JoAnne Skelly: Plant care claims: Fact or myth?

JoAnne Skelly

JoAnne Skelly

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I often notice horticulture claims that will supposedly benefit plants as I read the news. Two caught my attention this past week: use baking soda to control leaf and root diseases and coffee grounds to make plants healthy.

I wanted to explore the science behind these assertions, so I turned to my favorite urban horticulture researcher and myth debunker, Linda Chalker-Scott. Ph.D., at Washington State University.

Baking soda (sodium bicarbonate or SBC) is often recommended as a foliar spray for powdery mildew and rose leaf diseases. Does baking soda kill fungi? Chalker Scott says no (“Miracle, Myth or Marketing: Baking soda will fungi fail, and roses rejoice?”

What actually happens is that, in a laboratory, baking soda applications can reduce fungi from germinating and growing, but do not kill existing organisms. How it works it that baking soda raises the pH of the plant surface above the acidic level at which fungi species germinate best. However, this effect does not transfer effectively to greenhouse, nursery or landscape use unless applied at concentrations that can damage plants.

Still, it is promising and “researchers have combined SBC with other disease control measures, such as using it to increase the efficacy of newer, reduced-risk fungicides.” “Combined with horticulture oil, low concentrations of SBC may have some effectiveness on mild conditions of powdery mildew.”

Next, I read whether the addition of used coffee grounds around plants, either indoors or out, will “perk them up.” There are numerous claims saying grounds “aerate and acidify the soil, provide nitrogen, attract earthworms repel cats, kill slugs, prevent weeds…”

Coffee grounds’ acidity levels are not consistent, and pH can range from 4.6 (acidic) to 8.4 (alkaline) as they decompose. While disease suppression has been found in controlled tests, is unverified in gardens and landscapes. Crop benefits appear to vary by plant with some plants’ growth inhibited. This growth inhibiting effect may play a factor in supposed weed control.

Scientific data is not only incomplete, but conflicting. Chalker-Scott does recommend using coffee grounds at 10 percent to 20 percent of compost volume, but not more. She does not recommend using grounds alone as a mulch because they compact but feels a half-inch layer of grounds under four inches of coarse organic mulch such as wood chips is acceptable.

As a horticulturist, I always learn new researched-based information about plants, working with soil, plant maintenance and pest management when I go to Chalker-Scott’s website on horticulture myths, It’s great horticultural reading!

JoAnne Skelly is Associate Professor & Extension Educator Emerita at University of Nevada Cooperative Extension. Email


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