Piedmont Brew

6 May 2009

In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950

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How long has the spent [recycled] coffee grounds issue been percolating below the surface of garden lore? Many believers apply the moist grains directly as fertilizer, some as mulch and some as part of their compost mix.

There are many claims about what the dregs of the breakfast table will do for your garden but few have been attributed to scientific research. There is one credible investigator who has examined but admitted she has not exhausted the subject.

Recently, Linda Chalker-Scott, PhD, Washington State U., reviewed 67 journal articles about spent coffee grounds effects in the garden. Chalker-Scott did not list the other journal articles she sifted through to find those 67, but did say theres more research on the wastes of coffee production than the end product.

Now coffee may feel acidic on an empty stomach, but in the earth, the grounds are not acidic. Several studies found that over time, as short as 21 days, the pH or acidity of treated soil shifts from mild (4 to 5) to neutral (7) to alkaline (8); so, the acid jolt is transitory.

Coffee grounds do negatively influence some plants germination or growth: alfalfa, asparagus "fern" (Asparagus densiflorus), white and red clover, geranium (Pelargonium x hortorum), inch plant (Tradescantia albiflora), mustard greens (Brassica juncea), and mustard spinach (Brassica campestris).

The death mechanism is unknown; so it's best to keep coffee's residue out of the newly seeded garden and away from tiny volunteer shrubs and trees.

However, if coffee grounds prevent germination and squash seedlings in some plants, they may do the same to those we consider weeds.

Another advantage is, decomposing coffee grounds seem to control, not eliminate the usual fungi that plague our gardens, like Fusarium (potato, tomato), Pythium (turf grasses) and Sclerotinia (bean, cabbage, cucumber, lettuce, melon, squash, tomato).

The theory is that coffee grounds assist the beneficial fungi in their defeat of the wicked fungi during the composting process. The caveat is that these findings were reported under controlled laboratory conditions.

Research has also shown that along with the known benefits of organic mulch, composted and mulched coffee grounds may be the source for improved growth of cabbages.

Dug in coffee slag and earthworms partner to improve soil structure, a good thing for Virginias piedmont clay. As the coffee grounds decay, humus is produced, providing additional structural and chemical improvement to the soil.

A few warnings: mix no more than a five-to-one ratio by volume of grounds to other materials in the compost pile; mulch no more than a half inch, then layer three to four inches atop with a denser organic mulch, preferably arborist wood chips.

For those who want to know what the active fertilizing agent is in coffee grounds, its nitrogen, up to 10%, with a carbon to nitrogen ratio as low as 11:1, the perfect balance for the garden.

Thats the boost your plants are getting and if that's what youre looking for, then be judicious in your application and your timing.

Tip of the Week

Gardening is the second most popular leisure activity, after walking. in the U.S. According to the National Foundation for Infectious Diseases, 84% of Americans participate in gardening or yard work every year; 31% of tetanus cases reported between 1998 and 2000 came from garden, yard, or farm injuries. Reduce your risk. Get vaccinated. Wear gloves and protective clothing.