The Harvard Conservation Commission voted unanimously at its Sept. 23 meeting to ask the Community Preservation Committee for $50,000 to support a three-year program to combat the invasive weeds that infest many open-field conservation areas in Harvard.

Commission member Wendy Sisson said she expects the problem of invasive species to be a major focus for the Conservation Commission in coming years. Open areas are particularly at risk, and the Clapp, Powell, Williams, and Great Elms lands would be targeted for initial control measures.

On the Clapp land, for example, Sisson said, black swallow-wort is getting a foothold and it can quickly spread to nearby woodlands. Other invasive species that threaten to overwhelm the fields include Asian bittersweet, buckthorn, multiflora rose, and autumn olive.

"We are losing the value for which we purchased this land," Sisson said, explaining that some areas are becoming so choked with invasive plants that walking there is impossible.

According to the National Invasive Species Information Center, an invasive species is one that is "non-native to the ecosystem under consideration" and "whose introduction causes or is likely to cause economic or environmental harm or harm to human health."

The Williams land is one of the few open areas in Harvard that is extensive enough to offer a home for grassland nesting birds such as meadowlarks and bobolinks, according to Sisson.

Keeping the field open and grassy, she said, will encourage the birds to return.

Under the terms of the Community Preservation Act, which Harvard joined in 2001, grants from the fund can be used for the rehabilitation or restoration of open space.

The Conservation Commission arrived at the $50,000 sum through estimates from area organizations that do ecological restoration.