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Wednesday - July 30, 2014

From: Camano Island and Orcas Island, WA
Region: Northwest
Topic: Rain Gardens, Groundcovers, Grasses or Grass-like
Title: Groundcover for a wet hillside in the San Juan Islands
Answered by: Larry Larson

QUESTION:

I live on Orcas Island in WA state. We have a place on the water and want to plant something that is no taller than 2-3 feet, lower if possible, on a hillside. The hillside gets lots of sun, yet also has a lot of water in the ground since the mountain near us drains down toward the sea. We'd like to cover the hillside, about 3-5 feet wide and 20 feet long with a type of ground cover or plant that will fill the area. I looked at butterfly weed, but that doesn't seem to be a WA state type of plant. Any other ideas for wet ground, full sun, ground type cover plants?

ANSWER:

Mr Smarty Plants doesn’t have a lot of material on the San Juan Islands, so I’m going to look at similar areas and check if their recommendations work at Orcas Island. Here are a few Mr Smarty Plants question/answer pairs that have useful discussion and suggestions. I have extracted just a few of their suggested groundcovers that are adapted to full sun and wet or moist soil and are indicated to grow as a native in the San Juan Islands.

Farthest afield is “Plants to replace Polygonum cuspidatum ( Japanese knotweed)” which was actually written from Wisconsin. This discussion suggested both Arctostaphylos uva-ursi (Kinnikinnick) and Cornus canadensis (Bunchberry dogwood), which are also native to NW Washington and the Islands.

“Groundcover for Oregon gravel path” suggests Fragaria chiloensis (Beach strawberry) and Fragaria virginiana (Virginia strawberry) and even closer to home “Low maintenance, shade tolerant groundcover for Pacific Northwest” recommended Linnaea borealis (Twinflower) and Mahonia nervosa (Cascade barberry). All of these are groundcover or low shrubs, are adapted to full sun and wet to moist soil. In addition, they are indicated to be native to the San Juan Islands according to the USDA.

Another method we use is a direct search. The Wildflower Center maintains a set of "Recommended Species” for many states and ecoregions. This is the list for Washington State. On the left hand side of the web-page, one can search the collection for several attributes. When I did that for “grass & grass-like” aspect, full sun and wet or moist soil, the search revealed three more candidates. They are Carex stipata (Awlfruit sedge), Distichlis spicata (Saltgrass) and Trisetum spicatum (Narrow false oat).

Those are my suggestions for your groundcover. I also found these locally interesting question/answer pairs. “Grass for Seattle Arboretum”, is of course close, but has recommendations for different applications and climate. Resources for Camano Island is directly applicable, but “just” contains a number of excellent reference books and information for that area.