Perhaps you’ve inherited a mature Douglas fir canopy. Or, maybe your garden was sunnier a few years ago, but as nearby trees filled in, the light equation shifted. Many gardeners complain about gardening with trees, but certified arborist and horticulture educator Christina Pfeiffer wants us to celebrate this relationship.
Pfeiffer, co-author with Mary Robson of “Pacific Northwest Month-by-Month Gardening,” says that trees are ecological powerhouses — of all our garden plants, they support the most life above and below ground. The time they take to mature makes an adult tree canopy a gift, not a barrier to beautiful gardens.
Rather than fight the canopy that makes it harder for sun-loving roses, dahlias and tomatoes to bloom, shifting to a plant palette that wants to grow under the trees will foster a healthier, easier-to-maintain garden. Instead of classic English border, think magical woodland.
“Rather than seeing the tree as an obstacle,” says Pfeiffer, “think of the tree as the garden because when those big trees are gone, there’s a lot of impact to your own backyard and to our broader environment. It takes a long time for trees to reach their stature. They’re the longest-lived of plants on the planet, and as our guards, our trees should outlive us. By quite a lot.”
Putting trees first
At Dunn Gardens, a historic Olmstead Brothers-designed estate turned public garden in North Seattle, Director of Horticulture Rose Tobin says, trees come first.
“They are invaluable; they contribute so much to the landscape by providing beauty, shade, habitat, carbon sequestration and mental well-being,” Tobin says. “Many of the trees in Dunn Gardens are well over 100 years old, so they’re not just part of the landscape; they are the heart and soul of the gardens.”
Protect the root zone
“The root zone reminds me of an Achilles heel because something that happens there can have great impact on the trees’ vigor and health,” says Pfeiffer.
As she walks around the Washington Park Arboretum — where she worked for many years — she points out that many of the plantings begin 6 feet away from large trees.
Successful gardening under trees means disturbing roots as little as possible — and knowing where they are.
Key points include:
Tree roots, says Pfeiffer, generally extend far beyond the canopy.
Fine, fibrous roots within the top foot of soil are critical to water and nutrient uptake.
Large anchoring roots tend to grow outward from the trunk in a radial pattern.
Trees stressed from drought, pests or overwatering can handle less disturbance. Younger trees are more adaptable.
Use trunk diameter, not canopy, to gauge the root zone. Measure the trunk diameter at 4.5 feet high above soil grade. One inch of diameter equals a foot of radius, so an 18-inch diameter means an 18-foot radius. Protect the inner half of that zone the most carefully.
Ways we impact the root zone
We may be harming the root zone without realizing it — burying the trunk flare in soil or mulch; making significant grade, drainage or irrigation changes; bark injuries from mowers and string trimmers; over-fertilizing the soil; and tilling or repeated digging. Routinely wetting the trunk also endangers the tree.
Injuries compound and symptoms can take months to appear.
Construction poses particular risk; consult an International Society of Arborist (ISA)-certified expert before digging for that new patio or addition.
For low-stress planting, begin in fall or dormant seasons, keep mulch and compost away from the trunk flare, and water the roots instead of the trunk.
Pfeiffer mulches before planting, building outward, keeping a hand’s width bare from the trunk. “At the trunk I’m going to keep it bare, I’ll start my mulch thin — like one chip — and then I’m going to build up the depth as I move away from the trunk.”
Before planting, lightly dig to check for root density. Where roots are dense, it’s a no-win proposition. You risk hurting the tree by cutting too many roots, or the plant will suffer from root competition. Instead, she may plant a ground cover at the edge that roots into wood chip mulch, like native Woodland strawberry (Fragaria vesca).
Landscape designer Erin Lau suggests, “Notice what is already growing under a tree canopy — are there any weeds? If only leaf litter is present and no weeds, you can reasonably guess that other plants won’t thrive there either.”
When planting, she opts for 4-inch pots or smaller to minimize root disturbance. What if even the toughest plants fail?
“If you can’t get ground covers and dry shade plants to take hold under a mature tree,” she says, “let the leaf litter be the mulch and add a sculpture or boulder that doesn’t directly sit on obvious roots and doesn’t require much digging to install.”
Pfeiffer recommends soaker hoses or drip-emitter irrigation while plants and trees are getting established.
About thinning and ‘limbing up’
Pfeiffer says pruning lower branches to gain light rarely works as expected. You risk overthinking the tree, putting it at risk, and often the tree will grow back to replace the lost canopy it needs to support its trunk and roots.
“So, the lesson is: if the grass is no longer thriving, we’re either going to use a different strain of lawn, or we’re going to use a different kind of ground cover in that zone. If you try to force it, um, one or the other loses out,” she says.
Tree-friendly companions
“Design-wise, when you look at a bare new lot, and there’s a house, and there’s just a bunch of empty space, or a whole development of houses with empty space, it feels exposed and sad, and too hot,” Pfeiffer says. Trees give us comfort and security.
“Let’s give them good company,” she says. “To do that, you look at what happens in nature. Take lessons from places like the arboretum and other public gardens in our area, where you can see you can very successfully include new plants under trees.”
At Dunn Gardens, Tobin says, “Because our tree and rhododendron canopies are so established — we have rhododendrons the size of small trees — this allows us to rely on the mid- and upper-layer of our garden tiers for some of the flower power.”
Magnolias and dogwood trees lead, followed by peak bloom for 60-year-old rhododendrons in May. Underneath, snowdrops (Galanthus), hellebores, Erythroniums and Martagon lilies (Lillium martagon) bloom in sequence from February through June.
When it comes to choosing plants, Pfeiffer looks for plants adapted to summer-dry conditions and shade, pairing native trees and companions for the biggest ecological boost.
Ephemerals and bulbs take advantage of the extra light in early spring, while certain sedums and ferns can thrive amid thirsty roots and heavy leaf canopy.
At Dunn Gardens, Tobin finds ferns and bulbs a winning combination, specifically Cyclamen (Cyclamen coum and Cyclamen hederifolium) planted with Western Oak Fern (Dryopteris disjunctum), a spreading ground cover type.
“As the ferns die back for the fall and winter, the Cyclamen take over the show until the following spring. It keeps the seasonal interest going,” Tobin says. “Snowdrops, Erythroniums and Martagon Lilies also do great in dappled shade, and woodland environments; they put on a show from February through June.”
A favorite combination is native Western sword fern (Polystichum munitum), Western meadow rue (Thalictrum occidentale), Salal (Gaultheria shallon), Salmonberry (Rubus spectabilis), with Vine Maple (Acer circinatum).
Some of Pfeiffer’s favorites, many of which are Washington natives, include:
Bulb and corms: Allium, Colchicum ‘Water Lily,’ Cyclamen coum and C. heterophylla
Spring ephemerals: Trillium ovatum and Yellow avalanche lily (Erythronium grandiflorum)
Perennials: Epimedium, Pacific Coast iris and hybrids, Fragaria chiloensis and hybrids, Fringe cups (Tellima grandiflora), Woodland strawberry (Fragaria vesca), Western bleeding heart (Dicentra formosa), Heuchera, Hardy geranium, Geranium sanguineum, Helleborus orientalis, Oxalis ‘Iron Cross’ and Oxalis oregana, Kinnikinick (Arctostaphylos uva-ursi), Vancouveria planipetala, V. hexandra, False lily-of-the-valley (Maiathemum), Sword fern (Polystichum munitum) and low Oregon grape (Mahonia nervosa)
Shrubs: Sarcococca confusa and S. hookeriana var. humilis
Erica Browne Grivas: is a Seattle-based freelance writer; ebgrivas.writer@gmail.com.
Comments are closed.