Not long ago, the cobblestone streets and federal mansions of Alexandria's Old Town were punctuated by dozens of bright gardens, both large and small, that augmented the city's historic sights.

Although some of them remain -- bursting with azaleas, tulips and lilies -- most of the largest sites have fallen victim to the huge surge in development that has caused land values to soar in Old Town over the past few years.

As property becomes more valuable and taxes go up, there is a growing fear in the city that the few green spots remaining will soon be lost.

"We just cannot afford to see those gardens in our town disappear completely," said Andrea Dimond, president of the Old Town Civic Association. "But with the price of land here and the great desire so many people have to move to this area, there is just a tremendous effort to build."

House values that had already been higher than the rest of the city climbed more than 7 percent in Old Town in the past year, acccording to the city tax assessor. Land is increasingly scarce and average prices for homes are normally said to be more than $200,000. At those prices, anyone who owns vacant land has a large incentive to build on it.

Unfortunately, the size of many houses in Old Town -- sliverlike town houses strung in rows -- can encourage development on even the smallest parcel of land. The minimum width allowed to build a town house is 18 feet by 16 feet. For people who have gardens that size, the pressure to sell can be powerful.

In order to prevent the development of the few remaining parcels of green space -- the city has 16 square miles and less than 4 percent of the land is undeveloped -- city officials and members of the Virginia Historical Landmark Commission are encouraging owners with plots of open space in Old Town to consider putting easements on their property.

These easements are contracts that stipulate that land will never be developed, even after the current owner has died. Normally the land is passed to the state, or a historic association in Alexandria.

When a landowner puts an easement on his property, his tax bill can be reduced by more than half. For many property owners saddled by skyrocketing tax assessments that have doubled land values in some parts of Old Town, there can be significant financial advantages to turning over the rights to development.

"It's really the only way we can keep intact significant plots of open space, and it would be a tragedy to see them lost forever," said Calder Loth, senior architectural historian with the State Landmark Commission.

There have been at least two notable successes for the commission in the last two years. The Lord Fairfax House at Cameron and North Saint Asaph streets, and the Justice Black house, on Franklin Street, each with majestic gardens in Old Town's most valuable neighborhood, have had easements placed on their property by owners who do not want to see their gardens become yet another cluster of quaint, but modern, row houses.

But there are many other private gardens that Old Town residents fear will disappear over time. Although the city does what it can to stave off this last gasp of growth in an overrun area of town, Alexandria officials recognize that their ability to broaden the tax base -- a much desired goal in this city where development is hampered by lack of open space -- is dependent on bringing new business and residents to the city.

"This is a major problem for us," said one city official. " . . . It is damn tough to get attention for a few little gardens."