Garden Detective

Lots of people -- and until recently I counted myself among them -- are simply baffled by hydrangeas.

The most common type -- mopheads (above) -- are gorgeous shrubs that sport pompon blooms of white or varying shades of pink or blue. Lacecaps (below) are more delicate in appearance. Oftentimes, one purchases a pink variety, only to plant it in the home garden and watch it bloom blue, or vice-versa. How could such a thing happen?

Turns out, its all in the soil. If your rose-themed garden puts out blue hydrangeas, you needn't settle. Changing the pH of your soil -- and as a consequence, the color of your hydrangeas -- is indeed possible. Since outside factors (like nearby concrete walkways that leech lime) can alter your soil's pH, and because soil amendments must be continually added in order to maintain the ideal pH for the color blooms you desire, this is most easily done if your plants are in a soil-less potting medium (like Pro Mix) and in containers . But it can be done -- with perseverance -- in a garden bed.

Soil must be alkaline in order for hydrangeas to bloom pink and acidic to bloom blue. (pH testing kits can be purchased at most garden center or big box stores. Or you can dig up a small sample and bring it to your county's cooperative extension office for advice.)

To make your soil more alkaline -- and your hydrangea bloom more pink -- you need to add dolomite or lime several times a year in order to achieve and maintain a pH of 6.0 - 6.2. Feeding with a fertilizer that's high in phosphorus also would be effective, as phosphorus blocks the uptake of aluminum, which makes the flowers turn blue.

Increasing acidity will lower the pH of soil and result in blue or purple flowers. Shoot for the 5.2 - 5.5 range. This can be done by increasing aluminum in the soil by adding coffee grounds, compost or aluminum sulfate. If using the latter, read and follow package instructions carefully to avoid burning the roots. Also be sure to water the plants thoroughly before applying. Speaking of water, if you can't seem to get blue flowers, have your water tested. It shouldn't have a pH higher than 5.6.

The process isn't a one-shot deal, and unless you're growing hydrangeas in pots, must be repeated diligently to maintain the desired bloom color. White blooms, on the other hand, can never be altered.