The dirt on geraniums has nothing to do with their everlasting popularity, their bright and cheery blooms, or their fancy and fussy leaves. Those geraniums that are ubiquitous in window boxes throughout the summer months are exceedingly disease-prone and require a lot of special care in production.

In the last two years alone, millions of geraniums were destroyed under orders of the U.S. Department of Agriculture after they were linked to diseased cuttings imported by a California grower. The cuttings were infected with a bacterial disease, ralstonia solanacearum, that is on the government's bioterrorism list and is a potential threat to crops such as potatoes, tomatoes, and peppers.

Karl Batschke, production director at Oglevee in Connellsville, Pa., the country's largest geranium producer, said his company's farms have never had a disease problem, but he said the outbreak "doesn't help the industry as a whole."

In the late 1970s, Oglevee developed a rigorous scientific process to eliminate bacterial and viral pathogens that had in the past threatened to destroy the geranium industry. The process is designed to prevent disease among the 30 million annual cuttings the company produces in the United States, Kenya, and Mexico and ships on to wholesale growers, who produce tens of millions more for the consumer market.

According to Oglevee, the process has become nearly standard throughout the industry and has allowed the development of geraniums that are far more vigorous, free-flowering, and disease-resistant than those of the past.

Although geraniums have been around for more than 200 years, probably the most popular was the "Martha Washington" geranium, referred to today as regal geranium or pelargonium x domesticum. Today's regal geraniums bear little resemblance to the Martha Washingtons likely grown by your grandmother.

Regals are an upright geranium with stiff, deeply toothed, pleated leaves and large, rounded blooms that come in a multitude of colors aside from the standard rosy red: bicolor white and dark purple, hot pink, raspberry, lavender, salmon, burgundy, and many more. Regals bloom heavily in the spring and sporadically in the summer and fall.

New types, however, are under development all the time. Batschke of Oglevee says the regal geranium is once again "gaining in popularity because of its ability to re-bloom much more readily than it used to."

The most common garden geranium is the zonal geranium, pelargonium x hortorum, known for its almost continuously blooming flowers and circular leaves with distinctive darker markings, or "zones."

These geraniums, too, come in a whole host of colors with either single or double flowers. The main reason for their popularity, according to Batschke, is that "consumers know it's a plant that's pretty forgiving." Not only are they drought- and use-tolerant, he said, but "people can go away for a weekend, come back and find their plants are still in good shape instead of flat on top of the pots."

Besides the upright geranium varieties, growers have bred many hybrids. Ivy-leafed geraniums (pelargonium peltatum) can grow up to 4 feet long and are perfect for cascading out of hanging baskets or trailing over the sides of a window box. They usually have medium green leaves, with or without darker markings, and flowers in the full geranium spectrum.

Scented geraniums are particularly nice if planted near a seating area, where their delicate aroma can be appreciated. The plants have trailing, mounded, or upright habits, but they all emit a noticeable scent from their leaves. The fragrance might be rose or lemon, apple or peppermint, nutmeg or cedar, depending on the variety. Most of them have small, charming flowers of five petals in colors ranging from white through pale and rosy pinks and lavenders.

Oglevee also produces a "pillar" geranium, a zonal variety that grows aggressively upright, if given some support. Planted inside a tomato cage on a rose pillar, they'll produce an entire column of colorful blooms.

One thing to keep in mind when designing with geraniums is the temperature outside. Meredith Hubel, a horticulturist at the Smithsonian Institution greenhouses in Washington, D.C., says geraniums "like bright light, but they don't really like hot temperatures, so they're most showy in early summer and again in the fall."

Hubel says there is a kind of "busy look" to geraniums, but they can easily be used with other kinds of plants. "I think they tend to go well with something a little bit more serene, like a foliage plant," she said. "Or something with a simpler look to it and maybe a spike in the center of the container because geraniums are so low growing."

At the Smithsonian, geraniums are even used for indoor arrangements during special events like luncheons or press conferences. "They're nice and showy," said Hubel, "so they're good if you're having a party, but they will only really last for a day or two."

Geranium care is fairly simple. Asked for the best advice on geraniums, Batschke of Oglevee says,

"Don't kill them with kindness. They would much rather be a little on the dry side than over-watered." And although geraniums can be brought indoors for the winter and kept on a windowsill, they tend to stretch in low light. It might be better to just wait for spring and the crop of new varieties.

Oglevee's new geraniums this year include two introductions from Europe, "Victor" and "Victoria." Victor has dark red, semi-double flowers and large, deep green leaves. Victoria has bright scarlet red semi-double flowers and medium green leaves with a dark zonal band throughout the center of the leaf.

Batschke says one of the most interesting new varieties is called Catalina, which has a green leaf with a white edge around it and a very hot pink flower. "It really is an eye-catcher," he said, "and it's one that I think is going to be extremely popular."

Jane Berger is a partner in The Garden Design Group LLC of Alexandria, Virginia.