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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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April 17th, 2012

The Best Soil for Strawberries

My order of 50 bareroot strawberry plants will arrive at the end of April. The supplier recommends mixing 10-10-10 chemical fertilizer into the soil. Can you suggest an organic alternative? I have some horse manure, but it doesn’t appear to be fully rotted to use at this time.

blog-dougStrawberries grow best where the soil is fertile, well-drained, and amply enriched with organic matter. If you have been using compost, organic mulches, and cover crops in your garden for several years, you may need to do nothing further before planting the strawberries. A soil test will give you an accurate reading of how fertile your soil is.

If your soil falls short of the ideal, I recommend you dig in some organic amendments before you plant. Composted horse or steer manure is excellent for this purpose; it’s sufficiently composted when it looks and smells like soil. Yard-waste compost is another good source of nutrients and organic matter. Sphagnum peat moss as a soil amendment adds organic matter but little in the way of nutrients. Ideally, you should prepare the soil a few weeks before planting the strawberries.

Water thoroughly after planting, then give each plant one cup of a half-strength fish emulsion solution or compost tea to jump-start growth. Follow up 3 weeks later with another application if growth is slow.

Let your supply of fresh horse manure decompose longer before using it. You can layer it with leaves, kitchen scraps, and other garden debris to make compost, if you wish, or simply let it age in a pile. Next spring, use it to top-dress the strawberry bed when the plants begin to bloom.  —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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December 13th, 2011

Sugar as a Soil Supplement

Someone told me that sugar is good for plants. I sprinkled some around my vegetable garden this year but didn’t notice anything different. How much should I apply?

blog-dougPlants don’t need you to add sugar to their soil; they make their own. Through the process of photosynthesis, which is powered in nature by energy from the sun, plants turn water and carbon dioxide into sugars. Plants use their self-made sugars as a fuel for growth and reproduction.

Sugar you add to the soil will instead feed soil microorganisms, such as bacteria and fungi. These naturally occurring microbes are nature’s recyclers; they help to nourish plants by breaking down the bits of organic debris in soil into their nutrient components—including the potassium, magnesium, nitrogen, and other elements that are essential for plant life. In this sense, sugar could benefit the plants in your garden by boosting the microbial population, thereby speeding up the rate at which nutrients become available.

But plants already have a process for encouraging microbial life. Soil scientists have discovered that plant roots exude sugars—sugars produced by photosynthesis—as a way of developing mutually beneficial relationships with microbes. By controlling the types and amounts of sugars they release, plants can select which kinds of microorganisms will colonize the soil around their roots. Not coincidentally, plants choose to feed the microbes that will provide them with the nutrients they need most. Compared to this sophisticated à la carte system, a sprinkle of processed sugar from your pantry is just junk food for bacteria.

By the way, there’s another reason some gardeners haul the sugar canister out to the garden. Sugar added to the planting holes of vegetable transplants is said to discourage root knot nematodes, a destructive soil-dwelling parasite that plagues many Southern gardens.  —Doug Hall

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October 25th, 2011

Forked Carrots

My carrots this year were a freak show! They were bent and branched and bizarre—nothing like the perfectly straight carrots you see at the grocery store. Where did I go wrong?

blog-dougCarrots are pickier about soil preparation than almost any other vegetable. They grow best in sandy loam that is loose and porous to a depth of at least a foot. Unfortunately, most of us don’t have perfect soil; one way of getting around this is to grow carrots in a raised bed, where you have greater control over the soil quality.

If you garden where the layer of topsoil is shallow or heavy with clay, choose varieties that form short, stubby roots or one of the globe-shaped varieties, such as ‘Thumbelina’ or ‘Parmex’. Work the soil thoroughly, breaking up clods and mixing in plenty of finished compost. Sow the seeds where they are to grow; don’t try to transplant seedlings. The root of a carrot seedling may fork if it encounters a stone, clod, or even a chunk of un-decomposed plant debris. Excessive nitrogen fertilizer is another cause of branched roots.

Be sure to thin the seedlings promptly to avoid the twisty, conjoined roots that result from crowded rows. Thin to 2 inches apart if you intend to harvest full-size carrots, or half that distance if you’ll harvest at the “baby” stage. Use scissors to snip off the excess seedlings instead of pulling them out; even the slight root disruption caused by thinning can lead to forked roots in the remaining seedlings.  —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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August 5th, 2011

Hot-Weather Wilting

On hot or windy days my hydrangeas wilt completely, even if the ground under them is moist. What’s going on?

blog-dougWhen plants wilt despite plenty of moisture in the soil, it’s not water they lack—it’s roots. Soil that is rich in organic matter, deep, loose, and porous, promotes extensive root systems that sustain plants through brutal heat and dry spells.

Unfortunately for your hydrangeas, the time to work on creating ideal soil is before you plant. It also helps to locate plants that are prone to wilting, like hydrangeas, where they will get sun in the morning and shade in the afternoon. Other chronic wilters, like impatiens, are more likely to stay hydrated in all-day shade.

Now that summer has set in, help your hydrangeas through the heat with a cooling layer of organic mulch on the soil. Mulch maintains soil moisture, reduces competition from weeds, and slowly adds organic matter to the soil. Your hydrangeas might also benefit from a cooling spray of water at midday, if you are so inclined.  —Doug Hall

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