A Coffee Break For Your Garden

The soil in my high-plains garden is nothing like the rich, chocolate cake loam I grew up with in southern Idaho. No, I’ve got two extremes—sandy and dry, and heavy clay. So now I’m constantly on the lookout for free materials to build up my soil. I keep a compost pile by the garden for grass clippings and yard debris, and I haul in manure from my neighbor’s cows every fall.

Recently, though, I’ve started using another soil amendment in my garden—coffee grounds. I don’t drink coffee myself, but my local coffee shop lets me drag away all the coffee grounds I want for free. I love anything free, and I also like the idea of creatively using waste. After all, Starbucks alone generates over 200 million pounds of coffee grounds every year, so finding a way to reuse them is a great idea from a conservation standpoint. If you drink coffee at home, you probably generate as much as 100 pounds of grounds per year.

Even though I don’t drink coffee, I love its earthy smell in my garden. The coffee grounds also seem to accelerate plant growth and improve the texture of my less than ideal soil, but don’t take my word for it. Linda Chalker-White, Associate Horticulture Professor at Washington State University, has found the following benefits of coffee grounds in the garden:

Nutrition. Coffee grounds are a good source of phosphorus, potassium, copper, magnesium, and calcium—all nutrients plants use for strong growth. They’re also a reasonable source of nitrogen.
Improved Growth. In trials, coffee grounds in the soil have been shown to improve plant growth, particularly in soybean and sugar beet crops. Many gardeners claim coffee grounds work especially well for roses, boosting their growth and blooms while also improving leaf color.
Microbe Growth. Coffee grounds have been shown to attract beneficial microbes and fungi to the soil, which improves the soil structure. Earthworms also consume decomposing coffee grounds and carry them deep into the earth. Over time, this action aerates and improves soil texture.
Suppress Disease. Coffee grounds have been shown to suppress fungal and bacterial diseases, including Fusarium and E.coli in research gardens that grew tomatoes, beans, and cucumbers. If you live in a moist, humid area prone to fungal diseases, this could be a major gardening boon.
Pest Control. Some studies have found that coffee grounds are effective pest repellents. Slugs and snails probably dislike its texture, and its strong smell lingers in the air for several days after application. Spread a thin layer of coffee grounds around vulnerable plants to deter garden invaders.
Alters Soil pH. Studies have found that coffee grounds in the soil have a pH level around 6.2, so theoretically, they can lower the soil pH, although this condition is probably temporary. Use coffee grounds as you would peat moss. If you already have acidic soil, you’ll probably want to use coffee grounds carefully. On the other hand, if you have alkaline soils, coffee grounds may perk up acid-loving plants like hydrangeas and rhododendrons.

Put Your “Garbage” To Work!

Considerations

Research into the effectiveness of coffee grounds as a soil amendment is still fairly new. Before you cover your entire garden in grounds, consider the following potential downsides to coffee grounds:

Coffee grounds have been shown to inhibit growth in some crops, including brassica crops like Chinese mustard and ornamentals like asparagus fern. Researchers believe the grounds give off a toxic substance during the decomposition process that may stunt plant growth. The good news regarding this is that coffee grounds have been found to inhibit weed growth as well. So, your best bet may be to use coffee grounds to amend soils in mature landscapes. It won’t harm trees and shrubs, but it just may keep the weeds down.
Because coffee grounds have a very fine texture, they tend to become compacted. When used as a mulch, they form a crust that can prevent moisture absorption.

Recommendations

If you’re a coffee drinker or you have access to free coffee grounds, go ahead and use them. They’ll add nutrients to your garden, while improving soil texture. Here’s how:

Dig them into the soil in the spring. Spread one inch of coffee grounds, along with compost and manure, on the earth and dig it in to a depth of six to eight inches.
Side dress plants with a sprinkling of coffee grounds in July or August to perk them up and improve growth and blooms. Use about ½ inch of grounds and dig it lightly into the soil.
Add coffee grounds to compost. This is probably one of the best ways to use coffee grounds because you still get the benefits of disease suppression and plant growth without the potential pitfalls of overly acidic soil or compaction. Combine 10 to 20 percent coffee grounds with manure, grass, leaves, and other yard debris. A ratio of 20 percent coffee grounds has been found to provide good disease suppression, while over 30 percent coffee grounds can actually be harmful.
If you want to use coffee grounds as a mulch, mix them with a coarser product like cocoa hulls or wood chips or spread a thin layer of coffee grounds on the earth and then cover them with wood chips.
When adding coffee grounds to the garden, don’t add more than one to two inches per year. This ratio is enough to add nutrients and improve soil texture. Adding more probably won’t help and may actually harm plants.