's been exciting news. But, gardeners should wait for more results before trying caffeine as a pesticide," cautioned Ward Upham, K-State Research and Extension horticulturist.
The researchers' results underline the point that caffeine can sometimes be a fairly effective poison. The challenge now is to determine safe, but effective application levels for particular plants and soils, Upham said.
"If you've been around many school science fair projects, you understand why," he said. "Sometimes treating plants with caffeine hurts the plant. Sometimes it seems to help the plant. Sometimes it has no effect. All kinds of factors could explain such mixed results, but you have to suspect plant species is one."
Soil absorption and runoff rates could be important, too. Historically, coffee plantations start poisoning themselves when years of decomposing plant litter raise soil caffeine levels beyond a certain point.
"So far, all that gardeners need to remember is this: Drenching soil or spraying plants with a caffeine solution is simply not the same as the better-known practice of spreading around used coffee grounds," Upham said.
For example, recent research at the University of Aberdeen, Scotland, found low-dose caffeine solutions killed all three earthworm species being studied. In contrast, gardeners who currently raise earthworms often add used coffee grounds to their vermiculture soil – "with mixed, but apparently no negative results," he said.
Experienced gardeners are naturally attracted to caffeine's potential, Upham said. Land snails and slugs are destructive plant pests, particularly in irrigated gardens and wet weather. They're also difficult to control.
The creatures can ravage bulbs, chew on roots and make seedlings disappear. They're often the culprit behind irregular holes in hosta, dahlia, ivy, cabbage, viburnum and other leaves. They create gaping wounds that invite bacteria into such fruits and vegetables as tomatoes, strawberries, potatoes and green beans.
"If you find some under a board or they come out after a rain, you learn they also have a 'yuk' factor," he said.
Snails carry their shell on the outside. Slugs may have an inner, vestigial shell. However, most slugs have a mantle (extra fold of skin) on their back, positioned like a woman's shrug or shawl.
Other than that, snails and slugs are basically the same naked-looking, pulsing cigar of stretchy flesh. They're covered in a sticky, sometimes drippy slime that soap can't clean from hands or clothes.
"Before now, caffeine has never received much attention as a potential organic pesticide," Upham said. "Perhaps that's because we humans have consumed so much for so long. We almost automatically cut back when we develop overdose symptoms – dizziness, twitching, irregular heartbeat, trouble sleeping."
The USDA Agricultural Research Service team who discovered caffeine's slug-disturbing effects was hunting for something different: a natural or easy-access way to control tiny tree frogs (invasive from Puerto Rico). The frogs can breed year-round. Their night-long mating calls can reach the sound level of a back hoe.
The scientists had tried such off-the-shelf products as soaps, Tylenol, pesticides and nicotine. They finally got good results when they tested a soil-applied, diluted caffeine solution in greenhouses where the frogs gathered.
A faster and totally unexpected outcome, however, was that slugs began to surface and die.
"In the Hilo studies, just treating cabbage leaves with a 0.01 percent solution led to a significant drop in slug feeding," Upham said. "To put that in perspective, instant coffee is about 0.05 percent caffeine. Brewed coffee is somewhat stronger … which could make applying leftover coffee a tempting idea, if we only knew more."