I am printing this out to take with me to the nursery. Interesting point about the hostas and tree roots.

consider Carex 'Ice Dance'
"once extablished" I've had it do quite well under cedars and hemlocks.

I'd like to know more about this raised bed. How big is the bed and how much is it raised? Does it go all the way around the tree? While an established cedar will have a root system that will extend far beyond a raised bed, it is NOT a good idea to raise the soil level around a tree by any significant amount. Or to increase the soil level around the base of the trunk. It can smother feeder roots and starve the tree of oxygen and moist soil or mulch against the trunk can promote rot. It's not necessarily an immediate result but you can kill or seriously compromise the health of the tree by doing this.

Under normal conditions (no raised bed), planting under a cedar calls for plants tolerant of dry shade. Obviously various natives like salal, mahonia and sword ferns will work. Geranium macrorrhizum is an excellent choice. So is lamium, the vinca, Euphorbia robbiea, dwarf sarcococca, epimediums and wintercreeper (Euonymus fortunei ctvs.). Dry shade is not a situation that encourages a lot of showy flowers -- look for your color in foliage rather than flowers.

The Western Red Cedar can handle soil on top of the roots better than most other trees. It's natural habitat is river valleys where it is subjected to periodic flooding and silt buildup. A young tree planted too deep will root from the trunk. It's called advantatious rooting. Older trees will do the same when backfilled.

Remember, the Cedar will take over all raised beds with its own thick root system. If your plants can't handle that, a permanent, non-penetrable barrier is needed. Remember, this will prevent water from getting to the cedar's roots in that ares, but can be tolerated in a small bed. Also, there is usually little water reaching the area near the trunk, because of the tree's own foliage - ours gets very dry in the summer. To the tree's benefit, her roots extend more than twice her perimeter diameter (her trunk is 3 feet thick), so she gets plenty of water from other places, and defenitely benefits from the shade provided by plants above.
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Being acid-soil plants, strawberries were placed a couple feet outside the drip line (so they can receive rain), and they are THRIVING this year, no barrier used. We learned cedar branches as mulch result in cedar-flavored berries... not so desirable. We're waiting to see what "flavor" strawberries we get as the roots invade more each year.
God Bless your Garden...

The Cedar is very well established, probably at least 30 years old, if not more. The house was built 90 years ago and it is a good 30 feet tall. Would be taller if someone hadn't topped it early on (split trunk leads us to believe it was topped quite awhile ago).

It has a huge root system that go underneath the yard and the alley behind the house several yards in every direction.

The bed is maybe 10'x20' at it's widest. It is the back corner of our lot with a fence on two sides, so that part is a triangle and then curves in the front. Basically the yard was slightly sloped and we leveled it and put in a retaining wall around the tree to give it some space from the lawn and highlight the tree. It's maybe a foot difference from the base of the wall to the top. There are 3 rose bushes against one side of the fence with the tree on the opposite fence wall.

It just looks really empty right now. I prefer native plants since they will thrive with little maintenance or additional watering.

Attached are some photos to give you a better idea of the area.

Here is a link that might be useful: Photos of yard

YOu would be wise to ditch the planter bed, then to select plant which tolerate root competition, low light and low water conditions.

Then plant in pockets here and there where wanted. Irrigate for the first year or two to get them well estalblished, then wean off the supplemental irrigation.

Natives are a good choice for such a planting.

It will take a while to fill in but this method will be much more safe for the tree.

Other than watering them in when planting, none of my 3 gc in my first paragraph get or need any supplemental water, and do fine with root competition. They are very aggressive and spread well. Very few weeds can compete, though I'm having to deal with some native blackberry coming up in my Vinca major. Vinca minor may need a little more care if planted at a dry time of the year. I prefer to transplant in fall when the rains start rather than in spring when it may start to get hot and dry.

Gardengal48 is totally right about raised beds around trees suffocating the tree. When started this yard did not know that and killed off a Colorado spruce tree with a new raised bed around it. Science of roots should be taught in elementary school!

Have many varieties of vinca here, some variegated and very pretty. Vinca spreads fast and is aggressive but easy to prune. It tends to mound over other plants. Good because survives, but needs work to manage it.

Have sarcacoccas, sweet box, smell fantastic in winter; they like water and shade. They are growing fast here. Variety of ferns, also like water. Of course thuja plicata LOVES water! Pulmonaria does really well with shade and water here, will be planting more. There's a native wild ginger that also works well for shade under trees.

Growing lots of miniature ivies here, non-invasive, a bit slow, easy to prune and control, including persian & algerians. They can handle water, shade, sun and dryness.

Slug control is an issue with shade and water. Using a lot of slug bait (the organic stuff) and praying the possum that devoured all the slugs last year comes back.