Hennepin County will move 500 hardy young trees to county properties as part of its fight to preserve the area's canopy in the face of spreading emerald ash borer infestations.

The trees — 12 kinds, all native to North America — were grown in a gravel-bed nursery at the Hennepin County Adult Corrections Facility in Plymouth.

On Monday, the county began the first in a series of relocations of the young trees, transferring them from the nursery to a more permanent place on the facility's property.

The county grew the trees in the gravel bed, the cost of which has been about $23,700 so far, to help them develop stronger root systems, said Mike Reinikainen, a Hennepin County tree specialist. The hardy and diverse trees will be used to diversify the canopy and to replace ash trees. The county is now home to about 1 million ash trees.

The county will plant hardier trees from the gravel bed at the Hiawatha greening project and flowering trees at the Cepro green space, both in Minneapolis.

About 30 trees will go to the Orono Work Station woodlot. And 10 to 15 trees will line the bike path near the new Vikings stadium in downtown Minneapolis, replacing ash trees removed because of construction.

The gravel-bed trees are expected to have a higher survival rate than commercial nursery trees.

"It is a cost-effective tool we are using to enhance the county canopy," Reinikainen said.

The county will plant two to three trees for every imperiled ash tree.

Minnesota has instituted a quarantine on moving ash wood in Ramsey, Hennepin, Houston and Winona counties. About 998 million ash trees grow in the state.

Hennepin County is working with the University of Minnesota to research trees that can survive in urban areas.