Q uestion: We grow tomatoes, peppers and other vegetables in the same spot each year and dump our fireplace ashes (a lot) into the garden and till them in with a rototiller. The plants are not doing well! Also, we are surrounded by tall trees. Is this why the garden is not doing as well as it did 20 years ago?

Answer: I’m sorry to hear that your garden isn’t doing well. I suspect that your yearly additions of fireplace ashes are the main reason why, though the trees may be playing a role as well. Let’s take a look at both possible issues.

Gardeners have dumped fireplace ashes into their vegetable patches for decades, but the practice isn’t necessarily a good one. Though ashes are high in calcium, potassium and several other plant nutrients, they can alter the soil’s pH to a point that’s not optimal for supporting plant growth.

The pH scale runs from 0 to 14, with 7 being neutral. Measurements between 0 and 7 are acidic, and measurements between 7 and 14 are alkaline. Most vegetable garden plants require a very slightly acidic pH of around 6.5 because that is the level at which the broadest diversity of nutrients is available for plant use. If your soil falls too far from that number, plant health and production can suffer.

In gardens where soils are very acidic, the addition of ashes can help raise the pH closer to that target 6.5. But, if you put ashes on soils that are already above pH 7, you’ll be doing more harm than good. Fireplace ashes are very alkaline with an already high pH level, so do not add them to soils with a pH above 6.5. Since you’ve been adding a lot of fireplace ashes to your garden for so many years, I suspect that your soil pH is now too alkaline to support optimum plant growth.

The only way to confirm this is to take a soil test to measure your pH. You might also have a toxic amount of potassium, magnesium or other plant nutrient in your soil from adding so much ash for all these years. Again, a soil test will confirm this and give you suggestions on how to remedy the issue. Soil test kits are available from all of the Penn State Extension Service offices. There’s one in every Pennsylvania county.

For Allegheny County, the number is 412-263-1000. Westmoreland County is 724-837-1402. The cost of the kit is minimal, but the information the test provides is priceless when it comes to growing a healthy vegetable garden.

If your pH is above the target 6.5, stop adding those fireplace ashes altogether. In fact, for other gardeners who might be considering adding ashes to their gardens, I recommend avoiding the practice. It’s difficult to know whether you’re adding too much and tipping the pH scales too high. Instead, I suggest adding the ashes to your compost pile which will help neutralize their pH, but still allow the nutrients to make their way into the garden via the finished compost.

As for your trees, root competition could be playing a factor in the health of your vegetable garden, as could shading from the trees’ canopy. Trim back any overhanging branches to allow maximum sunlight to reach the garden. If you encounter any fibrous roots in your garden, don’t be afraid to remove them for the sake of your veggies. Trees can withstand having some of their feeder roots removed from time to time without any ill effects.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to tribliving@tribweb.com or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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