It’s pouring rain outside, a gully-washer, a yellow blob on the doppler radar, a river in the gutter. I’m reminded that the majority of soil erosion occurs on days like this, especially on steep slopes devoid of vegetation. Preserving soil is the main reason that I have decided to leave in place several old-growth patches of “Seattle’s worst weed.”
Plants in the Ivy family (various Hedera species) were once widely used on slopes to prevent erosion, but I only found one source on the web (here) that still promotes that use. Other sources (e.g., here and here) state that the use of Ivy for erosion control and slope stabilization is “overrated.” The King County Noxious Weed website explains that because of their shallow root systems, entire mats of ivy and soil can slide downhill when water from heavy rain begins flowing beneath them.
If time and money were no object, I might remove the Ivy and replace it immediately with dense plantings of trees, shrubs, ground covers, and sheet mulch, recognizing that the very act of area-wide planting on steep slopes would completely disrupt the existing soil structure. Further disruption would be caused by the need to go back on the slopes to irrigate the new plants for at least two summers.
For now, I’ve decided to use a modified Bradley Method to contain the Ivy and gradually replace it with plants recommended for erosion control in the Pacific Northwest. The Bradley sisters’ original model of “bushland regeneration” relied primarily on natural regeneration to fill in the narrow bands of cleared ground separating the restored and non-restored areas. Because natural regeneration is slow in the Pacific Northwest, I’m supplementing it with plantings. I’m starting at the bottom of the slopes, clearing a band of Ivy about 3’ wide, and putting in rescue plants, live stakes, and divisions obtained from the nearby forest. I’ll do some minor earthwork with each band to help control runoff and hold water around plants, including small boomerang berms, rocks I dig up placed on outer lips, and fallen limbs laid horizontally. When those plants have become established in 2-3 years, I’ll remove the next band of Ivy. Over the next decade, working up the slopes, the size of the Ivy patches will shrink, and eventually the last bits left will join the rest in the compost heap.
It’s far from a perfect solution, partly because it will take so long, but I think the existing blankets of Ivy are better than bare slopes. Heeding the words of Arthur Lee Jacobson, however, from his book Wild Plants of Greater Seattle, I am treating the Ivy “like an outlaw” and allowing it to grow “only under strict supervision.”