Are Coffee Grounds Good or Bad for Rhododendrons and Azaleas?

Coffee grounds have many uses in the garden, including as a popular compost item. But are they good for rhododendrons and azaleas, or is this a practice that you should avoid? In this article, gardening expert Jill Drago examines if this practice is one you should actually do, or skip this season.

Written by Jill Drago Last updated: October 19, 2023 | 4 min read

Gardener using coffee grounds with rhododendrons in the garden

Contents

Azaleas and rhododendrons belong to the same genus: Rhododendron. These spring blooming shrubs love acidic soil. In fact, they thrive in it. So, what is one to do if you do not live in an area with acidic soil?

The internet runs rampant with gardening tricks and myths. Admittedly I love reading about them. Who doesn’t want a trick to make their garden more lush and beautiful with an at home hack as easy as adding coffee grounds to your soil?

Does adding coffee grounds to your garden actually work to acidify your soil? Will it actually benefit Azaleas and Rhodoedndrons? What are the pros and the cons? Let’s find out.

The Short Answer

The short answer here is, well….maybe? There is evidence that supports the use of coffee grounds for acidifying your soil. However, there is also evidence that coffee grounds can stunt plant growth. This is a “choose your own adventure” garden myth. There is a lot of conflicting evidence and at the end of the day this decision should be made by you once you are knowledgeable about the soil in your gardens.

The Long Answer

Close-up of a blooming lush Rhododendron bush in a sunny garden. This evergreen shrub has thick, leathery, oblong, oval-shaped, dark green leaves. Rhododendron flowers are large and showy, purple in color, grow in large spherical inflorescences. The flowers are funnel-shaped, have 5 ruffled petals and long protruding stamens from the centers. — To grow azaleas, acidic soil with pH of 6.5 or less is best, and adding coffee grounds to the soil can acidify it.

Azaleas and rhododendrons grow best in acidic soil. This means that your soil should have a pH of 6.5 or less. Many people already have acidic soil and they do not need to amend their soils at all. If you are unsure, the best place to start is to get a soil pH kit or a soil test.

Either way you will learn pretty quickly what is going on in your soil. If your soil is pH 7 or above, you may want to acidify your soil.

Coffee is an acidic beverage, if you drink it then you already know that. So at first thought it seems to make a lot of sense that adding something acidic to your soil will make your soil more acidic.

What is unknown is how much you should be using in your soil, and what form of coffee should you be using: used grounds, fresh grounds, or brewed coffee.

The Argument For

View from above: close-up of a square white tray filled with pressed ground coffee leftovers, on the ground in a garden, surrounded by Rhododendron leaves. Coffee is dark brown in color and consists of many tiny grains pressed into round disc-shaped clumps. — Adding coffee to soil is an organic, cost-effective method to acidify soil.

The best thing about adding coffee to your soil is that coffe is an organic material. It is also nearly free if you are a coffee drinker. If you are ready to add coffee grounds to your garden, let’s get into it.

Purely for the acidic value, fresh grounds or black coffee is the way to go here. The downside is that these two forms still have the full amount of caffeine which can cause stunted growth, more on that later.

Most of the acidity remains in fresh grounds or brewed coffee. Brewed coffee can be diluted with water in a watering can and applied the way you would apply any other liquid fertilizer.

If you prefer to use coffee grounds you can sprinkle them around the roots of your azaleas or rhododendrons and work them into the soil the same way you would work granular fertilizer into your gardens. Do not leave them to sit on the top of your soil. This can form a barrier that does not allow water to permeate the soil appropriately.

There is also some evidence that critters do not like the smell or taste of coffee. The aroma has been shown to deter animals from coming into your gardens.

The Argument Against

Close-up of a flowering lush Azalea bush in a sunny garden. The shrub has upright stems covered with lanceolate leathery bright green leaves with a smooth and glossy texture. The flowers are large, funnel-shaped, consisting of 5 white petals with bright pink throats. The upper petals have pink strokes and dark pink dots. — Adding coffee grounds to soil may stunt plant growth and cause water retention issues.

There are a few cons to using coffee on your azaleas or rhododendrons.

The first is the evidence that the caffeine from the coffee will actually stunt the growth of your plant. Because this is a garden hack there is not a ton of evidence on how much coffee is too much.

This is exactly why I don’t love the addition of coffee grounds to your soil. You do not want to damage your plants while you are attempting to help them.

The next con is that they can improve water retention in the soil so much so that it becomes an issue. Sure, water retention sounds great especially when it comes to plants that are shallow rooted and can dry out quickly such as azaleas and rhododendrons.

Unfortunately, soils that hold on to too much water can cause root rot as well as other fungal issues that can wreck havoc on your azaleas or rhododendrons.

Other Options

Top view, close-up of a young flowering Rhododendron bush in the garden, with soil strewn with granular fertilizer at the base. The bush is low, dwarf, has large leathery oval oblong leaves of dark green color and spherical inflorescences of large lilac bell-shaped flowers with protruding stamens and purple dots on the upper petals. Some flowers are dry, brown in color due to lack of nutrients. — Use soil acidifiers or add compost for a natural nutrient boost.

If you are a little weary of using coffee grounds in your garden there are other products you can use to acidify your soil.

When you get to your local garden center or favorite online retailer look for a soil acidifier. These products will be made up of sulfur or aluminum sulfate and are safe to apply to your soil. Use the label instructions to help you determine how much product you need to use for your space. Sprinkle the sulfur evenly over your soil every two months until you have achieved the pH you are looking for.

Adding compost to your garden is another great way to lower your soil pH. This may take a bit more time, but you will be adding a lot of nutrients to your garden at the same time and most likely will be able to skip any extra fertilizers.

Compost can easily be added to your gardens at any point of the growing season without any adverse effects. Add an inch or so to your garden beds in the fall or spring to provide your plants with a natural boost and you will be greatly rewarded.

Best of Both Worlds: Composting

Top view, close-up of a bio-container with various organic waste for recycling and composting. It contains waste such as ground coffee, many branches, banana peels, apple peels and other vegetables and fruits leftovers. — To provide nitrogen to compost, add used coffee grounds and paper filters to your compost bin.

If you are not quite ready to start adding coffee grounds directly into your garden beds, you can add your used grounds as well as the paper coffee filter right into your compost bin. Coffee can be a great provider of nitrogen to your compost.

This is simple enough to do, just take your filter and leftover grounds and chuck it right into your kitchen composter or your outdoor pile. Even though the grounds are brown, they should be treated as green material in your compost bin due to the amount of nitrogen they contain.

Be sure to balance them out with brown material such as dried out leaves, shredded paper, and those paper coffee filters! Tend to your compost as your normally would, don’t forget to turn it regularly!

Final Thoughts

Whether you are brave enough to add coffee to your gardens or not is up to you. I think that if used sparingly in your gardens it is safe to use coffee grounds to acidify your garden. However, if you need to lower you soil’s pH drastically I would go with a more tried and true method.