Dirr to speak on noble trees, new plant introductions


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Michael A. Dirr has a love affair with plants that has propelled him on a straight-line trajectory through life.

"I think I jumped out of the womb and I wanted to be a gardener," he said during a recent phone interview.

He is particularly passionate about trees, and he advocates planting large shade trees, or noble trees, which he describes as "anything that spans generations, has a long life, supports wildlife, fixes CO2, spits out oxygen, prevents erosion and increases property values. We need large trees."

Stormwater mitigation is high on his list of tree benefits as well. "Trees contribute so much to everyday life, the quality of life."

As keynote speaker of the Garden & Landscape Symposium next Saturday, he will be sharing his passion for noble trees as well as new plants he's developing that are particularly suited to growing in our region. His lecture "In Praise of Noble Trees" begins at 9:15 a.m. at Shady Side Academy in Fox Chapel.

Now retired from the University of Georgia, where he taught horticulture for nearly 30 years, Mr. Dirr is widely acknowledged as the leading expert on landscape trees and shrubs. He has authored at least a dozen books on the subject, including the "Manual of Woody Landscape Plants," now in its sixth edition and the most widely used reference and textbook on the subject in the U.S.

In 2006, Mr. Dirr, along with two partners, formed Plant Introductions (www.plantintroductions.com) to breed, evaluate and introduce new plants for the nursery industry.

"We started with an abandoned pig farm. We cleaned it all up ourselves. We built all the structures ourselves. We've used our own private money to build a company along the way. It's been an experience."

During the symposium, Mr. Dirr will discuss how he develops new varieties of plants.

"Many of our plant introductions, particularly our tree introductions, are by serendipity, just by somebody with a keen eye and a decent background that can say, 'Hey, this is different. This is better. Let's test it. Let's see if it's worth introduction.' "

Mr. Dirr has been a pioneer in breeding plants such as crape myrtles for northern zones and hydrangeas guaranteed to bloom. There must be a demand, he says. "You can breed all day long, but if nobody wants it, it doesn't make any difference."

Mr. Dirr says retailers and their customers are looking for plants with long-lasting flowers.

"One of the key things that I hear from retail people is, 'Does it re-bloom?' They want every flowering shrub, for example, to start flowering in April/May and to be flowering in October/November.

"It's definitely happened with hydrangeas. It's hard to sell an old-fashioned Hydrangea macrophylla anymore" if it isn't one of the re-blooming varieties.

Mr. Dirr mentions some exciting new trees and shrubs for the Pittsburgh region:

* 'Beacon' swamp white oak (Quercus bicolor): "It's a fastigiate [upright] tree. We found the tree in Virginia."

* Corylus fargesii, which came into the U.S. in 1996 from China. "It's a remarkable plant. It has an exfoliating bark much like a river birch. It's absolutely gorgeous."

* Tupelo (Nyssa sylvatica) "is an unheralded native tree that is finally receiving recognition. We have weeping forms, fastigiate forms, better fall color forms, more disease-resistant forms, on and on and on."

He also gives the green light to plant new Dutch-elm resistant American elms (Ulmus americana) such as 'Princeton,' 'Valley Forge' and 'Prairie Expedition.'

At 70, Mr. Dirr's enthusiasm for his life's work remains palpable, and he continues to be highly engaged in teaching, writing, developing new plants and working in his own garden. "You're continually a student. You're always in learning mode."

Mr. Dirr, who lives near Atlanta (Zone 7), believes this year's harsh winter will serve as a reality check for those who are planting marginally hardy plants in their gardens.

"We got hammered, too. We had the same polar vortex," he says.

He expects to see damage on broadleaf evergreens such as rhododendrons and azaleas.

"The easiest way to test it is to take your pocket knife or pruning shears and do a little bark peel and see if it's still green or watery. If it's brown, it's gone. We've lost crape myrtles down here this year, which is pretty much unheard of."

Mr. Dirr has lost none of his passion for planting and has no plans to slow down.

"When I fall over, I [will] have one hand on a plant, one hand on a shovel and I'll be looking for a spot to put that plant in the garden. I'm going to keep at it until my last day on planet Earth."

The Garden & Landscape Symposium & Marketplace will run from 9 a.m. to 4 p.m. next Saturday at Shady Side Academy Senior School Campus, Fox Chapel. Cost is $120 per person and includes meals. Information and registration: 412-473-2540 or http://extension.psu.edu/garden-landscape-symposium.


Martha Swiss is a Penn State master gardener. Columns by master gardeners sometimes appear in place of the Garden Q&A by Sandy Feather, a Penn State Extension educator.

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