If you ask different gardeners if roses like coffee grounds, the answers run the gamut, from touting coffee grounds as a welcome nutrient booster to warnings about the high acidity of coffee grounds that can burn—and even kill—your roses.
The correct answer lies somewhere in between: coffee grounds can be beneficial to roses if used correctly. Coffee grounds have antioxidant and antimicrobial properties, which gives nutrients to plants, and as coffee grounds break down in the soil, they suppress plant diseases such as Fusarium wilt.
Read on for more information about what coffee grounds can do for your roses and how to use them.
Benefits of Coffee Grounds for Roses
Coffee grounds are an organic material that contains carbon, nitrogen, and other compounds. These nutrients are used by beneficial soil microbes in two ways: microbes feed on them, and they transform nutrients so they can be absorbed by plants. The grounds also have antioxidant and antimicrobial properties. As they break down in the soil, they can suppress plant diseases such as the fungus Fusarium wilt.
Roses do best in slightly acidic soil with a pH between 6.0 and 6.5. Contrary to widespread belief, adding used coffee grounds, which have a pH of around 6.5, to the soil does not make it more acidic. When it comes to adding coffee grounds to roses, using them in moderation is key because adding too much can turn the benefits into harm.
Should I Water Roses with Leftover Coffee?
With a pH of 5.5, brewed, liquid coffee is ten times more acidic than used coffee grounds. While adding leftover coffee to acid-loving houseplants is a common practice, pouring coffee on your roses on a regular basis is not a good idea, especially for potted roses. Instead, if you don’t want to pour the coffee down the drain, do the same as with used coffee grounds—pour it on your compost pile.
How and When to Apply Coffee Grounds on Roses
You can either add the coffee grounds directly to the soil around your roses, or you can compost the coffee grounds first. Adding the coffee grounds directly has more limitations. As explained above, when the soil microbes break up the coffee grounds, they use up nitrogen so there is a temporary nitrogen shortage in the soil although coffee grounds contain nitrogen, but it is not yet in a form that is available to your roses. That’s why it needs to go through the decomposition process first.
Instructions
Sprinkle no more than ½ to 1 inch of coffee grounds on the soil and work it into the top couple of inches of soil with a cultivator or hoe.
Working the coffee grounds into the soil is important because if you leave more than a very thin layer on the soil surface, the particles will lock together and form a water-resistant barrier. Rain or irrigation water will simply run off while your roses remain thirsty.
Using Composted Coffee Grounds
The second option, using composted coffee grounds, works better if you want to make your daily coffee grounds benefit your roses without worrying about any of the negative effects of adding too many.
Coffee grounds should not make up more than one-fifth of the compost in volume but that should be easy to follow even for heavy coffee drinkers. Coffee grounds are so dense and compact that they get lost in the mix of vegetable scraps and other green compost materials.
Once the coffee grounds are fully composted, they can be used in liberal amounts around your roses without any concerns (keeping in mind that the organic matter in any soil should not exceed 6 percent).
Coffee Grounds vs. Fertilizer
Roses need phosphorus to bloom, and coffee grounds are not a significant source of phosphorus—but rather, they contain high amounts of nitrogen. Therefore, coffee grounds are not comparable to a complete fertilizer (organic or inorganic), which has all three macronutrients nitrogen, phosphorus, and potassium.
Coffee grounds, even if they have been composted, should not be used as the sole fertilizer for most roses. The only exceptions are roses that grow in the wild and don’t need fertilization. It certainly does not harm amending the soil around your roses with coffee grounds, but you shouldn’t rely solely on coffee grounds if you expect your roses to thrive.