Pacific Northwest Gardening: Class C Noxious Weed, 1 by Katye

Katye wrote:
Oh yes - Vinca! Is there anywhere it won't grow?

One thing I have made a practice of over the years is to observe where weeds are growing, and which family they are from. A veritable smorgasbord of them can be found under WRC's, the majority having a preference for dryer soil.
I think that the shape of the WRC's branches & the low, sweeping manner in which they hang precludes much from growing there. However, if you limb them up a bit, all manner of weeds will take advantage of the setting, some obviously more well-adapted than others.
If you are looking for ground covers under the trees listed earlier, there shouldn't be too much of a problem. Shrubs & many perennials are a different story under the cedars. The firs (true & dougs) are more amenable to a wider variety of understory plants.
Again - Hostas, when established are sturdy buggers, so I would not hesitate to try this type location, as long as you are willing to monitor their moisture/nutrient needs. I think the bigger pain is digging & preparing the holes. However - Hostas are so worth the effort, so - go for it!

The Hosta in the pic was located at the very base of a large Doug Fir - up against the trunk. It was healthy/happy for years and survived the tree being felled, the stump being ground: never missing a beat. Now it is in full sun, so it will be moved elsewhere. The lesson for me was realizing that many plants have the ability to adapt - much better than we tend to think.