Should You Use Spent Coffee Grounds as Fertilizer?

The internet is full of suggestions on how to use your household waste in the garden. From wood ash to entire fish heads, it can be difficult to decipher how useful these hacks actually are.

One of the most well-established kitchen waste recommendations for the garden is spent coffee grounds. It is used often as a soil amendment, mulch, and even as a fertilizer. From using coffee grounds to fertilize tomatoes or using them to help fertilize orchids, there are plenty of gardeners both in favor and against the practice.

This trick is so popular that many coffee shops now offer their spent coffee grounds for free to cut down on waste. But are coffee grounds as useful as they are believed to be? Let’s find out.

The Long Answer

Like many controversial gardening hacks, there are pros and cons. Understanding how coffee grounds actually impact the growth of your plants will help you decide whether it is worthwhile or not.

Potential Benefits

There are several benefits that many gardeners will tout as logical reasons to use spend coffee grounds as fertilizer which are highlighted in some peer reviewed studies. First, let’s take a look at why this might be a good idea for your garden.

Sustainable

The most often repeated benefit in the garden is that they are sustainable. Although gardening involves nature and the environment, in practice there are many elements of the industry that don’t benefit the environment.

Things like harsh chemicals or single-use plastics known to negatively affect the environment are still used widely in gardening. When it comes to fertilizers, there are luckily many natural fertilizer options that don’t have a negative impact on the planet.

However, overuse of synthetic fertilizers does have some negative impacts, from an increase in greenhouse gas production to soil imbalances that impact the natural functioning of the local environment.

That’s where coffee grounds come in. It’s a great way to make use of your waste in a way that may benefit the environment. It also limits your need to use harmful synthetic fertilizers.

Cost-Effective

Gardening is not always the cheapest hobby around. It certainly can be inexpensive if you choose your plants and products wisely, but there are many specialized products on our wish lists that don’t quite fit into a tight budget.

Many fertilizers aren’t considered expensive when you think about how little you need to use at a time, but when you have lots of plants to feed, the costs begin to add up. So, what could be more cost-effective than spending no money at all by using coffee grounds?

As we’ll discuss later, they cannot completely replace fertilizer use because of the balance of nutrients in the grounds. However, it can supplement fertilizer use, lowering your overall costs. This will save some extra money in your gardening budget to spend on what we all prefer splurging on – the plants themselves.

Nitrogen-Rich

The previous benefits are quite general. Now, let’s look at the benefits for your plants. After all, that is the purpose of fertilizing.

To understand the ways coffee grounds can be used as fertilizer, you first need to understand how plant nutrients work. You’ve likely seen the letters NPK on fertilizer packaging in your local nursery. These numbers refer to the three plant macronutrients – nitrogen, phosphorus and potassium.

There are also many other nutrients plants need to survive, from secondary nutrients like calcium to micronutrients like iron. But the three macronutrients are those plants need in the largest amounts to function correctly and keep themselves alive.

Studies of spent grounds show they contain around 2% nitrogen. Nitrogen has many functions in conjunction with the other nutrients, but it is largely responsible for strong leaf and stem growth and the maintenance of chlorophyll.

Leafy plants typically need more nitrogen, while flowering plants need slightly more phosphorus and potassium. They do contain traces of these other macronutrients, but not in high enough amounts to have an impact on growth.

The best way to reap the benefits of this nitrogen is by using them in compost. In your compost pile, the grounds will break down to release nitrogen over time. Combined with other kitchen and garden scraps that add other nutrients to the mix, your compost will become the perfect balanced fertilizer for your plants.

Using spent grounds on their own can lead to a nutrient imbalance in the soil. Excessive use can also have a few negative effects on growth – one of the major downsides.

Potential Drawbacks

There are documented drawbacks to using spent grounds as fertilizer. From attracting pests to harming the growth of your plant, let’s take a look at some of the reasons why you might avoid this method.

Negative Effects On Growth

As good as the benefits seem, there are some serious risks involved. One of the major downsides is the risk of inhibiting growth.

Due to the interest in using coffee grounds in the garden in the past few decades, many studies have conducted experiments to determine their usefulness.

Although most were conducted on vegetables, the results can be extrapolated to the other plants in your garden too. These studies largely found that excessive use will actually hinder growth rather than help it.

This issue relates to the caffeine content. While spent grounds do have less caffeine than fresh grounds – around 20% less on average – this caffeine level is still quite high for plants. It can be useful in some cases, but just like with humans, too much caffeine is not a good thing.

The way to get around this downside is to follow the phrase ‘less is more’. Don’t replace your compost entirely. Mixing in a small amount can have some benefits, but it should never be the majority of what the roots are planted in if you want to prevent damage.

Impact On Moisture Levels

One of the benefits of coffee grounds is that they are known to retain plenty of moisture, great for water-loving plants that can’t be left to dry out. On the other hand, this does come with a big downside.

When spent grounds dry out, they really dry out. If they are left to dry out completely, they become hydrophobic, repelling water rather than absorbing it. When used in thick layers or as a mulch, they can also form a dense layer that blocks any water from reaching the soil below. 

If you want to try using coffee grounds in your soil mixes as fertilizer, make sure you don’t miss any watering, or you will create even more problems with moisture retention down the line. In other words, if you’re a gardener who often forgets to water, it’s best to avoid using them completely.

When applying spent grounds as fertilizer, never leave thick layers on top of the soil either. Instead, mix the grounds in with the soil to break up the particles and prevent them from compacting.

Attracting Pests

The final downside is one that is most common indoors – pest problems. Using spent grounds increases your chances of soil-borne pest problems that will end up negatively impacting the health of your plants.

Fungus gnats are one of the pests to watch out for as they are attracted to moist environments. Thrips are also more likely to reside in moist and humid environments, typically found in houseplant soil amended with spent grounds.

Using them doesn’t guarantee you will have pest problems. However, it does mean you’ll need to be more vigilant. Pest problems need to be tackled as soon as they are spotted, or preferably prevented, to save your plants from permanent damage.