Walking in Forest Park recently, admiring an area of dense native groundcover that was devoid of weeds, I wondered how it came to be. Perhaps when the area was last cleared of trees a century ago much of the perennial groundcover growing at that time survived. Over the decades that followed, those plants gradually filled in the scars left from logging, and no significant disturbances of the ground occurred later that would invite weeds.
A robust groundcover in Puget Lowland forests includes– Salal, Sword Fern, and Cascade Barberry (a.k.a. Low Oregon Grape). Salal (Gaultheria Shallon) grows in Pacific Coast forests from southeastern Alaska all the way to southern California. Some of the plants in this stand may have been growing for a century without significant disruption.Spreading mainly by rhizomes, Salal grows and spreads slowly in shady forests, but over time creates dense stands several feet deep.Cascade Barberry (Mahonia nervosa), like Salal, is shade and drought tolerant and spreads mainly by rhizomes. By August, each plant has added one or more new groups of compound leaves.According to the U.S. Forest Service (here) “on 03 March 2016, the scientific and common names of this species were changed from: Berberis nervosa, dwarf Oregon-grape to: Mahonia nervosa, Cascade barberry.” Sword Fern (Polystichum munitum) may not be quite as drought tolerant as Salal and Cascade Barberry. However, dense installations of Sword Fern on sites that retain soil moisture through long dry seasons can create weed proof groundcovers. Sword Fern doesn’t spread like Salal and Cascade Barberry. (Clones of Sword Ferns remain attached to the mother plants.) This shady area under Western Redcedars was cleared of blanket Ivy in 2015 and planted with Sword Ferns. They have survived and grown, but since the ferns were not planted densely enough, and since they won’t spread, the area will continue to invite invasive weeds.This 2018 dense planting of Sword Ferns in deep, moist soil has remained essentially weed free.This 2014 dense planting of Salal and Cascade Barberry has only recently achieved enough density to deter weeds. The Douglas Fir is a welcome volunteer in this mix.Lady Fern (Athyrium filix-femina), though not evergreen, contributes to a diverse groundcover. Extremely widespread, it grows in temperate regions throughout the northern hemisphere, spreading by spores and rhizomes.Trailing Blackberry (Rubus ursinus) can form a thick groundcover, especially in sunny edges.The ideal groundcover includes a mix of species. Even this Bigleaf Maple sapling with powdery mildew adds diversity. This young Osoberry shrub adds to the diversity of the groundcover and provides a bit of color as the dry season commences and individual leaves turn yellow one-by-one. Osoberry is a prime example of the way trees and shrubs reduce leaf area to manage water stress.By August, the perennial groundcovers have added another layer of leaves to the gradually accumulating depth of vegetation in the understory. A deep, dense, undisturbed groundcover is the best deterrent to weeds.
