We have Oregon Grape planted on the north side of our house under some cedar trees. We want to fill in the area but the plants are not doing well. Can you suggest what might be wrong with them. Also, can you suggest other plants we might use with these. The area get some sun in the morning but otherwise in the shade.
The leaves of your plant are nice dark green. The damaged areas are most probably caused by slugs. Slugs do their damage at night. During the day slugs hide out in dark moist areas protected from the drying sun. You can provide the slugs with sanctuaries made up of shingles, boards, plastic lids, etc. Mornings you can find and destroy the slugs. Baits include yeast solutions, like beer. If you decide to use chemical baits the iron phosphate-based ones are much less toxic. This article has additional information, How to Control Slugs in your Garden https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9155.pdf. Other possible causes of the damage include: 1. drought stress, especially in the heat we've had the last few summers, and caused at least in part by the cedar tree taking up most of the soil moisture, 2. fungal infection of the leaves, you may have seen black or rust colored fungal spores at the time of the infection, or 3. herbicide damage, especially splashing, maybe from weed and feed used on the lawn.
Oregon grape (Mahonia spp) grows best in moist, cool, well-drained soil in sun or light shade. Trees take up an enormous amount of water, especially when it's hot. Our western red cedar prefers moist conditions, they've struggled in the heat of the last few summers. It doesn't leave much moisture for any plants growing under the tree. Water underneath the tree using a soaker hose around the drip-line, the edge of the branches, for 2-3 hours every month when it's hot and dry.