by Mark N. McCann
While holly, Ilex aquifolium, can provide a welcome whit of green during the gray periods near the holidays, this invasive species can be an unwelcome sight the rest of the year. This introduced European ornamental is an aggressive plant that can grow to 35 feet tall and out competes native species by casting a deep shadow within the bush and creating a thick barrier against other plants. This eliminates competition by absorbing the bulk of the nearby sunlight and physically preventing the seeds of other species from germinating within the plant’s dense growth.
Holly grows readily in our climate and propagates both vegetatively and by seeds, normally cast by birds after eating the berries. Once the plant’s limbs come in contact with the soil, it re-roots quickly, spreading out from a single menace to become akin to the hedge around Dante’s fourth or fifth circle. I often see such hedges on the rainy side of the Cascade Mountains and admit they do serve a purpose—the deflection of runaway trains.
The shiny leaves and red berries of the holly make it easy to spot, but there are many variegated species, in addition to the solid dark green. It is the dark green, sometimes called the English holly varietal, that presents the greatest problem. Holly is not a problem in and of itself—it’s the plant’s aggressive reproductive nature that makes it a menace.
When birds consume the berries, the stones of the fruit are passed out between a couple of hundred yards to a distance twice that far. An attempt to find out the name of scientists who study the actions of a bird’s anatomy nearest the tail feathers drew looks of disgust or belly laughs, but within Tryon Creek State Natural Area (TCSNA is near Portland) the previous distances are quite accurate.
If you must have one of these in your yard, keep the berries from forming; by trimming them as they appear. This will keep it from spreading in your yard or to nearby properties.
Some brave souls may have tried to remove a large holly with nothing more than a hand saw and enthusiasm. They may find themselves tackling a moderately hard wood that sends up shoots year after year from the tear stained stump. Killing this pest usually requires more assertive actions.
Plants less than two inches in diameter should be dug out, or pulled out for the young and dramatic gardener. Anything beyond that size might require a backhoe or a cast of thousands. I attacked a four-inch size trunk in the TCSNA that had a root ball similar in size and shape to a black bear. Then, under that massif, I encountered a one-inch diameter tap root. These plants are the cockroaches of Northwest vegetation. I reburied the tap root under a bale of depleted plutonium and jube-jube beads I picked up in Haiti. It may have been the beads that worked best.
Like cockroaches, with an ability to lay eggs after the head is gone, the holly can become a doppelganger of itself. The green plant will often re-root if left in contact with damp soil for an extended period. Usually moving the plant a couple of times in a season is enough to prevent the woody material from putting down roots.
Cutting a tree and painting the freshly cut surface with a systemic herbicide can be most effective. To maximize the effect, I put a plastic bag over the stump to keep the herbicide in place until the compound becomes dormant.
Oddly enough, the largest of holly trees seem to be the easiest to kill. Those plants greater than eight inches in diameter can sometimes be killed by cutting them down and then periodical removing the suckers that appear at the base. This may require an entire year of attention but in the long run is the easiest method.
As with most plants, cutting them down during late spring and summer decreases the possibility of survival with the decreased amount of rainfall.
Some gardeners love the holly as an evergreen visual barrier but there are alternatives, including some natives.
Conifers: Noble fir (Abies procera) seems to grow somewhat slower in this area than other conifers, keeping it a more manageable size for a yard plant. Conifers have a whole group of negatives, including rapid growth, allelopathic needles, shedding and generally making it impossible to grow turf nearby. The needles of most conifers contain high acid levels that inhibit many other species.
Golden chinquapin (Castanopsis chrysophylla) is an overlooked Oregon evergreen. It is an attractive tree, often maintaining a conical growth habit. On the minus side, they do shed catkins and may require more sunlight or better drainage than conifers.
Oak trees (Quarkus garriana) like the Oregon white oak do not fill in like some other trees but is an attractive small tree. It does require lots of sun and good drainage.
Madrone (Arbutus menziesii) is similar in regard to growth habit as the oak.
Some bird lovers find the holly provides forage for wing-weary travelers commuting to warmer climates. The snowberry (Symphoricarpos albus) retains a long lasting, waxy white berry that is attractive to many native species of birds and animals throughout the winter months, but is poisonous to humans.
Written by Mark N. McCann
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Mark N. McCann is currently working on a book about Green Space and Stream Stewardship within the Vancouver-Seattle-Portland-Eugene (VSPE) Corridor. Based on his five years of experience removing invasive species, and replanting and propagating natives species within the Tryon Creek State Natural Area (TCSNA) as a member of the Adopt-A-Plot program. He has been a Citizen Member of the Tryon Creek Watershed Council, Friends of Tryon Creek and presently is a volunteer at Berry Botanic Garden, specifically in the Native Plant’s Trail and in the Propagation Department.
Photograph by Debbie Teashon