How should I sheet sheet mulch my clay hill side in preparation for a lavender bed? Should I add sand to the straw? Oyster shell? What plants would make a lavender guild?
I don't think lavender would survive in clay soil. Clay holds too much water for too long, and lavender needs really good drainage.
I can't help but think it would do better in raised beds or large tubs.
The best advice I ever got for growing lavender: plant it in well-drained soil with some lime, then ignore it.
I wouldn't add anything special unless your pH is especially off one way or the other.
When I tore out my front lawn I didn't bother enriching the soil much - I've got heavy clay that was pretty depleted after 50 years of growing lawn (5 yrs on my watch without any chemicals or pesticides). I dug a hole, added a handful of 10-10-10 general purpose fertilizer, and threw in a 1gal lavender bush. I planted about 6 of them. Full sun, poor soil, very little water - about 3x a week in high summer here in California (temps around 90-100+ and dry). I mulched heavily with whatever shredded leaves I had on hand and fertilized about 2x/yr.
Some but not all of the lavenders did fine. The Spanish (with fancy spider-web-like leaves) died almost immediately - actually all the Costco lavenders died. The French lavender and the ones with the tough needle-like leaves did great. Now, had I planted them with more care, amending the soil, watering generously, I'm sure they would be much bigger. But 2 yrs later they are about 2' high and 2' wide and doing just fine. Clay holds water, yes, but sometimes that's a good thing.
What do you want your guild to do - prevent erosion? Attract bees? Deter deer? Create shade? Promote edibles? Colorful? How much water do you intend to donate to this hillside?
Things for the groundcover/understory:
I'd put down a layer of clover and yarrow to prevent erosion and enrich the soil.
Manzanita comes in a low-growing variety that grows quickly, doesn't use much water, and is great cover for little critters.
Things about the size of the lavenders:
Offhand I'd plant lots of salvias - there are quite a few that are water-hardy, including edible sages. Bee balm comes to mind. Blue flax is a real winner - if you can get it established, it's just gorgeous in the spring and has an extra long taproot that goes down into clay, looking for water. I've got seeds for red flax but haven't yet gotten them to sprout.
Sticky monkey flower is always a good native that will give you blooms throughout the summer. They come in a large variety of colors and are readily available. Coyote mint is a great native that will die back in the winter and come back in the spring.
As for bigger bushes/trees:
Butterfly bushes do well in poor soil, although they might need a little extra babying the first year while they establish. Past that, though, they need very little water.
Ceanothus is an excellent bush to plant - bees love it and the plant actually fixes nitrogen. I particularly love "dark star" for its deep purple blooms. And once established they don't need extra water.
Flannel bushes have beautiful blooms early in the spring and are extremely drought tolerant too. Such interesting leaves.
You might look into guomi bushes too. The berries are edible, the bush fixes nitrogen, and it grows in terrible dry soil. I've never grown it myself, but plan to put some in soon.
Renataka, thanks for the advise on the lavender guild. I've already got a 400 sq ft plot of nicely growing Grosso that I planted in good garden soil a couple of years ago. I plan to use these as mothers and am making clones in the fall. I love lavender's drought and deer resistance, it's easy on me as a farmer and on the land as well as having the potential to turn a small profit. The thing about lavender is that I notice it thrives on oyster shell which makes too base a ph for many plants (forget roses and blue berries), hence my questions about a guild. I plan on donating nearly no water to this guild, just enough to keep the lavender going for the first couple of years. Not needing irrigation and heavy fertilizer is one of the reasons I love this plant so, (oh and of course the euphoric scent, lovely color, bees and butterflies). It's going to be a 500 sq ft area so I plan on taking time and care in its preperation. I am going to check out guomi bushes on the web right now. Thanks so much for your time and please post more info if you think of anything.