Michael Dirr

Shared his passion at the Pittsburgh Garden & Landscape Symposium

By Martha Swiss ©2014
Penn State Master Gardener

Dr. Michael A. Dirr, Ph.D. 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 stated during a recent interview. He is particularly passionate about trees, and 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.” 

Storm water mitigation is high on his list of tree benefits as well. He maintains, "Trees contribute so much to everyday life, the quality of life."  He shared his passion for noble trees, as well as new plants he’s developing that are particularly suited to growing in our region, “from Acer to Zelkova” during Spring 2014 in Pittsburgh. 

Now retired from the University of Georgia where he taught horticulture for nearly 30 years, Dr. Dirr is widely acknowledged as the leading expert on landscape trees and shrubs.

Dirr Garden
Dirr garden outside Atlanta, Georgia

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, Dr. Dirr along with two partners formed Plant Introductions, Inc. to breed, evaluate, and introduce new plants for the nursery industry.

“We started with an abandoned pig farm. It was just dilapidated. 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.”


Development of New Plants

During the symposium, Dr. Dirr discussed 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.”

He elaborates on the process: “You make a cross today or this year. You’ll harvest the seed maybe at the end of this growing season. You germinate the seed over winter. You plant it out. It takes you two to three years to get the plant in flower or to some stage where you can say it has good foliage or good form. You propagate it again; you vegetatively propagate it by grafting or cuttings. You test it some more and you’re in the fifth, sixth, seventh year. And you finally make a judgment call, and say, ‘OK, it’s time to go or it’s time to throw.’”

Michael Dirr on the Polar Vortex

Gardeners are beginning to see the effects of the harsh winter on their plants. Dr. Dirr, who lives near Atlanta, says, “We got hammered too, we had the same polar vortex.” He thinks damage on broadleaf evergreens, such as rhododendrons and azaleas, is likely.

“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, or if it’s brown it’s gone. We’ve lost crape myrtles down here this year, which is pretty much unheard of.”

He believes this winter will serve as a reality check for those who are planting marginally hardy plants in our area.

Dr. Dirr has been a pioneer in breeding plants like crape myrtles for northern zones and hydrangeas guaranteed to bloom. He says one of the pertinent questions in breeding plants is knowing what to breed. “You can breed all day long but if nobody wants it, it doesn’t make any difference,” he says.

Today, nurseries and other plant retailers are responding to customer desires when selecting what plants to offer. Dr. Dirr describes an important trend: “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. They want something to be sort of, retail acceptable year around. It’s definitely happened with hydrangeas. It’s hard to sell an old-fashioned Hydrangea macrophylla anymore…” if it isn’t one of the new re-blooming varieties.

Updates for Pittsburgh

During his afternoon presentation at the symposium, Dr. Dirr discussed some exciting new trees and shrubs for the Pittsburgh region. He lists a few of his favorites:

“There are some new oaks coming out that are unique in growth habit... Quercus bicolor—swamp white oak. It’s a fastigiated [upright] tree. It’s called ‘Beacon.’ We found the tree in Virginia.”

He is enthusiastic about a new filbert, 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. I think it’s one of the up-and-coming new trees. It’s absolutely gorgeous.”

“Nyssa sylvatica [tupelo] is an unheralded native tree that is finally receiving recognition,” he says. “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 for gardeners to consider planting new Dutch-elm resistant American elms (Ulmus americana) such as ‘Princeton,’ ‘Valley Forge,’ and ‘Prairie Expedition.’

Michael Dirr says hydrangeas have been his most important plant breeding work. This new variety, ‘Bloomstruck,’ will flower reliably in our climate since it blooms on the current year’s growth, a characteristic of newer hydrangeas that Dr. Dirr says has changed the market forever.

At the age of 70, Dr. Dirr’s enthusiasm for his life’s work remains palpable and he continues to be highly engaged in the field, teaching, writing, and developing new plants, as well as spending time in his own garden. “I think one of the great things about my career has been wisdom is acquired with certainly a sort of passion for your subject matter. You’re continually a student. You’re always in learning mode.”

Surprisingly, for someone with such a deep love of trees and who has travelled the world, he didn’t make his first visit to see the magnificent redwood trees of California (Sequoiadendron sempervirens) until last August. Upon seeing them, he says, “I just stood there and the magic word was, awe. I could hardly move, could hardly talk. I think trees inspire certainly. They are very spiritual. Obviously we need to plant and preserve hopefully forever.”


He sums up his love of gardening, “My goal in life is to be planting and when I fall over I have one hand on a plant, one hand on a shovel and I’m 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.”

Favorite plants in the Dirr garden

Dr. Dirr maintains that he is, most importantly, a “dedicated and humble gardener.” He and wife Bonnie tend three and a half acres outside Atlanta, “where all manner of ornamentals, fruits, and vegetables are nurtured to their fullest genetic potential.” He says, “Sometimes you question in the middle of summer, when it’s a hundred degrees and the humidity is ninety-nine percent, and you think, ‘Why are we doing this?’ Well, we’re doing it because we love it.”

Noble trees
  • White Oak (Quercus alba)

  • Sugar maple (Acer saccharum)

  • Tupelo (Nyssa sylvatica)

  • Magnolia (Magnolia)

  • Sweet Birch (Betula lenta)

  • Witch hazel (Hamamelis intermedia)

  • Viburnum (Viburnum). “I have over fifty viburnums in the garden right now. It’s hard to beat viburnums as a group. They give you multi-season interest.”

  • Crape myrtle (Lagerstroemia).”For summer bloom from June until literally August/September, how do you beat crape myrtles?”

  • Hydrangea (Hydrangea). “Hydrangeas, by goodness, hydrangeas…”

  • Chaste-tree (Vitex). “It’s a hummingbird plant, it’s a bee plant, it’s salt-tolerant. Unbelievable. Flowers on new growth. It’s another one of those multi-season interest plants. I just think they’re great.”

Fool-proof perennials
  • Coneflower (Echinacea)

  • Black-eyed Susan (Rudbeckia)

  • Aster (Symphyotrichum oblongifolius)


Books for Gardeners

Longing for Longwood

Best Bloomers


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