Coffee Grounds and Ashes: Uses and NON Uses

Q. Brian in Schwenksville PA (home of the fabulous Philadelphia Folk Festival) writes: "I've been binge listening to past episodes of You Bet Your Garden as I drive across Pennsylvania, Ohio, and Indiana (87 episodes left before I'm caught up) I've learned a lot, thank you Mike!

"I'm raised bed gardener (beds are currently 8" to 10" high; but based on what I've learned will reframe them to 12"). In addition to growing annual vegetables and herbs I grow asparagus (200 sq ft.), raspberries (200 sq ft.), alpine strawberries (100 sq ft.), ten blueberry bushes, and three apple trees.

"In one of your past episodes you discussed the dangers of using cow manure (or was it horse manure?) as a nitrogen enrichment in vegetable gardens. If I understood you correctly a safer/better nitrogen enrichment would be coffee grounds. Was this only the case if you are mixing it in, or does this hold true for spreading it on top? I was planning on placing composted manure on top of my asparagus bed this spring. I was told that asparagus is very nitrogen hungry. Would I be better off using used coffee grounds?"

A. Great questions. Let's start with manure, as many listeners feel that it is what I am most full of. Cow manure is not nitrogen rich; it is actually a very balanced fertilizer, but it is weak in all of its nutrients and is almost certainly collected from animals that are factory farmed. Many years ago, my old friend Dr. Andy Weil was asked 'what's the most unhealthy food you can eat'? And he instantly replied "anything cooked by an angry person". And that's why I don't use cow manure; those are very unhappy animals.

Horses are more like cats; they enforce their own personal happiness. And their manure IS extremely high in nitrogen, which is somewhat tempered by the composting process. But fully composted your horse manure must be, as raw horse manure would burn plants to death and is loaded with weed seeds. (And yes, I learned both of those lessons the hard way. At least I know what it's like to grow wheat and timothy instead of tomatoes.)

FULLY composted horse manure (which was called 'well-rotted' in the old days) is the perfect fertilizer for nitrogen-hungry asparagus.

Now, on to coffee grounds. Rich in nitrogen as well as calcium and many micro and macro nutrients, spent coffee grounds are the best source of garden nutrition that comes out of your kitchen. BUT they are best used as a source of 'wet green material' in a compost pile. (And yes, I am aware that they are black in color; they're wet green material; OK??!!)

Anywho, they are the perfect complement to shredded fall leaves in a compost pile. The leaves are carbon. The grounds are nitrogen. Together they give birth to the BEST compost. The red wrigglers in a worm bin also thrive on spent coffee grounds as part of a balanced diet of other kitchen waste.

But using grounds alone is dicey. Their high nitrogen content grows big plants but inhibits flowering and fruiting. And their acidity limits their fresh use to plants that require an acidic soil, like blueberries, azaleas and rhododendrons. But these are flowering plants and there's that tricky 'inhibits flowering' thing. Theoretically, you could spread coffee grounds under azaleas and rhododendrons in the Spring, because their flower buds are already formed. With blueberries, you would apply them after harvest, to feed and acidify the soil and not get twenty-foot-high plants with no berries.

Hey; mix them into your shredded leaves and everybody goes home happy.

Q. Bob in Wardensville West Virginia writes: "I've got a lot of wood ashes and wonder if I can use them in the garden and if so where?"

A. The unfortunate short answer is 'no', Bob. However, the soils in West Virginia (and everywhere that receives a lot of rain) are naturally acidic; and ashes from a hard wood stove's are alkaline, so theoretically the ashes can be used to adjust the soil pH up to the optimal range for most plants of 6.5 to 7.

BUT everyone's soils are different. If you can test with certainty that your soil is overly acidic, you can use wood ashes to raise the pH. A professional soil test will indicate a certain amount of agricultural lime to use to adjust the pH to neutral. You may use 30% more wood ash by weight to achieve this, as wood ash, like lime, is rich in calcium but not as rich by weight.

However, if your soil is already neutral, the addition of wood ash will turn it into a killing field where nothing will grow except West Coast weeds that thrive on Death Valley alkalinity.

The best use for wood ash is on lawns with a low soil pH.

Many people foolishly 'lime' their lawns every season without having a clue. State subsidized soil tests will deliver an accurate pH reading (as well as almost criminal recommendations for overuse of chemical fertilizers which I urge you to ignore and then complain about). But they will also make a liming recommendation. So, again, use approximately one-third more wood ash than lime and you'll raise the pH as you add nutrients that lime does not contribute.