If trees could talk, they’d probably start by saying, “Enough with the insults.”
In more than 30 years of working with trees, Christopher Roddick has made it a habit to listen to their unspoken language – and to show respect for some of the largest and oldest organisms among us.
Mr. Roddick is the chief arborist and foreman of the grounds at Brooklyn Botanic Garden, a position he will be leaving this summer when he and his wife, Rebecca McMackin, the director of horticulture at Brooklyn Bridge Park, move north. Their first stop: a fellowship she is doing at Harvard.
But Mr. Roddick will not forget the trees in the Botanic Garden—old friends by now—and what they taught him. Especially about how we ignorant people expect them to adapt to our landscaping and home improvements, rather than the other way around. Too often we don’t consider the needs of our trees.
“As an arborist, I usually don’t get a call until a tree is in trouble,” he said. “And I ask, ‘Has something in the neighborhood changed?'”
At first, the answer is almost always “No” – and then something like, “Well, we put the patio in the neighborhood. But that was five years ago.”
“A tree can hold its breath for a few years, but then—” said Mr. Roddick, walking away, as if he didn’t want to think about all the bad results. “If you are attacked, you defend yourself. But for a tree that means without being able to get away.”
Trees have adapted, making compounds to prevent herbivores and fungal infections. But there’s no chemical strong enough for some of what we’re doing: we dig in or drive over their root zones. We do not prune because a tree needs it, but because a garden bed underneath has become too shady for our liking. We strand a tree in a sea of lawn, where it is exposed to unwanted fertilizer and an automated watering schedule.
“I’m not a big fan of planting a tree in the middle of the lawn,” Mr. Roddick said. “Most of the shade trees we use are woodland species and they often grow better in a group.”
For example, understory trees, such as flowering dogwood (Cornus florida), are not suitable for baking in full sun.
Beyond floriculture
The way Mr. Roddick looks at trees has changed since he first studied horticulture.
“I learned about trees from the point of view of selecting species for their ornament — from what’s in them for us to look at or eat,” he said.
But the appearance of a tree is not the whole story.
“If you look at a tree, you only see part of it,” he said. “But one-third to one-half of its mass is underground — huge root systems usually located in the top 18 inches of the soil.” In compacted urban soil, they may be in the top six inches.
Today he is more concerned with ecological landscape architecture than with ornamental horticulture. He takes an ecosystem view and recommends species that help create habitat.
He wonders, “Can I use some of the insights I’ve seen in nature to inform my use and care of trees?”
That may mean placing several trees close together, underplanted with native sedge (Carex). And maybe leave fallen trees to decompose on the spot. As they break down, these nurse strains, as they are called, put nutrients back into the soil and can support seedlings, the next generation.
The way he thinks isn’t the way most of us think, but trees might be happier if they did.
What creates resilience in a tree?
An important conclusion for Mr. Roddick: Not all species or individuals respond to change in the same way. Similar actions by gardeners can produce very different effects depending on the type of tree, its life stage and its overall health.
Trees with very heavy, woody root systems, such as white oak (Quercus alba), are not as flexible about disturbance as a fibrous root variety such as dawn redwood (Metasequoia glyptostroboides).
“If I knew I was going to build a garden, and I wanted a plant that I could easily work around, I’d be quite comfortable with Metasequoia,” he said. “I won’t be able to dig around trees like white oak much. Especially as it gets older, it deals with disturbances much less.”
A fairly young tree, with more resources to draw from, can be more forgiving.
Where damage or disease occurs, species-specific differences in resilience are also visible. Some trees are better at compartmentalizing – at shielding an injured or decaying part to protect the larger organism. Oaks are quite adept at this, Mr Roddick said, which contributes to their generally longevity.
At the other extreme are willows, cherries and magnolias, which are very bad at compartmentalization. As he put it, they are “more the live-fast-die-young model.”
The reality (and gift) of shadow
Trees cast shadow. That’s the good news, especially in a warming world. But many gardeners are startled when the shade plans foil for growing plants such as tomatoes or roses.
mr. Roddick recalls a client who dreamed of a rose garden in her new home, which had a mature maple tree (Acer pseudoplatanus).
“I suggested doing shade gardening instead to preserve the tree, but it wasn’t to be,” he said. “People want more light for the lawn or garden, as opposed to designs around the established shade trees.”
A common request: couldn’t he lift the tree or open the canopy to let in more light? His answer is invariably based on the species, its health and its age.
“It’s better to train a sapling to fit into a garden, rather than trying to change an old tree,” he said. “If we have to prune part of an established tree to open it up, does it have enough resources reserved to remain a healthy and safe tree?”
The customer got her rose garden, but at the expense of the tree. It deteriorated, became a safety hazard and eventually had to go down.
Making a living mulch layer
Often the problem is “death by a thousand cuts,” said Mr. Roddick, when we garden too aggressively in the root zone. Or worse, if an irrigation system is installed, which repeatedly cuts through the tree’s lifelines.
He is a proponent of “living green mulch, not a ring of bark mulch.” But turning an area under established woody plants into a ground cover herbaceous layer takes a gentle hand and patience.
“You have to start with small plants or divisions and dig very small holes,” he said.
Think of a ground knife, not a shovel: with larger tools (or plants) the tree roots are chopped into pieces. “A couple is not bad,” he said. “But if you disturb a lot of roots, it can affect the tree.”
Another all-too-common insult: adding soil over tree roots to allow for planting.
“Now the tree roots are buried and they’re not getting oxygen,” Mr. Roddick said. “How water moves through the root zone is going to change.”
The roots can grow in the new soil to find those resources, but that can backfire if they dry out or get too much water, leading to a number of problems, including root rot diseases.
Some Favorite Trees (But Skip the Manure)
Rarely has Mr. Roddick fertilized the trees under his care.
In a forest environment, nitrogen isn’t very abundant, he said, so trees don’t need much of it. Applying high nitrogen fertilizers (even accidentally, when feeding an adjacent lawn) can be costly to a tree. ‘You don’t feed it; the tree makes its own through photosynthesis,” he said.
“Nitrogen stimulates growth,” he explained, but that means the tree “must draw from its reserve and pay for that growth. You could even deplete its reserves by forcing it to grow artificially.”
An exception is a tree grown in a large container, which you water often, which will wash out the nutrients from the soil. But with trees growing in the ground, there’s a better approach than applying fertilizer: Leave fallen leaves in place in the fall so they gradually return nutrients to the soil.
When Mr. Roddick gives lectures these days, he often ends with an ecological message: “Plant trees, mostly natives – and preserve as many old trees as possible.”
He recommends, among other things, sourwood (Oxydendrum arboreum), which is often promoted for its summer flowers and fall foliage, although he loves “the wintery aesthetic, all gnarled, with seed pods hanging down.” This is a tree he likes to see planted in a group.
In the native flora garden of Brooklyn Botanic Garden, he came to appreciate the showy bark of the shade-adapted striped maple or elk wood (Acer pensylvanicum).
The botanical garden also has a collection of redbud (Cercis canadensis), and outside their Brooklyn home, he and Mrs. McMackin planted the purple-leaved cultivar Forest Pansy, for the hundreds of hearts hanging from the branches.
Good for small gardens, fringetree (Chionanthus virginicus) has fragrant white spring flowers and yellow fall color (as well as blue fruit on female plants). One drawback: It is sensitive to the emerald ash borer.
Pawpaw (Asimina triloba) has it all: flowers, fall color and fruit, with a custard-like texture and a complex flavor.
And Mr. Roddick wouldn’t be without quick-establishing sassafras (Sassafras albidum), which has distinctively shaped leaves that flare up in the fall. Yes, it requires management to discourage colony formation.
But then again, a whole bunch of sassafras wouldn’t be so bad, he admitted, “I’d just sit under that all my fall.”
Margaret Roach is the creator of the website and podcast A way to gardenand a book of the same name.
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