When I was a kid, my mom would sprinkle her philodendrons with a combination of coffee grounds and egg shells. The plants grew like wildfire, especially the one on the kitchen table. It got an additional jolt from the mixed vegetables my brothers and I buried in there during dinner time.
We were raised in a home where you weren’t excused from the table until your plate was clean. Mom and Dad would leave the table, having finished their meal, and us kids would sit and stare at each other until one of us came up with a brilliant plan.
We each took turns serving as lookout as the others deposited a spoonful of mixed veg into the planter. To this day, lima beans still make me gag. We hated them and the plant was an excellent way to dispose of them. Mom discovered our little secret a year or so later when the plant grew so large and was in need of a bigger pot and a soil refresh. Thank God she has a great sense of humor and didn’t have the heart to punish us. She waited for us to come home from school and revealed her discovery. Potting soil full of petrified veg. None of us denied it. How could we?
I’ve always reserved my used coffee grounds for the garden. Either by sprinkling them at the base of my rose bushes, as my mom did, or adding them to the compost bin. So far so good. But then it occurred to me that perhaps too much of a good thing may not be so good. I did it because my mom did it and it worked for her. I simply never questioned why. If coffee can wake me up, does it do the same for plants? Or is the whole coffee grounds in the garden thing a myth we believe simply because, well, mom did it?
Take this giant bag of used espresso grounds from Starbucks. I thought I’d hit the jackpot when I came across it…FREE! Very cool. But having a little knowledge about how to use them in the garden and the compost bin can save you a lot of heartache. Too much of a good thing CAN wreak havoc on your hard work.
Coffee Grounds As An Acidifier
Truth is, they aren’t that acidic once they’ve been used. Their acidity transfers to the water. Experiments have revealed that the pH of decomposing coffee grounds ranges from approximately 4.6 (slightly acidic) to 8.4 (slightly alkaline). As the grounds decompose, their pH continues to fluctuate. There’s no hard and fast rule for exactly how much acidity, if any, you’re adding to the blueberry or hydrangea bushes if you’re relying on coffee grounds to do the job.
Better to use an organic fertilizer like Espoma’s Holly-tone for acid loving plants. This way you’re certain of the amount of acid the plants are getting provided you follow the application instructions on the bag.
… as Mulch
Avoid mulching around plants with a thick layer of coffee grounds. Their fine texture and tendency to compact mean that water may not be able to penetrate the barrier and air circulation will be poor. Better to lay down a thin layer, no more than half an inch, and cover with a coarse mulch. Avoid mulching seeded areas and seedlings with coffee grounds as they have been shown to inhibit germination and growth of certain crops. However, there is some anecdotal evidence that coffee grounds can help reduce the germination of weed seeds around trees and shrubs. Makes me wonder if this could help my bindweed issue. It’s serious.
… as Compost
Despite their appearance, coffee grounds count as a green ingredient, the nitrogen source. In a perfect compost pile (something that continues to evade me as I am a lazy composter), a 30:1 ratio of carbon to nitrogen makes for a quick cooking compost. Nitrogen suppliers include fresh, wet ingredients like your kitchen scraps, grass clippings and tea leaves. The carbon source, aka the browns, includes things like shredded newspaper, paper egg cartons and fall leaves. Too much green, the pile gets stinky. Too much brown and the temperature needed to cook the compost lowers and the process slows.
Coffee grounds should account for no more than 20% by volume of the compost pile, according to this article written by Dr. Linda Chalker-Scott.
Use coffee grounds sparingly. It’s tempting to dump a 10-pound bag of used grounds around your plants and think you’re doing for them what coffee does for you. While they add insignificant levels of nutrients such as magnesium and phosphorus, they aren’t the magic soil amendment of myth.