How To Use Coffee Grounds for Plants: A Gardener's Guide

If you don’t drink coffee, visit your local mom-and-pop café—many give away used coffee grounds for free. Starbucks also offers them through its Grounds for Your Gardens program.

What Are Coffee Grounds?

It might seem like an obvious question, but it helps to understand the difference between fresh and used coffee grounds—never use fresh grounds on plants—and its nutrient content, pH, and texture.

• Fresh coffee grounds are high in caffeine and acidity, which can burn plant roots. However, used grounds are low in caffeine and generally have a neutral pH (around 6.5 to 6.8), so they won’t acidify your soil. 
   
• Used coffee grounds are made up about 2% nitrogen, .06% phosphorus, and .06% potassium—plus, a small amounts of micronutrients like magnesium, iron, and calcium. However, the nutrients can not be taken up by plants immediately. The grounds need a few months of decomposition before nutrients become accessible.
   
• Composed coffee grounds can improve soil texture and boost organic content if properly incorporated. Don’t apply in thick layers—such finely textured particles tend to compact and form a crust that blocks water and air from reaching roots. (This is why they are not recommended as mulch.)

Benefits of Using Coffee Grounds

• Great for composting: Coffee grounds are superb for composting. Microorganisms break them down over a few months, releasing plant-available nitrogen. Their fine texture helps them mix and rot quickly. Just make sure coffee grounds make up no more than 15–20% of your total compost volume.

• Worm-approved! Coffee grounds provide grit for compost worms’ digestive process. Worms will also work coffee grounds deeper into the compost, ultimately helping improve soil structure and aeration. They’re nature’s plow!  Note: Some experts advise against adding grounds to worm bins, aka vermicompost bins, as the binds are too constricting, and the worms can die; others state that it depends on the type of worm.
   
• Natural slug repellent:  Coffee grounds help deter slugs and other pests. Their grainy texture and caffeine content are unappealing—and potentially toxic—to soft-bodied pests. According to Oregon State University, a 1–2% caffeine solution caused 100% of slugs to leave treated soil.

• Prevents disease: The “good” bacteria and fungi that help decompose coffee grounds help prevent the “bad” or pathogenic fungi from establishing. And that’s a very good thing!

Note: Caffeine can be toxic to dogs. While used grounds have lower levels, avoid surface application if your pup tends to snack on what’s on the ground. Composting or burying the grounds is a safer option.

How to Use Coffee Grounds for Plants 

As mentioned, it’s best to let coffee grounds sit for a few months—ideally in a compost pile (whether alone or with other kitchen scraps). For beginners, composting is the breakdown of organic matter (like leaves, grass, and food scraps) aided by microbes. A good compost recipe: 3 parts leaves, 1 part fresh grass clippings, and 1 part coffee grounds. Avoid exceeding 20% coffee grounds.

Turn the compost weekly with a pitchfork; it should be ready in a few months. Once composted, you have an excellent soil amendment. 

Tip: Make sure your grounds are cool before adding them to an outdoor compost pile. Hot grounds can kill the beneficial organisms that are doing all that work.

If applying composted grounds directly to soil, stick to a thin layer—no more than ½ inch. Just sprinkle on top or rake into the top two inches of soil. Don’t turn it under. Then, cover with a 4-inch layer or organic mulch—wood chips, bark, or leaves—to prevent compaction and protect the soil.

Tip: Test your soil before adding any type of fertilizer or even compost. If your soil is already high in nutrients or alkaline soil, hold off on adding coffee grounds—they’re not always needed, and too much nitrogen can harm plants.

Which Plants Do You Put Coffee Grounds on?

It’s a myth that used coffee grounds are acidic and great for acid-loving plants like blueberries, azaleas, and rhododendrons. In reality, once brewed, the grounds are nearly neutral in pH (6.5–6.8).

Composted coffee grounds are at their best as a soil amendment that feeds all your garden plants. They feed beneficial microbes, improve drainage, and contribute low nitrogen levels (about 1–2%). Note: You can’t rely on coffee grounds alone for nitrogen; pair up composted grounds with other nitrogen sources, like aged manure.

In addition, nutrient-hungry plants like roses may benefit from a light layer of coffee grounds, and mulched over with coarse mulch. Other heavy feeders include leafy greens (spinach, kale, lettuce), corn, tomatoes, cucumbers, squash, broccoli, and Brussels sprouts. 

Tip: Plants that are hungry for nitrogen have stems and stalks that often appear light yellowish-green in color, as well as leaves that show yellowing starting at the tip of the leaf and moving downward.

Which Plants Do NOT Like Coffee Grounds?

Never use coffee grounds when planting seeds or on seedlings or very young plants—especially with tomatoes and other delicate crops. 

Grounds can inhibit seed germination; caffeine (even low) stunts early growth. High doses of nitrogen applied directly can burn roots. 

Tip: Nitrogen is best added when plants are in full growth mode (eq, for tomatoes, that’s about 30 days after seeding).

Final Thoughts

Coffee grounds are a valuable, free resource for gardeners! If you wish to use grounds to increase the nutrients in your garden soil, it’s safest and most effective to add to your compost. And try it as a slug repellent! 

How do you use your coffee grounds? We’d love to hear your tips and tricks!