ALTAMONT— At the entrance of the Cornell Cooperative Extension, off Route 85A, there is a 250-square-foot garden that looks to horticulturalist Sue Pezzolla’s boss, like a “puddle with dead plants in it.”

“He’s a farm guy,” she laughs, “and he doesn’t appreciate horticulture.”

The kidney-shaped bowl of dirt is actually a rain garden. It’s a simple gardening strategy being adopted on a national scale by those with green and black thumbs alike whereby a garden is built to capture water runoff from roofs and roads in order to stop pollutants in the water from reaching America’s waterways.

“Water runoff rarely goes through filtration plants,” says Pezzolla. “In Albany County, polluted water goes straight into the Hudson River. And it’s a lost opportunity that the water is going somewhere else instead of seeping into the ground and recharging our own aquifers.”

Last year, through a grant from the Department of Environmental Conservation, master gardeners at the Cornell Cooperative Extension worked with locals to plant four rain gardens — one each in Bethlehem, Cohoes, Colonie, and at the extension. This spring, as the perennials in the gardens poke through the soil, the extension will erect signs explaining what the gardens are, what they do, and who to contact for further information.

Rain gardens are actually a flowery name for one type of bioretention, a technique invented in the early 1990s in Prince George’s County, Maryland for using plants and soil to combat water pollution. The technique is so simple and effective that, according to Pezzolla, many new building developments are requiring rain gardens as a way to minimize the disruptive effects of developing the land. The state of Minnesota even has a slogan for it: “Saving the Great Lakes one rain garden at a time.”

Pezzolla is quick to weed out the most common question from people considering a rain garden: Will the garden breed mosquitoes? The answer is no. Rain gardens are dug only a few inches deep. They are designed to hold water for between four and 12 hours, depending on the soil, while mosquitoes in the Northeast need seven to 10 days to breed.

Rain gardens are typically between 150 to 300 square feet, depending on how much water runoff they are meant to capture. While there are formulas for calculating the garden size, Pezzolla is quick to point out that any size helps. Internet bloggers also dole out do-it-yourself advice, saying the best size for a rain garden is the size the gardener is able to manage.

While the shape of rain gardens vary, their composition is uniformly made up of native perennials. These hardy types of flowers, which most nurseries stock, soak up large amounts of water effectively but don’t need additional watering during dry spells. Brown-eyed susans and little blue stems are two native flowers that Pezzolla says are effective, but the extension provides a lengthy list of others to choose from that most nurseries stock.

“Using native perennials is often reason enough for people to make a rain garden,” says Pezzolla. Native plants are not only hardier and therefore less work for gardeners, but they are also less likely to overtake a garden like a non-native species. “People with kids also like rain gardens because it’s an educational experience for them to learn about local ecology,” said Pezzolla.

Along with native perennials, the other identifying feature of the rain garden is the flat bottom of the garden, which is more like a saucer than a bowl. The flat shape makes the water uniformly soak in so that the roots, compost, and soil can work their magic of filtering out toxins from pesticides or roads.

Since the rain garden captures water and slowly drains it into the soil, it is also important to construct one at least 10 feet away from a house’s foundation and away from septic systems.

Gardeners like rain gardens for what they do and how they look but they are not the only ones. Unlike other perennial gardens, rain gardens are not cut back in the fall but instead are allowed to decompose naturally through the winter. This provides food and shelter for what Pezzolla describes as “beneficial insects” like praying mantises, certain types of wasps, and ladybugs, which feast on aphids while in the larval stage. Once the plants are established, the rain garden can even be mowed in the spring before the new shoots come up, making it the lowest maintenance garden on the block.

Pezzolla says that, because of the use of native plants, rain gardens are an accessible project for most levels of gardeners, although she cautions beginning gardeners to “find buddies to plant one with who have a little more experience because no one wants to not succeed the first time out.” She also suggests that those interested in planting a rain garden stop by the extension at 24 Martin Road off of Route 85A to see the garden on site, and to get their soil tested. The extension also has how-to manuals and native perennial plant lists available for pick-up.

“A rain garden is a simple solution with a lot of merit,” says Pezzolla. “It isn’t the only answer but it’s a good answer.”