In a field behind the adult correctional facility in Plymouth, about 750 tree saplings stand in mounds of pea gravel. Twenty-seven varieties are represented, with some trees stretching more than 12 feet high.

The nursery bed looks like a simple one — a pile of gravel corralled by old concrete traffic barriers and surrounded by an irrigation system that waters the trees four times a day. But just a few kicks in the gravel reveal the rapid root growth underneath the pebbles.

The gravel-bed nursery began last year as part of Hennepin County’s efforts to prepare for the loss of ash trees to the emerald ash borer. More than 15 percent of the county’s trees are ash trees — about 1 million in total — and the majority of them are within 15 miles of infected trees.

Growing saplings without soil offers a quick and cost-effective way to raise hardy trees to transplant to other county property and diversify the canopy.

“Who would have thunk it?” said Peter McLaughlin, Hennepin County commissioner. “I just kind of marvel at how this nursery is working. Successfully planting trees in urban areas is a tall order, but this seems to be successful.”

The goal is to expand the nursery and grow and transplant 1,000 trees each year.

“It’s a great model and is really unique,” said Dustin Ellis, a forester for the county. “And it saves us money, too. Overall, it’s just a responsible and proactive part of the solution.”

Buying a balled and burlapped sapling might cost $100 to $300, while a bare-root sapling is only $20 to $50, Ellis said.

“The state is going to spend money on this no matter what,” said Karen Zumach, director of community forestry with Tree Trust, a local nonprofit that promotes tree planting. The gravel bed nursery, she said, “is a great solution to get trees in the ground to help make up for what we are losing.”

Pulling trees out of gravel is a lot easier too — with just few tugs, Ellis can lift a young elm tree without a shovel. “It makes transplanting simple and means we can have volunteers come help us, allowing for education and engagement,” he said.

The emerald ash borer was confirmed in Minnesota about seven years ago, making the next few years critical in preventing the mass loss of ash trees, Zumach said.

The Minnesota Shade Tree Advisory Committee is meeting with state agencies next week to talk about budgeting for a comprehensive plan to combat the pest. Once the legislative session begins, the committee will again bring the issue to the State Capitol.

“This is when we are going to really see an uptick in the death rates,” Zumach said. “We are in that sweet spot to do something.”

McLaughlin agreed.

“This really is a coming crisis,” he said. “We are talking about losing 1 million trees. That has a huge impact.” Dead ash trees are brittle, and falling limbs pose a danger, he said. Plus, he added, “There’s just the aesthetic of it. We don’t want to lose that.”

Hennepin County has not yet positively identified ash borers on its properties but plans to start removing ash trees this winter. “I’m sure we will find it when we start that process,” Ellis said.

Even the areas surrounding the lot with the gravel bed nursery have ash trees that need to be replaced, he said. This fall, about 100 trees will be moved from the nursery across the fence to the woods surrounding Parkers Lake in Plymouth.

As he walked through the rows of saplings on a recent afternoon, Ellis pointed at his favorite varieties. There’s the spindly-looking Kentucky coffee tree that will fill out and grow wide. There’s the Liriodendron, commonly known as a “tuliptree” for its blooms. Then there are the more familiar elm and oak trees.

In a few months, each will be planted around the county as a piece of what McLaughlin, Zumach and Ellis hope to see as a more comprehensive plan to save the tree canopy from ash borers.

“As a forester, this gravel bed nursery makes my life a whole lot easier,” Ellis said.