Are Coffee Grounds Good For Tomato Plants?

Creative gardeners are always looking for ways to boost production. One topic that gets a lot of buzz in garden forums is using coffee grounds to fertilize plants. Coffee grounds can provide benefits to many plants, including tomatoes, but the impacts vary depending on how they are used. Learn how to effectively incorporate coffee grounds into your gardening routine to supplement tomatoes and other garden plants.  

The Effect Of Coffee Grounds On Tomato Plants

Gardening myths abound regarding the use of coffee grounds in the garden. By looking at the chemical properties of coffee grounds, we begin to flush out facts from fiction and expose the true benefits and disadvantages of applying coffee grounds to tomatoes. 

First, it is important to understand the differing acidity levels of coffee and coffee by-products. While brewed coffee itself is acidic, the spent grounds that remain after brewing are only mildly acidic to neutral. During the brewing process, the acidic components in coffee beans are dissolved and end up in your mug, not in the spent grounds. Studies have shown that applying coffee grounds to soil or compost does not consistently lower pH. This is good news for most gardeners, but if lowering soil pH is your goal, you’ll need to use a different method.   

Spent coffee grounds can contribute to healthy soils by adding organic matter and nutrients. The spent grounds are composed of nitrogen-rich proteins and contain potassium, small amounts of phosphorous, and other essential plant nutrients including calcium, iron, magnesium, and zinc. However, these nutrients must first be broken down by soil microbes through natural decomposition or composting before that are available to plants. This can take many months. In the meantime, coffee grounds will not provide an immediate effect on tomato plants. Likewise, the overall concentration of nitrogen and other nutrients in coffee grounds is relatively low and will not replace fertilizers needed to provide tomato plants with the nourishment they need. As such, coffee grounds are best used in compost as a soil builder.

To that end, compost is the ultimate soil conditioner for both heavy clay soils and loose, sandy ones. It improves soil structure, allowing air and water to move readily, while enhancing drainage and water-holding capacity. Compost releases nutrients slowly over time, feeding plants and beneficial soil microbes alike. One study showed coffee grounds applied to tomato plants as part of a compost mix suppressed common fungal pests. This research was conducted under controlled conditions and results in the garden setting have yet to be studied.

One common suggestion for using coffee grounds in the garden is to apply them as a top dressing around plants. This is a case where the potential drawbacks outweigh the known benefits. Like any mulch material, coffee grounds can help retain soil moisture and moderate soil temperatures. However, due to their very fine texture, coffee grounds easily compact and create a barrier to moisture and air movement, especially when applied in thick layers. In addition, studies have shown coffee grounds inhibit seed-germination in certain plants, which can provide limited weed suppression, but also negatively impact some desirable plants, such as geraniums, clovers, and mustards. Further research is needed to compare the pros and cons of top-dressing tomato plants with coffee grounds. Until more is known, it is best to limit such applications. 

How To Use Coffee Grounds For Tomato Plants

Given the above information, we can identify best practices for using coffee grounds in the garden. Research demonstrates the single best approach is to add coffee grounds to the compost pile or tumbler. This allows microbes to break down nutrients into forms usable by plants and mitigates
potential negative impacts associated with direct applications like top dressing. Apply finished compost to the garden to provide supplemental nutrients as well as organic matter. 

As a rule of thumb, limit the ratio of coffee grounds in compost to 10-20 percent of the total volume. Strive for a healthy mix of green (kitchen scraps, coffee grounds, grass, and garden clippings) and brown materials (straw, dead leaves, sawdust) in your compost bin. Allow coffee grounds to cool before adding them to your compost. Paper filters will also decompose in the compost pile. While earthworms are shown to use coffee grounds in the garden, grounds are not recommended for the confined spaces of vermicomposting bins. 

To supplement tomato plants, apply a one-inch layer of finished compost to the surface of established garden beds once per year in spring or fall when adding fertilizer and other soil amendments. Work compost and other amendments into the upper six to twelve inches of soil using a garden fork or tiller. For new garden beds, you can increase the amount of compost to 3- to 4-inches. Remember, compost alone will not provide all the nutrition tomatoes require for growth and production. Likewise, do not rely on composted coffee grounds to alter soil pH.  

Top dressing with fresh (uncomposted) coffee grounds is not recommended. However, if you have no other way to use grounds, you can make limited applications by top dressing plants with a very thin layer, no more than one-half inch thick. Rake grounds into the upper soil layer to avoid creating an impenetrable mat on the soil surface. Or better yet, mix coffee grounds with coarser mulch materials to reduce the likelihood of compaction. Do not use uncomposted coffee grounds in beds or containers where seeds are to be planted.

Risks Of Using Coffee Grounds With Tomato Plants

If applied as a thick mulch or top dressing, coffee grounds can form a dense mat that reduces water and air penetration into the soil. Additionally, coffee grounds should not be used in seed-starting mixes as they may inhibit germination.

Other Considerations For Tomato Plant Care

Tomatoes thrive in loamy soils with good drainage and high organic matter content. Adding composted coffee grounds to planting beds is a great way to build healthy soil for tomato planting but won’t provide all the required nutrients. When preparing soil for tomatoes, work in 2 to 3 pounds of complete fertilizer (e.g. 5-10-10 or 5-20-20) per 100 square feet of planting bed. Tomato plants are heavy feeders, but too much nitrogen early in the season will promote big leafy plants with few fruits. Wait to fertilize again until the developing fruits are about the size of a golf ball, at which time a nitrogen-rich fertilizer can be used to side dress plants.  

Tomatoes are heat-loving plants that thrive in the sunshine. Plant tomatoes in an area that receives at least 6 hours of direct sun per day. Wait until the threat of frost has passed before setting out plants in spring. You can get a jump start by starting seeds indoors or purchasing transplants from a reputable nursery. Water plants deeply to promote healthy roots, supplementing rainfall as needed to provide one inch of water per week. Plants in sandy soils or extreme heat may require more frequent irrigation. Maintain consistently moist soils for the best quality fruit.