Spilling the Beans About Using Coffee with Hydrangeas and Other Plants

Love a steaming hot cup of coffee? Don’t throw away the grounds you use to brew it, or the extra liquid that remains in your coffee pot. Some of your plants will appreciate your taste for “the bean”. You may already be aware of this when it comes to using coffee near Bigleaf Hydrangeas. However, Roses, Begonias, Hibiscus, Holly, Clematis, and other plants that like acidic soil and need a boost of nutrients—a nitrogen boost, in particular—will thank you for being a coffee drinker. Many more plants will appreciate the fact that used coffee grounds can improve soil aeration.

If you’re ready to share your love of coffee with your plants, just keep the following in mind:

*Nutrients vs. Acidity: Don’t count on used coffee grounds to acidify your soil to any great extent. Coffee (fresh grounds, brewed liquid) is acidic, but the pH of ground coffee gets changed in the brewing. This can make it impossible for used grounds to make an appreciable difference to soil pH. Instead, count on used grounds to increase nitrogen, plus potassium and phosphorous. They can also be used to aerate the soil. Aeration will be achieved with the help of worms, traveling and tunneling in response to this soil amendment.

*Use Moderation: Even if you have a BIG taste for coffee, introducing it to your plants’ feeding routine should be done in moderation. Add used grounds around the base of certain plants once every two weeks or so. Moderate soil amendments will be most easily broken down in the soil, and this will be appreciated by your plants. This goes for outdoor plants such as Hydrangeas and Roses, houseplants such as Philodendrons, and Begonias which find themselves in and outdoors.

*Understand What Nitrogen Does: Nitrogen helps plants achieve green, leafy growth. But too much nitrogen can reduce their budding and flowering. Amend your soil with care, and with a watchful eye.

*Mellow Is Best: If possible, use grounds that have had time to age and mellow (think, compost). If you’re not yet a fan of composting, soak them overnight in clear tepid water, strain them out, and use the remaining mild “tea” (forgive the term, coffee lovers) on your plants. Water this fortified liquid into the soil near your Roses, Clematis, Hydrangeas, and certain other perennials. Then consider adding some to plants that are part of your indoor display. For most of your houseplants, this will serve as a nourishing drink.

*Use a Heavier Touch on Bigleaf Hydrangeas: If you have Bigleaf Hydrangeas—those with flowers that can be turned from pink to blue—you can add used grounds to the soil to aerate it. But to promote blue bloom color with the help of acidifying the soil, consider pouring cooled brewed coffee near your Hydrangeas. This method achieves the blue flower color well, when soil pH drops to around 5.5, and aluminum, present in the soil, is taken up by the plant.

*Know Your Plants, in General: Succulents such as Hens & Chicks don’t want to know about your coffee-drinking habit. They love alkaline soils, so even though the used grounds and diluted coffee-water may not be major soil-acidifiers, it makes sense to steer clear of using either with these plants.

*Know Which Hydrangeas You Have: The Bigleaf Hydrangeas, Hydrangea macrophylla, change from pink to blue with manipulation. On the other hand, Panicle Hydrangeas and others will stay whatever their starting shades are. Although you might expect a blush or ruddiness to appear on the blooms of your Panicle Hydrangeas as they age, more drastic changes will not occur. But you can still mix some coffee grounds or use a diluted "tea" to improve the soil near these others.

*Avoid the Roots: Amend the soil near your plants, but not right up near their roots. Treat used coffee grounds as you would treat any fertilizer: Assume that they can burn roots. You will want to add space between below-ground plant growth and this soil amendment. This entails mixing the grounds with the soil around your big Hydrangeas in the garden, around your Roses.... In smaller container plantings—say with Begonias—you'll want to mix, as well. This mixing is important not only to avoid fertilizer burn but also for two additional reasons: 1) so that decomposition and aeration begin quickly, and 2) so that your dogs or cats don’t get interested in the grounds, which can cause them health issues.

If you happen to make Swedish-style brews with an eggshell tucked into your coffee grounds, you can include the shell as part of your soil amendment. The shell will introduce some calcium. However, it will take time (months to a year) for the shell to break down, and it will rob a limited amount of nitrogen from the soil as its decomposition unfolds. Still, the calcium addition can eventually improve your soil. But unless this is part of your favorite way to brew coffee, don’t think you have to add anything to the grounds. And don't worry if your coffee already has its own addition or two. For example, if your coffee grounds are flavored—hazelnut, vanilla, whatever you like—it’s okay to use them. If they’re decaffeinated grounds, it’s also okay.

Utilizing some of your kitchen waste to nourish plants is a tried-and-true, resourceful way of ensuring good plant growth, as long as you don't make this the only fertilizer your plants will get. Saving the used coffee grounds from your morning "cuppa" can improve your garden grounds, if you know their effect (and the limits to their effect) on soil and on plant growth.

If coffee grounds contribute to your good gardening and you begin to light up your neighborhood with unusually healthy and gorgeous plants (big blue Hydrangeas and more), then be generous: Spill the beans about how you achieved such a garden. Invite your neighbors in for coffee; fill them up, and fill them in.