Much of the reason that he is so well known is his book Manual of Woody Landscape Plants. This is an essential book for garden designers, and there's not a day that goes by that I'm not looking up some plant or another to check on growth habit or information on cultivars.
For his morning lecture Dirr presented The Flowering Shrub Revolution from Abelia to Zenobia: Our Clients Love Color. The talk covered some of the most popular nursery plants, where they come from, customer expectations, and what interesting new plants are on the horizon. It was an very interesting review of shrubs, some familiar, some not. Topping his list of the biggest selling shrubs was boxwood. He seemed a little skeptical of this fact but anyone living here in the tidewater area would not be surprised. One shrub he mentioned that I was unfamiliar with was distylium, which he said would make an excellent substitute for evergreen shrubs like Otto Luyken laurels and dwarf hollies. Dirr also showed several plants that were in development like dark, almost black-leaved crapemyrtles.
It was a bit difficult taking notes, as Dirr was speaking a mile a minute (while his wife Bonnie cracked wise from the back of the lecture hall). I did learn one new word though : remontant. It means reblooming. I'm not exactly sure if there's a technical reason why you would say remontant rather than reblooming, but it does sound smarter and it's one letter shorter. Remontantisminess (is that a word?) is becoming ever more important for new plant introductions.
Dirr spent many years teaching at the University of Georgia, but is now retired and is started Plant Introductions Inc. several years ago to pursue his passion for introducing unusual new plant cultivars into the nursery trade. As Dirr pointed out, plants are shaped by economics as much as genes. New cultivars need to be developed with regularity and having a catchy name is crucial. A simple change in names can cause a plant to explode in sales. 70% of sales are from March to May, so plants have to be bred to be in bloom and looking their best at this time. Or they have to be grown in greenhouses to get a head start on the growing season. Or breeders can cheat a bit by using growth inhibitors to manipulate a plant's growth. You may have noticed a plant that ended up looking a bit floppy when it went from the pot into the yard without the special chemicals it was used to.
You're probably familiar with one plant Dirr helped to make famous: Endless Summer Hydrangea. Dirr came across the selection in a test field at Bailey nurseries in Minnesota. The plant had been ignored for years, but the nursery certainly took notice when Dirr became interested in it. Bailey's had it patented and went on to sell millions of plants. And now, new improved varieties like Twist-n-Shout are on the way (and beating Endless Summer in the reblooming department).
The holy grail of hydrangeas right now is a pink version of 'Annabel' with strong stems that can hold up the enormous flower heads. Some attempts are available, but Dirr said we don't seem to be quite there yet. It's only a matter of time though. Looking at all the hydrangea pictures was great, but alas, I can rarely use them as a designer. They are much too susceptible to deer damage to be used in most gardens around here.
In his afternoon lecture Dirr presented In Praise of Nobel Trees. He talked about his passion for trees, describing their inspirational, spiritual and generational qualities. He also went into a bit of detail about how they are misused through single species plantings and poor species selection. It reminded me of what Joan Crawford used to say:
Most of the trees he mentioned were familiar, like oaks, but he also praised less commonly used species and varieties like Kentucky coffeetree and parrottia (anyone know of any good examples of parrotia in the tidewater area?). A recent maple hybrid he recommended was Red Pointe. And keep an eye out for 'Bonnie & Mike' swamp white oak (named after the Dirrs).