Coffee grounds, banana peels and wood ashes are believed by many to provide a big boost to growing plants, and University of Missouri horticulturists agree--to a point.
"I think coffee grounds and banana peels should be composted first," said David Trinklein, chairman of the University of Missouri horticulture department. "It's a good way to recycle, if you want to look at it that way, but I don't think there's anything magical about coffee grounds or banana peels."
Coffee grounds are relatively acidic, he said. "I have heard of people advocating you put your coffee grounds at the bottom of the plant. The only thing that does immediately is to promote acidity."
MU horticulturist Mary Kroening said hydrangeas, rhododendrons, azaleas and blueberries are among "the acid-loving plants" that coffee grounds could help. "Roses are acid loving, too and coffee grounds tend to increase the acid. Our water tends to be alkaline, so it would probably be beneficial."
Chris Starbuck, MU extension woody ornamentals specialist, sounded a note of caution. "People tend to get carried away," he said. "They think, if a little bit is good, a lot must be better. It's usually not a good idea. You can get the minerals out of balance if you overdo it."
Banana peels provide calcium, magnesium, sulfur, phosphates and sodium--all essential plant nutrients, Kroening said. But perhaps their best quality is "they're one of the quickest things to decay and rot, so the nutrients break down and the plant's able to get those nutrients quickly.
She said some gardeners "recommend putting banana skins in the holes before you plant roses or vegetables."
Wood ashes are one of the elements most commonly recycled in lawns and gardens. Unlike coffee grounds, they tend to raise soil pH, increasing alkalinity.
Starbuck strongly recommended a soil test before applying wood ash. "Most people will find that their pH is fine or too high already. So it's not a good idea in a lot of cases." If the soil is acidic, he said, "wood ashes are about half as effective as lime at raising pH, on a per-pound basis."
Cultivated soil tends to become more acidic over time, Trinklein said, and an occasional application of ashes can alleviate the problem. "It's the same as applying lime, but it's free, and it's something we have to dispose of anyway. But watch out not to get the soil too alkaline."
Wood ashes are likely to help in some situations, Kroening said. "If you've got fairly virgin soil that hasn't had agriculture on it, and a heavy canopy of oak, you might have pretty acidic soil, and ashes could help." First, she said, "get a soil test, and if the soil is slightly acidic, the wood ash could be beneficial."
Organic materials such as coffee grounds and wood ashes are "slow-release," Trinklein said. "They're not as apt to burn the plants. The bottom line is, it takes a lot of the material to give an adequate amount of any one nutrient for the plant."
On the other hand, over-application of organic material can harm plants. "Moderation is a good rule with all this stuff," Starbuck said. "You don't want a big pile of anything around your trees--not even mulch."
Trinklein agreed: "You find a lot of home-brew remedies, and we probably shouldn't thumb our noses at all of those things. But you wonder if the plants responding well did so in spite of the application of the coffee grounds or because if it."
Overall, composted material is almost always better for most plants than old coffee grounds, raw banana peels or other kitchen scraps, Kroening said. The myth that they have magical properties will survive, just as some gardeners cling to traditional practices that have long since been disproved.
"Some people say tea roses are supposed to smell better if you water them with very diluted tea," she said. "You just never know."