The biggest plants in our gardens often get the smallest share of our attention. And it’s not because trees don’t need or want attention — or because we intend to neglect them.
Maybe it’s because they look so strong, holding most of their foliage overhead and not making their needs known near ground level, where we are busy paying attention to everyone else. Or maybe we just don’t have much tree-care confidence.
At The Morton Arboretum in Lisle, Ill., Julie Janoski and her Plant Clinic colleagues respond to gardeners’ and green-industry professionals’ questions — about 17,000 a year. And many of those questions are about trees.
From that sampling, the team can infer that people find trees a little enigmatic.
In 2020, they received calls, emails and visits from residents of 48 states, requesting help with plant and pest identification and treatment, pruning, arborist referrals and more. The questions are answered free of charge by two full-time staff members and about 45 volunteers, many of them master gardeners who receive 30 hours of training at the arboretum, which is about a half-hour west of Chicago and welcomes more than a million visitors a year.
The Morton’s mission is to act as “a champion of trees.” The 1,700-acre institution, preparing to mark its centennial in 2022, is a research center, conservation and educational organization — besides being an arboretum and public garden with more than 222,000 plant specimens representing some 4,650 species and varieties.
Ms. Janoski, a former landscape designer, has been the Plant Clinic manager for three years, working under the mandate she learned as a volunteer for five years before that: “To teach gardeners the best practices in plant care, based on the latest science,” she said, “unbiased and research-backed.”
The Very Hungry Caterpillars
Some questioners arrive in person, bearing exhibits with evidence of the crime.
The other day, a man walked into the clinic with photos of a five-gallon bucket of bagworm bags he had handpicked off a besieged arborvitae. The larvae of these native moths in the genus Thyridopteryx feed on tree foliage, including his Thuja, junipers and even deciduous trees, combining their silk with the foliage to make bags that resemble tiny pine cones. When the larvae mature, they hang the bags from branches and pupate inside.
“Because they use so many needles, between eating and making bags, they can defoliate a tree,” Ms. Janoski said. With evergreens, which don’t regenerate their foliage as often as deciduous trees, this is especially taxing.
Fall webworms (the larvae of another native moth, Hyphantria cunea) are appearing now as well, forming large, gauzy enclosures on the ends of branches of a wide range of plants. The advice: Any harm caused will be cosmetic, so view them as bird food, and don’t panic.
No such reassurance, sadly, is offered in response to inquiries about the invasive, destructive gypsy moth (Lymantria dispar). Arboretum researchers conduct trapping each year to identify where populations are, and how serious the risk.
At the clinic, callers seek guidance in spring about spraying the biological insecticide Bacillus thuringiensis var. kurstaki (Btk), but it’s too late for that now. From summer throughout fall, the advice is to scout for the tan-colored egg masses on tree trunks, branches, firewood piles and even buildings. Using a putty knife or similar instrument — not your hands — scrape the eggs into hot, soapy water.
“The more that we destroy now,” said a recent issue of the clinic’s popular Plant Health Care Report newsletter, published weekly during the growing season, “the smaller the population for next year.”
Choosing the Right Trees
Those seeking advice on choosing plants, including trees, are urged to do on-site observation and plant research before shopping. The Morton Arboretum’s online plant search includes detailed profiles of about 265 tree possibilities.
Do an honest evaluation of the long-term space you can offer a tree, said Ms. Janoski, who often hears from people wanting to retrofit a tree that has grown too big alongside the house, using some miracle of pruning that just doesn’t exist.
“People think about height, but forget to ask about the mature width of trees,” she said. “That river birch they’re considering may be 30 feet wide at maturity; siting it eight feet from the house won’t work.”
Ask what you want from a tree, and what will it ask from you. Besides matching your soil moisture and type, light conditions and space, think about its intended role.
Questions about trees that will block an unwanted view are common, and most of those asking assume that an evergreen wall is the solution. Ms. Janoski encourages them to consider other possibilities. Deciduous shrubs and small ornamental trees will do the job of screening, say, a patio that you don’t use in winter from a neighbor’s. “You don’t need 20 arborvitae,” she said.
Where you may need an evergreen or two is to hide something like the neighbor’s trash cans. But the rest of the screen could be more dynamic, alive with plants of various shapes and sizes.
And there are other reasons to consider diversity: “Planting another maple in an area full of maples is not the best plan; look at other options,” Ms. Janoski said. In a changing climate, a diverse palette can help minimize potential wholesale losses.
Do I Need to Buy a Big, Expensive Tree?
Trees can require a major investment in time and money, but the clinic has good news, based on research.
Younger, container-grown trees, up to two-inch caliper (the trunk diameter measured six inches to a foot above ground), settle in and start growing faster than field-dug specimens that may take three to five years to reestablish root systems and resume growth.
Whatever size you transplant, are you thinking that fertilizing when you plant will speed things up? Don’t do it, the experts say.
With a transplant, promoting root growth, not canopy growth, is the immediate goal. Until the root system has time to reestablish itself, that fertilization will be ineffective.
Be Kind to Surface Roots
Most of a tree’s roots are in the top two feet of soil, where they can access water and air. Clinic clients often express frustration that some trees develop prominent surface roots, which can be a tripping hazard and make it difficult to mow the lawn; the roots, in turn, can be injured during mowing.
“They may be annoying, but you need to protect them,” Ms. Janoski said. Cutting a surface root damages a tree’s ability to take up water and nutrients, and may open wounds that provide entry for disease.
Some trees, including various maples, are prone to developing surface roots, which are more common in hardpan clay soil, or when erosion occurs because trees are on sloping sites. Frost heaving caused by bouts of alternating freeze and thaw can also expose roots.
“I’ll just add soil on top,” people say, “and then grow grass right over them.” But piling additional topsoil over roots can harm the tree, Ms. Janoski cautioned.
Instead, apply two inches of compost or coarse-textured mulch, which are more porous than topsoil. Important: Don’t risk root damage by digging up the turf first. Just top-dress it with the compost or mulch, which will need replenishing as it breaks down.
Mulch Madness
Do we really need to repeat this? Apparently so. Never pile mulch against the trunks of trees.
The dreaded “volcano mulch” so often seen is damaging, Ms. Janoski said, trapping moisture against the bark and inviting pests, disease and decay.
Keep mulch about four or five inches away from the trunk. You want to see the root flare — the spot where the base broadens, just above the soil line. A maximum of three or four inches depth, using a medium-textured mulch, is recommended.
A living mulch layer — rather than rings of lifeless, bagged stuff — adds diversity to a garden, but digging beneath an established tree is tricky and potentially damaging, Ms. Janoski said. When you’re attempting to add ground cover, stick to small herbaceous plants (like landscape plugs or very small divisions) and use a trowel or soil knife, not a shovel. Planting shrubs beneath established trees is not recommended.
Trees Need Watering, Too
We water the tomatoes, and the pots on the patio, but when was the last time we watered our trees?
The northern Midwest had a 12-week drought this spring, and the clinic staff worked to spread an urgent message: “We remind people constantly that when it’s hot and dry for 10 days or so, even mature trees need water,” Ms. Janoski said. “Your lawn will come back, but trees may have long-term damage.”
She can’t give customers a precise prescription for how long to water, because water pressure varies, but except with small trees (or abundant patience), this is not a job for a hand-held wand. Instead, she suggested, set out sprinklers in the root zone, and use rain gauges or makeshift tools — repurposed tuna or coffee cans will do, as will any flat-bottomed pan — to measure rainfall. Your goal should be to provide the tree with at least one inch of water a week, unless nature does it for you.
More plant questions? Ms. Janoski and the Plant Clinic staff welcome them at plantclinic@mortonarb.org or 630-719-2424.
Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.