Are Coffee Grounds Good or Bad For Roses?

Most of us count on the caffeine boost from a cup (or 3) of coffee to start our day. In fact, Americans alone drink an estimated 980 million cups a day! With all these coffee grounds laying around, wouldn’t it be great to repurpose them in your rosebeds?

Coffee grounds contain a combination of macro and micronutrients that have the potential to benefit your plants. But, don’t rush out and pour them on the soil just yet. It turns out that despite anecdotes to the contrary, this gardening “hack” is a bit more nuanced than many gardeners are aware of.

So- let’s take a closer look at the facts and myths surrounding the use of coffee grounds and their impact on your roses. Are they good, bad, or something in-between?

A Brief Overview

Why are spent grounds such a popular soil amendment? Well, if you’re like me, you’re all about a DIY approach to compost, and want to make use of excess kitchen waste.  Economical and easy-to-access, they are a tempting soil amendment.

Coffee grounds contain about 2% nitrogen, .06% phosphorus, and .06% potassium. They also contain small amounts of micronutrients like magnesium, iron, and calcium. Roses need these nutrients to grow, so it seems like they would be an excellent addition to your soil.

They are claimed to boost growth, repel pests, acidify the soil, and suppress fungal growth. Unfortunately, research doesn’t back up all of these superpowers. While they can benefit your garden, how much you use and your method of application matters.

Let’s look closer at the common statements made about using coffee grounds when growing roses, and what research tells us about their actual impact.

The Long Answer

Similar to using Epsom salt with your roses, a quick google search will give you conflicting advice if coffee grounds are safe to use with your roses. Some gardeners will say that it stunts the growth of your flowers, but others that they boost it.

Some say they’re good for worms, others say they harm them. Before your head starts to spin, here’s some sleuthing about what coffee grounds actually can and can’t do for your roses and the creatures that help them thrive.

Raising Soil Acidity

Roses prefer soil with a pH between 6-6.5, just slightly on the acidic side. Since coffee grounds contain nitrogen, many gardeners hope application will increase the acidity of their soil.

Unfortunately, spent they don’t contain enough nitrogen to impact soil pH. The nitrogen content of coffee is water soluble, so most of it is brewed into your cup, rather than left behind in the grounds. In fact, spent grounds have an average pH between 6.5-6.8, making them neutral.

To increase soil acidity, apply a product like elemental sulfur, which will interact with soil bacteria to transform into sulfuric acid, giving your plants a quicker boost. As with all soil amendments, this is only beneficial if your soil needs it, which can be discovered with a simple soil test.

As a Fertilizer Replacement

We’ve already established that the pH of coffee grounds don’t raise soil acidity. But do the grounds provide other beneficial nutrients to our plants?

They can, but the amount and method matters. The truth is, we lack good studies on the effects of spent grounds on roses, and have to rely on anecdotal evidence and research conducted on other plants.

Studies based on broccoli, viola, sunflowers, leek, and radish revealed that the plants responded to coffee grounds with stunted growth, possibly due to caffeine content. 

Other studies indicate a negative impact to seed germination. High doses of nitrogen applied directly can burn roots. So, it’s reasonable to proceed with some caution before liberally applying.

Knowing that, how can you use them safely and effectively? If you want to use coffee grounds to increase the nutrient content of your soil, adding them to your compost where they can decompose and mix in with other organic matter is the best option. Evidence shows that keeping the amount around 20% of your compost is the ideal amount to provide an excellent source of nitrogen.

If you don’t compost, use spent grounds sparingly, and sprinkle on the soil around your roses. On top, place a mulch of wood chips, leaves, and other organic material that will help it break down.

Using as a Mulch

Coffee grounds are composed of very fine particles that have a tendency to become compacted over time, forming a mat that repels moisture and reduces airflow.

You don’t want a hydrophobic barrier around your roses, so they are not a good choice to mulch your rose garden.

Instead, sprinkle no more than half an inch of coffee grounds into the soil and cover with a layer of organic compost or wood chips. This will help preserve moisture (roses are thirsty plants), and keep the ground from becoming compacted.

Coffee Grounds and Worms

Worms love organic matter, and spent grounds are organic matter, so feeding them via the leftovers from your morning joe is a win-win, right? It’s true that coffee grounds can provide nutrients for worms, but they must be mixed with adequate levels of carbon- containing materials.

In studies of 3 different composting methods, worm mortality increased significantly when adding spent grounds. However, this effect was mitigated by mixing in lots of carbon rich material, like cardboard. Be careful when using them in confined vermicomposting bins, where they have shown to kill and harm worms.

When applied directly to the soil, small amounts are unlikely to harm or attract worms. Once broken down and in interaction with other nutrients, coffee grounds can nourish them. They’re just not a prime attractant as some claim.

Using as a Natural Pesticide

This claim has been extrapolated from studies where foliar sprays with high concentrations of caffeine were applied. The caffeine content is actually much lower, and will not kill pests.

Does it discourage pests from taking over your roses? Maybe. You can find videos of slugs crawling right over coffee grounds to get to prized food sources, but also videos of them avoiding coffee ground barriers.

While anecdotes abound, there is zero scientific evidence they repel rose pests like aphids, spider mites, and Japanese beetles. The jury is still out on this one! Give it a try this spring and let me know how it goes.

Fungal Disease Supression

Here we have another resounding maybe. There is some evidence that coffee grounds have antimicrobial and antibacterial properties that might prevent fusarium wilt and Pseudomonas on tomatoes, spinach and beans.

However, soil microbes are a good thing, and we don’t want to kill off anything beneficial in our garden. We need further study on the impact of spent grounds on common fungal diseases in roses. If you like to roll the dice, conduct your own research and give it a try!