Michael Dirr is launching a campaign for noble trees. These are the big ones, the trees you build treehouses in. They provide shade, hold on to carbon dioxide, give off oxygen and reduce water runoff. They are a major weapon in the battle against global warming.

Dirr is a god in the horticultural industry. He wrote the “Encyclopedia of Trees and Shrubs” that is used by all plant nurseries, and he has introduced to the market hundreds of new plants, including the “Endless Summer” hydrangea. He spoke in late January at the annual meeting of MeLNA — the Maine Landscape and Nursery Association — in Augusta.

Noble trees are the trees you plant for future generations, the ones “under whose shade you do not expect to sit,” he said, showing a picture of a tuliptree (Liriodendron tulipifera) that George Washington planted at Mount Vernon in 1785 and is still standing today. 

Tuliptrees are not native to Maine, although they are native to southern New England. According to Dirr, they are growing successfully in the Minnesota Landscape Arboretum, where the temperature drops to 35 degrees below zero. With the readjusted plant zones recently released by the U.S. Department of Agriculture, it might be worth trying a tuliptree in certain areas of Maine.

Tuliptrees have wonderful yellow flowers in the spring, and are the largest eastern hardwoods. A cultivar that Dirr likes especially well is “Emerald City,” which looks a lot like the species tree but has more lustrous leaves.

Big trees create an atmosphere that is comforting. Dirr said there has been a study that showed when parents bring students to visit a college, it is not the buildings or programs that most impress them, but the beauty of the campus.

And that beauty is often related to large trees. “Every college campus should be an arboretum,” Dirr said.

Trees have a lot of financial benefits. Dirr noted that in St. Paul, Minn., a study has shown that while elms make up only 1 percent of the trees in the city, they capture about 30 percent of the stormwater because of their large leaf surface and root system.

A website (itreetools.org/design.php) calculates the monetary value of any tree, based on its size and zip code. And this is in practical terms. A red maple in Cape Elizabeth measuring 15 inches in diameter is worth $20 a year for soaking up stormwater, cleaning the air and absorbing carbon dioxide.

Energy benefits can be calculated on the website, but you would have to create a design showing where the tree is located in comparison to your house. And it does not count the effects on property value and aesthetics.

Dirr has his favorite trees but doesn’t particularly care which ones people grow, except that monocultures should be avoided. He used the example of the elm tree. Many cities, including Portland, had elms everywhere, but when the Dutch elm disease hit, all of the trees died.

“You have to mix it up like jellybeans,” he said, showing a slide of a parking lot filled with different varieties of shade trees. “People don’t care what kind of tree their car is under, as long as it is in shade when they get out of work at 5 o’clock.”

Dirr is a big fan of redbud trees, which he admits are “not noble but noblette.” Redbuds (Cercis canadensis) traditionally have been only marginally hardy in Maine. But a lot of new native cultivars have been shown to handle colder temperatures.

He said a type called “Northern Strain” is doing very well at Coastal Maine Botanical Gardens in Boothbay. One called “Northern Herald” from South Dakota may be the hardiest, and when temperatures hit 35 degrees below zero in a field out West, “Silver Cloud” and “Whitewater” were still healthy while other cultivars were killed down to the ground.

Dirr would love to have a National Tree of the Year promotion, and his first nomination would be the redbud. “It’s a great tree, and it is one of our own,” he said.

He also likes oak trees — not only for their looks, but for all the things that are created from them, such as floors, furniture and barrels used for whiskey, wine and beer.

Dirr recently discovered a bicolor swamp oak that has an upright columnar shape. He is testing it in his own garden, and it will be introduced under the name “Beacon.” 

There may be too many red maples on the market, Dirr noted, but he likes one called Redpointe, which is 75 percent red maple and 25 percent sugar maple, and has excellent fall color.

Tupelos (Nyssa sylvatica) grow in Maine, but we are at the edge of their native range. Dirr said the northeastern-most native population he has seen is on the grounds of Coastal Maine Botanical Gardens. And the O’Donal’s catalog says Royce O’Donal found Maine’s largest tupelo growing behind the nursery property in Gorham.

While the American elm just about disappeared after the Dutch elm disease hit in the 1950s, they are making a comeback with a lot of resistant varieties. 

Dirr said the selection “Princeton,” which came from a tree on the Princeton University campus and has a resistance to the disease, is one of the classics for replanting. But others that have done well are “Jefferson,” “Valley Forge” and “New Harmony.”  Newer varieties that show promise include “Creole Queen” from Louisiana and “Prairie Expedition” from North Dakota.

“None are immune to Dutch elm disease, but they show resistance,” Dirr said.

He listed a lot of other varieties, mostly hardwoods, that he thinks should be used more often. And he made me proud to be a tree hugger.

HORTICULTURIST OF THE YEAR

Mark Faunce of Limington won the Al Black Award as Horticulturist of the Year at the MeLNA session. A past president of MeLNA and a member of many of its committees, Faunce is territory accounts manager for McHutchison Horticultural Distributors, and worked for many years as a nursery manager for Lucas in Portland.

Faunce also assists his wife, Susan, at Pond View Farm in Limington. The farm raises Icelandic sheep and Highland cattle, and has a wholesale greenhouse business.

Todd Bangs of Windswept Gardens in Bangor was named young horticulturist of the year. Windswept is owned by Todd’s father, Bob Bangs.

VOLUNTEERS NEEDED

The Portland Flower Show, scheduled for March 7-11 at the Portland Company Complex, is looking for volunteers. Assignments include staffing the various entrances to the show and providing support for vendors.

Volunteers are provided free admission for the day they are volunteering, and can get Master Gardener credits.

For information, contact Kerry Ratigan at 615-6271 or kratigan3@hotmail.com, or visit portlandcompany.com/flower

Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer who gardens in Cape Elizabeth and can be contacted at 767-2297 or at:

tomatwell@me.com