I started some strawberry plants this spring, and I was wondering if I could use some kitchen scraps to fertilize their soil. While I know that most fruit and veggie scraps should be good to use, I was curious about coffee grounds in particular. So, are coffee grounds good for strawberry plants, or do they have too much caffeine and acidity?
Coffee grounds are good for strawberry plants since they add nitrogen and acidity to the soil. However, some are concerned about the level of caffeine in the coffee and if will be harmful to beneficial insects and microbes in the soil. For this reason, it’s best to compost the coffee grounds before using them.
So, while coffee grounds are good for strawberry plants, how exactly do they benefit them and how much acidity can strawberry plants take?
How Do Coffee Grounds Help Strawberry Plants?
Coffee grounds contain a good amount of nitrogen, phosphorus, magnesium, and copper, all of which are important to maintain a healthy plant. They also increase the acidity of the soil, which is helpful for strawberry plants as they prefer more acidic soil with a pH of 5.4-6.5.
Like most plants, strawberry plants require three main nutrients: nitrogen, phosphorus, and potassium (also known as NPK). Coffee grounds already contain two of these primary nutrients, making them a suitable fertilizer for strawberries (if you’d like to add more potassium, try adding banana peels).
Secondary nutrients are also important but aren’t needed in the same quantity.
Some of the secondary, or trace nutrients found in coffee grounds include:
Also similar to most fruiting plants, strawberry plants prefer slightly acidic soil with a pH of 5.4-6.5. This is good news because coffee grounds also have an acidic pH, but the level of acidity depends on if you’re using fresh or used coffee grounds.
Fresh coffee grounds have a pH of 5.5-6.8, while used coffee grounds (ones that have already been brewed) have a pH closer to 6.8. This is because when brewed, the coffee grounds lose a lot of the acid and caffeine through the water.
Either way, you can see that the pH of both fresh and used coffee grounds closely matches the soil pH that strawberry plants like. This means that you won’t need to worry about making the soil too acidic by adding too many coffee grounds.
Still, if you’d like to be safe, you can always measure your strawberry plant’s soil pH every month or so.
The two common ways to check for soil pH are with pH strips or a pH meter. I personally like using a pH meter since they’re affordable and easy to use. If you’d like a recommendation on a pH meter, you can check out my recommended tools page.
However, what about the caffeine content in coffee grounds? Can caffeine be harmful to strawberry plants or the soil’s health?
Do Coffee Grounds Hurt Strawberry Plants?
Coffee grounds can contain some residual caffeine, which is a naturally occurring insecticide created by the plants. While caffeine can kill pesty insects, it can also kill beneficial ones such as pollinators. However, the science regarding the harmful effects of caffeine on insects is still debated.
So, even though coffee grounds can help strawberry plants, there are some mentions of it harming the beneficial life in the soil and around the plants.
In my research, I’ve found that used coffee grounds likely won’t have enough caffeine to do a lot of damage (of course this depends on the amount used). However, if you’d like to take the safe route on this one, consider composting your used coffee grounds first to allow for the caffeine and acidity to decompose further.
How to Use Coffee Grounds as a Strawberry Plant Fertilizer
If you have a handful of coffee grounds, you can apply it directly to the strawberry plant’s soil. However, if you brew coffee daily, and have 1 or more cups of used coffee grounds, it can overpower the soil. In this case, consider composting the coffee grounds first to reduce the caffeine and acidity.
When applying the occasional handful of used coffee grounds (think the size of an espresso brick), your strawberry plant and its soil shouldn’t be impacted and will benefit from the extra nutrients.
However, if you make regular pots of coffee, which can have an average of 1 cup of used coffee grounds or more, this can become concentrated in the strawberry plant’s soil. While it likely won’t do much harm, it’s a good idea to compost these grounds first.
If you don’t have a compost bin, don’t worry—it’s not too complex. You can easily start a cold compost bin by using a spare trash bin or container and adding some kitchen scraps to it (see image below). If you notice that flies, mold, or a smell is starting to develop, simply apply a 1-2 inch layer of dry soil. As you add more food and yard scraps, apply more dry soil as needed.
Additionally, you can start a vermicompost bin to accelerate the decomposition process. Just make sure not to add too many coffee grounds to overpower the worms. If you’re interested in having a worm bin, check out my recent post about the best worms to use for composting.
Either way, when applying, make sure to keep the coffee grounds and compost at least 3 inches away from the strawberry plant’s stems. This will help prevent mold and disease from being introduced to the plant.
How Many Coffee Grounds Should You Use on Strawberry Plants?
As a general rule, use no more than 15-20% of coffee grounds to the total amount of soil or compost. For the infrequent espresso brick, you can spread it out on top of the strawberry plant’s soil. For the daily pot, compost the coffee grounds first. Too many coffee grounds can imbalance nutrients and soil life.
For potted strawberry plants, I’d suggest using coffee grounds in the same amount as an espresso brick (about a couple of tablespoons worth). Since potted plants have a limited amount of soil, the coffee grounds can add up quickly and become too concentrated.
Even though it’s difficult to use too many coffee grounds, it’s still a possibility. For this reason, it’s a good idea to check on your strawberry plants weekly. Look for any issues with its growth. This includes mold or disease.
Remember that coffee grounds are considered a “green” compost, meaning that it’s high in nitrogen. To properly balance your soil or compost, you should also be adding some “brown” materials (those high in carbon), such as leaves, paper, or wood chips.
When Should You Apply Coffee Grounds to Your Strawberry Plants?
You can add the infrequent espresso brick to your strawberry plants any time of year. For more frequent coffee grounds larger than 1 cup, add them to a compost pile in the winter. After 2-3 months, in the spring, the coffee grounds should be decomposed and you can apply the compost to the plant’s soil.
If you’re growing strawberries as a perennial, I would suggest not using coffee grounds on them in the winter. This is because the strawberry plants are more dormant and don’t need many nutrients. In fact, too many nutrients sitting in the soil can chemically burn the plant’s roots. In this case, it’s best to compost the coffee grounds over the winter.
Once the coffee grounds have completely decomposed over 2-3 months, you can apply the compost in 1-2 inch layers in the early spring (after the last frost) and every 1-2 months during the growing season.
On the other hand, if you’re growing strawberries as an annual, you can apply coffee grounds every 1-2 months. Coffee grounds break down quickly and are absorbed in the soil rather quickly due to their smaller surface area (compared to banana peels, eggshells, and other kitchen scraps).
Remember to keep an eye on your strawberry plant’s soil pH. This is especially true after using coffee grounds and other soil amendments. For a good visual on how to use a pH meter, check out the video below by Alberta Urban Garden.