They are the flower thief's favorite quarry. Hydrangea blossoms have been known to vanish overnight from front-yard gardens and public parks. With just a few dried stems selling for more than $10, it's no wonder that these fabulous clusters known as "mopheads" are so highly coveted. Are the thieves just cash-strapped flower arrangers, or is there a florist's black market in hydrangeas that is driving the disappearances? Probably both.

What makes hydrangeas so different is their longevity. The mophead hydrangea creates a huge moplike head of sepals that are not fertile. This is the result of early French breeding of lacecap wild Hydrangea macrophylla that came west from China in the 19th century. With lacecaps, a cluster of insignificant fertile flowers is surrounded by a ring of showy colored sepals that attract pollinators. Hybridizers sought more sepals because they produced the color. Eventually, they bred out fertile flowers altogether, making mopheads sterile.

This has great significance in the garden. What makes all flowers fade is pollination. This signals the plant that it no longer needs bright color to attract a bee or hummingbird. With their purpose fulfilled, petals fade and dry up, falling away as the plant shifts gears to begin producing seed in the flower ovary.

With mopheads, the sepals are never pollinated, and therefore no signal reaches the plant to indicate that it's time to quit. The result is that a single mophead flower cluster can live on for up to six months. This means that summer-blooming flowers remain on the plants into autumn. As temperatures fall, the flowers change and become what growers call "antique." This means that the sepals take on new colors in irregular ways. For example, white may turn pale green and blues may even sport shades of burgundy. Compare that with the few weeks of flower life for other plants and you see what makes the hydrangea so incredibly neat.

On top of this, hydrangea flowers dry easily if they are cut and hung upside down in a dark closet. Not only do you enjoy extended garden life, the flowers continue on through winter indoors as dried material. This is why mopheads are stolen out of gardens. They can be dried and sold weeks, even months, later. When the hydrangea blossoms take on their antique fall color, theft rates rise dramatically.

Mophead hydrangeas originate in a coastal climate and benefit from steady moisture. Where conditions are drier, plants require generous daily irrigation. This ensures lush green foliage and the largest possible flower heads.

The availability of aluminum in acidic soil influences how blue a hydrangea flower becomes. In soils of high PH, around 7.0 to 7.5, you'll get the best reds and pinks. For good blues, a PH of 5.5 to 6.5 is ideal. Growers feed twice a year with aluminum sulfate to ensure that they get the bluest flowers.

Hydrangea macrophylla prefers winters of USDA Zone 6 or 7, but there are exceptions. New Endless Summer and white "Annabelle" are very hardy to USDA Zone 4. Blue "Penny Mac" and "Nikko Blue" are two examples of forms that take to winters in Zone 5. Varieties suited to your garden will be available in large sizes at your local garden centers.

In the garden, hydrangeas are truly magnificent bloomers that produce big color in the shade. This is a rare commodity, particularly in midsummer after rhododendrons have faded. Their preference is very bright filtered light or morning sun followed by afternoon shade.

This has made mopheads real problem-solvers for home sites beneath high canopies of very old street trees. Some of the finest old specimens of hydrangea can be found in Victorian neighborhoods, where they were coveted foundation plants.

But be careful if you plan to follow suit, because the hydrangea thieves are out there ready to steal your mopheads in the dark of night.

— Maureen Gilmer is a horticulturist and host of "Weekend Gardening" on DIY Network. Contact her at her Web site http://www.moplants.com or visit http://www.diynetwork.com.