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Q. When I was in a coffee shop recently I overheard a conversation about using coffee grounds in the garden. Do you have any information on this practice?
A. I have a longstanding habit of saving used coffee grounds, letting them dry out a little, and scattering them thinly around and under the blueberries, rhododendrons and other shrubs that prefer an acidic soil. The coffee grounds are acidic. They are also a source of nitrogen for the plants.
I’ve heard of gardeners using the grounds as a scented repellent to deter raccoons from raiding bush fruits. I’ve never used the grounds for that purpose, but it might be worth trying.
Because coffee grounds tend to cake over in water-repellent mode, and become mouldy when laid thickly on the ground, it is always best to scatter them thinly or scratch them into the soil. Coffee grounds are an excellent addition to compost heaps as well.
Q. In the container garden on my deck I have thriving compact rhododendrons and even fruiting plants like blueberries, but I am puzzled by a very healthy hydrangea that has produced flower clusters in a muddy pinkish-purple instead of the vivid blue they are supposed to be. Is there anything I can do to begin returning the blooms to the colour I want?
A. Substances that will acidify the soil will turn the flowers blue. Most commonly recommended is aluminum sulphate, but powdered sulphur could also be sprinkled on the soil and scratched in to begin the acidification process. Apply the sulphur away from the plant’s stems and closer to the pot rim.
Other acidic materials, such as used coffee grounds, can be scratched in to the soil in modest amounts at a time to help acidify the soil.
If you fertilize your rhododendrons with a product specifically for acid-loving flowering plants (rhododendrons, azaleas, camellias), use that, but apply it to container plants more sparingly than is recommended for plants growing in open garden areas.
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