He is also pained by the sight of trees being "horribly abused" during the erection of Frank Gehry's temporary pavilion in nearby Hyde Park. "When you see mistakes causing root damage like that, you know there's no arborist involved," he says.
It is such problems as these that Urban aims to prevent with his new book Up by Roots: Healthy Soils and Trees in the Build Environment - the result of five years' preparation and 30 years' experience in city greening.
"The book aims to bring information to local authorities, planners, city managers, engineers - anyone involved in putting trees in construction projects," he explains. "It's something an arborist can show to a local authority and say, 'Here's what you should be doing'."
The American covers the science of tree growth and its relationship to the soil ("only what you need to know or can control"), and derives from this 10 principles of tree-planting and management.Urban, who spoke at last week's groundbreaking conference on trees and climate change organised by Treework Environmental Practice, believes that greater appreciation of the role of trees in making cities liveable needs to be translated into practical improvements in their planting and care.
"There's concern about climate change at all political levels in the US, and an intuitive understanding that trees are part of the solution - everyone wants to be seen planting them," he says. "But when you try to make the budget changes at the Department of Public Works, you get resistance."
New York's mayor Michael Bloomberg launched the MillionTreesNYC programme last year, but Urban says the results could be mixed. "In Manhattan you can plant any old tree and still get great ones. But over in Brooklyn, once you've stomped on the marine soil there, nothing will grow."
Urban points out that a similar number were planted in Los Angeles for the 1984 Olympics. "There are not a lot still alive," he says. "Trees are simple to plant, but hard to get established. It takes a good five years before you know if you've been successful. The secret to the whole thing is soil, so you need to know what soil you have."
Analysing the site beforehand is vital, he says. "But in soil surveys it's often mapped simply as 'urban land'. So one of the best ways is to look at the plants growing there now.
"Old maps, planning records and photos can also tell you what past tree conditions were, and interviews with building managers will tell you if there are drainage problems. Plus you can just go out and take a soil probe to any hole you can find - there's always something under construction - and build up data points that way."
Soil differences also explain varying performances of street trees in Britain, he notes. "The soil in the Thames Valley is resistant to development pressures, and it was developed slowly - most of the ground is still within 1m of the grade it was in Saxon times."
He compares this to Milton Keynes, which he describes as an "arboricultural disaster". Having travelled there out of curiosity, he says: "I was shocked - I didn't see one successful tree. It will be a long time, if ever, before the town has a decent tree canopy."
This is partly down to its soil - a mix of clay and stone aggregate that Urban describes as "ideal soil to compact, then you're finished". The development of the new town also coincided with the arrival of large grading machines, which did precisely that, he says. "They flattened out the topography and killed the soil." He compares this to Columbia, Maryland, in the US - a new town from the same era, which is now "a green maze" of 15-20m-tall trees, though he attributes this to good soil rather than good planting. "They got lucky," he says.
Urban is also critical of some nursery practices that he believes diminish the long-term viability of tree stock. "Grafting can create a compatibility issue - the root may not even be of the same species. Grafted American red maples (Acer rubrum) often die 15-25 years later - eventually they reject the graft."
He is also sceptical of container-grown stock. "It's much worse than field-grown - repotting means you get root layering, and even with AirPots there is a problem with root circling. I have started sawing in half the rootballs of nursery trees and every one I've seen has had problems."
Nurserymen are also guilty of transplanting trees too deep in the soil, he has found. "But they have to be part of the solution."
- Up by Roots: Healthy Soils and Trees in the Build Environment is published by the International Society of Arboriculture and is priced at $86.95.
1971: Bachelor of landscape architecture, New York State College of Environmental Science and Forestry
1978: Founded Urban Trees + Soils, Annapolis, Maryland, US
1998: Inducted into the American Society of Landscape Architects (ASLA) Council of Fellows
2007: Awarded ASLA Medal of Excellence.