If you want a drought-tolerant garden, do not amend the soil. Drought-tolerant plants (such as your doug firs, ribes, and mountain ash) get their drought tolerance by forming symbiotic relationships with mycorrhizal fungi or frankia bacteria. These microbes efficiently deliver water and minerals to the roots, and also protect the plants from pathogens (the fungi and bacteria get carbohydrates from the plants in return). When you add nutrients (fertilizer, manure, etc.), the plants drop the fungi and bacteria from their roots, which leaves them vulnerable to pathogens.

More here: http://www.laspilitas.com/advanced/advroots.htm

http://www.laspilitas.com/classes/mycorrih.htm

The one thing that you can add is what has already been added: mulch. Bark mulch from drought-tolerant trees (fir, pine, redwood, and oak) will enhance the mychorrhizal "grid" in your yard.

As for drought-tolerant partial shade ground covers, I've always liked Mahonia nervosa. It's low-maintenance and most native plant nurseries carry it. Some of the lower manzanitas can take some shade, such as Arctostaphylos 'Emerald Carpet,' A. uva-ursi 'Point Reyes,' A. hookeria 'Monterey Carpet,' and A. hookeri 'Wayside.' I grow Salvia spathacea in partial sun, but I've read that it'll take partial shade as well.

I don't know what would work for you in deep shade besides ferns.

Yes: don't fix what ain't broke - the soil is already full of roots because the plants producing those roots are able to make use of the existing conditions.

In general you don't want to amend soil for permanent plantings anyway. Loosen what there is, plant in that, then mulch.

And keep it mulched.

What kind of ground cover ? is the area mostly shade ? partially sunny ?. If yes, I would recommend PACHYSANDRA . They love acid soil, shade and moisture. That is what we normally got in the area. PLUS, they are evergreens and can spread by roots. But are not INVASIVE>. For pachysandra, you just plant them , no preparation is necessary.

I forgot to mention Gaultheria shallon and, one of my personal favorites, Vaccinium ovatum. If they get a little sun, both will be held to 2-3'. Both will grow taller in full shade.

Thanks everyone for your responses! I used the word amend erroneously--I know I don't need to
amend what's already there, other than mulch.
I was specifically referring to the groundcover plantings. I'm just having a hard time imagining planting ground cover into such a seemingly sparse soil. Having only grown flowers and veggies, this is my first time with 'ornamentals.' I guess as bboy says, just loosen the soil up, pop it in, and mulch it. I love the info on the plant-fungal relationships; I will research it more and give more thought to just what it is I plant there.

Yes, if you choose a plant adapted to existing conditions you don't have to change those conditions.

And keep them changed - so often it is a fight if it is not a good match, with the plant ultimately being the biggest loser.

at my last house, i had two 50 foot doug firs on the edge of a slope. there was anywhere from a few inches to a foot or so of loose mulch below the trees.

it was extremely hard to get plants started under these conditions. lots of supplemental water, and they established very slowly.

you might loose a bunch of plants before you find the right ones. some ideas:

Adiantum venustum
Tolmiea menziesii
Luzula sylvatica

Tolmiea grows wild here in the damper areas. I doubt it will do well where it is dry unless given extra water.
Mike

I grow Tolmiea in those conditions and it does need supplemental water. But it can survive with surprisingly little- no need for deep watering.

Pachysandra also has fragrant flowers in early spring. Can root cuttings to make more of them, too.

You might consider fumewort, Corydalis lutea, bluegreen lacy foliage, tiny tubular yellow flowers late spring - frost, slug (deer, rabbit), reseeds freely, but less so in dry conditions. Prefers moist part shade, but tolerates dry. Will not tolerate dry sun for very long in summer & look good. Can be cut back if needed.

Any seedlings are easily identified in early spring & removed or smothered with mulch. Does a great job of covering the area with seedlings by the next growing season.

In a few places I've put some blue/purple flowering plants beside it & really liked the color combo.... Columbine, forget-me-not, vinca major, lavender, tall campanula.

If you want C. lutea & can't find it, contact me via Gweb. I regularly attend the Green Elephant swaps in Redmond.

I'm surprised at how little water Tolmie needs to survive also, considering what it likes best.
Here's some of my pachysandra by the driveway. Edged by Goodyear.
Mike

Here's my experience gardening under conifers:

I initially spread fine wood chips as mulch before planting. Since then debris (such as needles and cone scales) shed naturally add to the mulch.

These plants are growing in partial to full shade:

Linnea borealis - from a few starts, this has travelled many many feet. Much of it is in full shade.
Cardamine trifolia - dense, evergreen, looks good anytime of the year.
Cornus unalaschkensis (canadensis)
Vancouveria chrysantha
Pyrola asarifolia
Coptis lacianata
Galax aphylla (urceolata)
Mahonia nervosa
Blechnum spicant, deer fern

I also planted Gymnocarpa dryopteris, oak fern, which is a beautiful decidous fern, but after a few years it started poking up aggressively through its neighbors. Now I am trying to get it out and I'll re-establish it where it can spread to its heart content.

I have done a little sprinkling when it is hot and dry, but one summer I was unable to do this, and the plants survived.