Are Coffee Grounds Good For Tomato Plants? {Not So Much}

While there are some good uses for coffee grounds in your garden, the simple answer is that coffee grounds are ineffective at best and harmful at worst for your tomato plants. Coffee grounds should not be randomly applied in your garden, not even for nitrogen-loving tomatoes.

However, there is more to this story. Coffee grounds do have a place in gardening and composting. Let’s explore the role of spent coffee grounds in gardening, and especially in growing healthy and productive tomatoes.

Midsection Of Woman Holding Fresh Tomatoes At Market

How Do You Get Spent Coffee Grounds?

If you make one or more pots of coffee every day, it naturally follows that you’ll collect grounds over time, but another great source of coffee grounds that most people don’t consider is gas stations and coffee shops!

These businesses have huge amounts of fresh coffee grounds going to waste every day, and they’re usually delighted to donate to local gardeners. 

Spent coffee grounds are incredibly abundant and eco-friendly compost, adding valuable nitrogen and nutrients to the soil when it would otherwise just be thrown away.

Aside from composting, this frequently ignored organic material has a wonderful variety of uses.

Since coffee grounds don’t expire, feel free to collect them in large bins until you feel ready to use them.

green gardening boots and jeans on a woman walking through her garden and watering the plants

Are Coffee Grounds Good For Composting?

Coffee grounds are excellent to use for tomato plant compost.

They’re easy to use and are a wonderful source of nitrogen, which is essential for many plants.

The average healthy compost needs an efficient ratio of nitrogen-rich “green” items, food scraps like spent grounds, and fresh plant material like flower trimmings.

You can also add carbon-rich “brown” plant material like pine needles, dry fibrous materials like leaf litter, and processed paper products.

With coffee grounds, a little goes a long way. You should avoid including more than 20% coffee grounds in your compost material.

Ideally, your compost should have a 30:1 Carbon/Nitrogen ratio to encourage decomposition.

Studies have shown that any higher proportions can inhibit or even stall the decomposition of your compost into raw material.

For even better results, try saving your coffee grounds in reusable containers like tins and cans until you’re ready to add them to your compost pile.

You can add thin layers of coffee grounds in between your other compost materials like leaf litter and garden trimmings to add nitrogen and prevent evaporation.

Coffee grounds are great for composting, so feel free to toss your morning grounds and their torn-up filters into the compost bin. Just remember, moderation is a virtue.

womans hands and arm in a white shirt holding a pot of red tomatoes on a marble kitchen countertop

Can Coffee Grounds Be Used As Mulch?

While coffee grounds might seem like a great option for mulching, you’ll quickly realize with this material that there are more drawbacks than benefits.

You’re quickly going to encounter a handful of issues that show why coffee grounds aren’t a good mulching material. 

Firstly, coffee grounds have a light and fine texture that theoretically would thicken and help retain moisture in the soil.

In practice, however, the coffee grounds are so heavy and dense that they will form a thick layer only effective at blocking oxygen and moisture from reaching your plant’s roots.

Studies have shown that a thick layer of coffee grounds may impede plant growth and germination. In the GrassRoots Garden in Eugene, Oregon, the use of coffee grounds at a 25% ratio by volume for mulching resulted in stunted growth for lettuce seeds.

For best results, mixing a 1:6 ratio of coffee grounds to soil or mulch will result in a healthy boost of mycobacterial life and nitrogen deposits without suffocating your growing tomato plant.

This mulch should be applied in a 2 to 3-inch layer over the surface of the soil. If you prefer not to mix the coffee grounds and mulch, it also works to apply a half-inch layer of coffee grounds and then deposit 2 to 3 inches of mulch over it.

Remember that tomatoes require well-drained soil and that it’s important to ensure if you’re using coffee ground mulch that the water can reach the roots and that any excess water isn’t trapped. 

black pot of red heirloom tomatoes on a marble kitchen countertop

Can Coffee Grounds Be Used As Fertilizers?

While it depends on the stage of their growth cycle, tomatoes are generally needy plants.

Understanding when your tomatoes need extra support and how to fulfill those needs will give your garden a great boost.

The truth about coffee grounds is that they’re much better off being used as a supplement to your fertilizer than as a replacement.

Nutrients like nitrogen, potassium, calcium, and phosphorus are all essential to keeping your tomato plants healthy and vibrant, and coffee grounds are only able to supply a portion of these essential nutrients.

Your average coffee grounds contain many important nutrients, but not enough in the right proportions to sustain any plant.

They’re an excellent supplier of nitrogen, and release it into the soil in a delayed reaction as they decompose.

While your plants won’t survive off of this delayed nitrogen alone, it is an excellent way to supplement your tomato plant over a longer period.

If you’re going to use coffee grounds with your tomato plants, it’s important to squeeze out any excess moisture to ensure that you don’t lock in excessive moisture around your plant’s roots.

Otherwise, this could result in root rot or fungal diseases. Mixing dry materials like soil or leaf litter with your wet coffee grounds can also make an adequately dry mixture.

Once you have a damp but not wet mixture, you can loosely scatter it over the soil around your plants or mix it into your fertilizer.

The grounds should always be dispersed evenly and prevented from compacting, just to make sure that moisture and air aren’t trapped in the soil.

You can aerate the soil by stirring it together with the fertilizer or coffee grounds with a garden fork or your hands.

arms and hands in a white sweater holding a steel bowl of cherry tomatoes

Can Coffee Grounds Be Used To Acidify Soil?

Tomatoes are known to grow best in slightly acidic soil between 6 pH and 7 pH.

Coffee is widely known to be acidic, which makes using it for soil acidification seem like a great idea.

Unfortunately, this theory doesn’t hold up in truth. There’s a wide variance in acidity and alkalinity in coffee grounds, and it’s just not a good idea to make unknown adjustments to your soil pHl.

A high soil acidity, between 4.5 pH and 6 pH, has been shown to stunt growth and reduce harvest quality.

To ensure the pH is optimal for your tomatoes, you should look at using traditional and specially designed mixtures which are made to acidify the soil.

You can also use home testing kits to determine your soil’s acidity and make adjustments with significantly more specificity.

overhead shot showing numerous varieties of heirloom tomatoes

Combating Weeds With Coffee Grounds

While coffee grounds are sometimes suggested for fending off weeds, other techniques are much more effective and much less harmful to your plants in practice. It is better to save your coffee grounds for better uses with proven methods.

If you want to prevent weeds around your tomato plants, a good place to start is by adding a 2 to 3-inch thick layer of mulch.

This will block sunlight from reaching any seeds of unwanted weeds, while also giving your tomato plant’s soil a boost. 

Using coffee grounds for this purpose would result in harmful soil compaction and waterlogging.

garden harvest including tomatoes, carrots, peppers, squash, and herbs

Killing Slugs With Coffee Grounds

Slugs and snails are one of the oldest enemies of gardeners, and everyone from a lifelong career farmer to a hobbyist homeowner knows exactly why.

There are a lot of solutions marketed to get rid of slugs, and one suggestion is to use coffee grounds. Unfortunately, this technique has been disproven many times over.

While Robert G. Hollingsworth of the USDA and his team discovered that a solution of 1-2% caffeine kills slugs, the Journal of Agricultural and Food Chemistry reports that the minimum effect percentage of 0.01% is more than your coffee grounds will contain.

And while anecdotes suggest that the smell and texture of coffee grounds may deter pests, it’s cautioned that it just isn’t clear whether this also deters beneficial life.

If you’re looking to ward off slugs and snails, you’ll likely have better success using products that are proven to deter or kill snails.

There are many natural techniques and mixtures available which will keep your garden organic, natural, and pest-free.

See Also: Are Slugs Good For Plants? {Preventation & Eradication}

red and green heirloom tomatoes on a tree stump outside

Eliminating Fungi

Fungal diseases can quickly kill even the healthiest crops, and the sheer effectiveness of these diseases is enough to lead many gardeners to investigate supposed cures.

Coffee grounds are one of these supposed cures, but in practice, they can do more harm than good.

They’re an ideal substrate for large fungi like mushrooms, which can lead to nutrients being taken away from your crops.

And while garden soil infused with coffee grounds at a 0.1% caffeine concentration has been shown to suppress fungal growth, this is a higher concentration than is available in used coffee grounds.

Worst of all, the qualities of dense coffee grounds can cause fungal growth if the soil becomes waterlogged.

It’s important to ensure that the coffee grounds aren’t negatively impacting drainage if you decide to use them.

See Also: Are Mushrooms Good For Plants? {Garden Friends or Foes}

Are Coffee Grounds Good For Tomato Plants?

While there are some good uses for coffee grounds in your garden, the simple answer is that coffee grounds are ineffective at best and harmful at worst. 

You should avoid using coffee grounds with your tomato plants and opt for more proven methods where possible, but it is worth noting that coffee grounds are still great to use in a compost mixture.

Coffee grounds contain good levels of some important nutrients for tomato plants like nitrogen and combined with other compost materials it can be excellent for fertilizer.

Lastly, it’s important to watch out for the common issues with coffee grounds and make sure that water can adequately move through the soil.

Compacted coffee grounds can become hydrophobic, preventing water from reaching the roots or preventing excess water from dripping out of the bottom of the pot. You should check that the drainage is adequate.

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