Have you ever wondered if coffee grounds are good for plants? The home gardener will find plenty of sources online extolling their ability to “improve soil structure” and “promote plant growth.” But is it true? Will coffee grounds help plants? Yesterday, the Guardian’s James Wong published the results of a “rough-and-ready experiment” he conducted this summer to test the theory. Every day, Wong said, he sprinkled coffee grounds on one of two “identical vegetable beds containing a mix of tomatoes, lettuce, herbs and flowers,” creating an inch of mulch. The results? Not so good. Crop yield and growth was down in about two weeks in the bed that got the coffee grounds. Leaves yellowed, he wrote, and seedlings mostly didn’t germinate.

“While some species looked OK, none of the plants in the coffee group proved better than my basic control,” he wrote. 

What gives? According to Wong, the problem is caffeine. 

“One of the key functions of caffeine in the plants that produce it is allelopathy—the ability to reduce competition from surrounding species by suppressing their growth. Caffeine is packed into coffee seeds for the very function of suppressing the germination of other seeds,” he wrote. 

But some gardeners have had different results. I asked gardening expert Melinda Myers for her thoughts on the matter, and she said when it comes to coffee grounds, it’s all about how you use them.

“It can be helpful but it can also be detrimental,” she said. 

Some plants like coffee grounds, she said, while others don’t, so you have to be careful about where you’re trying this out. But in general, she said, she thinks grounds do more good than harm. She pointed me to research from Dr. Linda Chalker-Scott, extension urban horticulturist and associate professor at Washington State University, who found that, “when they are used properly, coffee grounds supply nutrients and provide other benefits that increase plant growth.”

The trick, Chalker-Scott claimed, is to use the right kind of grounds for the right purpose.

“In general, only composted coffee grounds should be worked in as a soil amendment, while either fresh or composted grounds can be used in a mulch layer. Fresh grounds are more likely to be phytotoxic, so keeping them away from direct contact with desirable plant roots is recommended,” Chalker-Scott wrote.

If you’re going to use coffee grounds directly as mulch, Chalker-Scott continued, apply a thin layer of the stuff—no more than half an inch—and then cover it with a thicker layer of wood chips or some other coarse organic mulch to protect the grounds from compaction. Use coffee grounds too much, she wrote, and they can “interfere with moisture and air movement in soils.”

Myers said she’s sprinkled a small amount of coffee grounds directly in her garden, and found it didn’t do any damage to her plants. Mostly though, she said, she composts the grounds first before using them, and she’d recommend other gardeners do the same if they’d like to put the byproducts of their coffee habit to good use. 

“If you have any concerns, that would be your safest bet,” she said.