Native landscaping under a walnut tree?
Thanks for your question about landscaping around walnut trees. Your request for 'native landscaping' presents a little more complexity to the question. Walnut trees are not native to the Pacific Northwest, so 'native' plants of the PNW have not had centuries (or more) to adapt to them so the walnut's toxicity will not kill them.
The greatest amount of information about landscaping around walnuts, therefore, is from the East Coast, where they have been established for quite a while. Here's a link to a Pennsylvania Extension article with loads of information about maple trees, and a list of plants common to that region that tolerate walnuts. If you scroll down to almost the end of the article, you'll see plants that we have here (some of which are native) that you could use. (If you want to see a great publication of PNW native plants, check out this link.) Comparing plants in both these resources will give you many common ones, not all of which are native in our region.
Those herbaceous plants that jump out to me include yarrow, hosta, iris, Shasta daisy, asters, astilbe, lobelia, snowdrop, stonecrop, trillium, heuchera, hellebore and zinnia. Shrubs include barberry, daphne, witchhazel, snowball hydrangea and currents. I have not cross referenced for shade tolerance, but the Metro publication has icons below each plant description with the plant's sun needs.
I hope this is helpful. Good luck!