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July 26th, 2012

How to Change Your Hydrangea’s Color

The hydrangea that was a gorgeous shade of blue when I bought it last year bloomed pink this year. How can I get it back to blue?

blog-dougThe flower color of certain types of hydrangeas depends on the amount of aluminum they draw from the soil, which in turn depends on soil pH. In general, they bloom pink when soils are alkaline or neutral, or toward the blue/violet end of the spectrum when soils are acidic.

Many of the beloved mophead and lacecap varieties of Hydrangea macrophylla respond to changes in soil pH by shifting their flower color. Gardeners have learned to manipulate the soil to take advantage. It’s too late to change the color of your hydrangea this summer, but here’s how to alter it by next summer:

To obtain blue flowers, increase soil acidity by adding sulfur, with your goal being pH 5.5 or lower. Sprinkle about ¼ cup (if the soil is sandy) or ½ cup (for clay or loamy soil) of garden sulfur at the base of the hydrangea and water it in.

For pink flowers, use dolomitic lime to decrease acidity; aim for pH 6.5 or higher. Start with about a pound of lime, scattered around the root zone of the shrub and scratched into the surface. Use half that amount if your soil is sandy.

It’s not an exact science. Some soils (and some hydrangea cultivars) are stubbornly resistant to change. You may need to repeat the application, and even then you may never get the color you want. And don’t expect immediate results; lime or sulfur applied in fall will affect the following summer’s blossoms. Some soils can maintain the color shift for several growing seasons while others will revert after a year.

I used to live in Kansas City, where naturally limey soils tend to keep hydrangeas pink, and gardeners go out of their way to achieve blueness. Now I’m in Pennsylvania, where blue hydrangeas are commonplace and discerning gardeners strive for pink. To my eye, they’re all beautiful.  —Doug Hall

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March 6th, 2012

Wood Ash as a Compost Ingredient

I’m not sure what to do with all my excess wood ash from the fireplace. I need to lower my soil pH, so I can’t add the ash to the garden or compost.

blog-dougThe ash from a wood-burning fireplace contains nutrients that plants need, including potassium, calcium, and trace amounts of zinc, magnesium, and phosphorus. But those nutrients come with a warning: Wood ash is strongly alkaline. If added to soil, it will raise the pH, or the measure of a soil’s acidity or alkalinity. Where the soil is too acidic (pH too low) and a higher pH would be beneficial, wood ash can be applied to lawns or gardens in place of agricultural lime to raise the pH. But if you are dealing with a neutral or alkaline (high pH) soil, adding wood ash would be a mistake.

The same goes for adding wood ash to a compost pile. Consider including wood ash as a compost ingredient only if the pile contains significant quantities of acidic materials, such as shredded oak leaves or pine needles. Even then, apply the ash with restraint, because too much can result in a loss of nitrogen from the compost. At a higher pH, the nitrogen present in the compost pile will escape to the air as ammonium hydroxide; if you smell ammonia near your compost pile, it’s losing nitrogen.

If you decide you shouldn’t use wood ash in your garden or compost, and you can’t find a farmer or gardener who can use it, I suggest you send the ash to a landfill with the rest of your household waste.  —Doug Hall

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October 18th, 2011

Acidifying Soil for Blueberries

My poor blueberry plants look yellow, stunted, and totally unhappy despite my efforts to acidify the soil with garden sulfur. Should I keep adding sulfur until they green up?

blog-dougBlueberries grow best in soil with a pH between 4.5 and 5.5. (On the pH scale, numbers lower than 7 indicate acidity; over 7 means alkaline, and 7 is neutral.) In the regions of eastern North America where blueberries are native, soils that are this acidic are not unusual. Elsewhere, gardeners try to accommodate acid-lovers such as blueberries by lowering the pH, using garden sulfur or aluminum sulfate.

Sometimes it works. But some soils have another trait that foils the gardeners’ attempts: high cation exchange capacity (CEC). Because CEC represents the ability of soil particles to stockpile mineral nutrients for later use by plants, moderately high CEC is usually considered a good thing. But soil particles can also cling to the types of minerals that increase alkalinity and make it more difficult for the gardener to acidify soil; high CEC soils are “buffered,” in effect, and resistant to pH change. Soils with plenty of clay or humus tend to have high CECs.

If you get a professional soil test, be sure to ask for an analysis of CEC. A measurement less than 20 means it will be fairly easy for you to lower the soil pH with sulfur. Between 20 and 40, pH change is possible but will require repeated applications and monitoring. Above 40? Grow your blueberries in pots.  —Doug Hall

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