What can be added to sawdust to make it a good mulch? Or should I add nothing?
Also, how deep should the mulch be for protecting plants in winter? We're in the PNW, zone 8, with temps occasionally down to 10-20 degrees F.
Nothing really needs to be added to sawdust to use it for a mulch except that sawdust does tend to cake when it gets wet. I usually stir the sawdust from my woodworking into the wood chip mulch in one of the beds that has a wood chip mulch. There may be some comment that you need to add some source of nitrogen to that sawdust so the sawdust does not "rob" your soil of nitrogen but that is not a problem as long as you use the sawdust as mulch and not as a soil amendment and mix the sawdust into the soil.
How deep to mulch depends on how well your soil drains. If you have clay soil that does not easily drain putting very much mulch may well cause too much soil moisture to accumulate and that could cause plant crowns to rot, not good. If you have sandy soil that drains really well 6 to 8 inches of a mulch may be necessary to hold even some soil moisture. Only you can decide based on your knowledge of your soil.
When you say mulch, we think of something that is different from winter protection.
However. Where in WA are you, and what are you trying to protect? Here in rainy western WA sawdust makes a horrible winter protection as it stays too soggy. Wood shavings and wood chips are ok, but not sawdust. Eastern WA is a different story, sawdust might be just fine as they get snow not rain, and temps tend to stay below freezing so snow doesn't melt and waterlog things.
But the question remains - what are you trying to protect, and does it really need protection. Rotting under 'protection' is a very real problem here. I don't use winter protection much myself. I put a layer of leaves over the dahlias to keep the water off the cut stems. I might pile some leaves around the bases of the roses if the planets align just right and inspire me. But I've found I lose as many roses to rot when covered, as I lose to freeze when uncovered, so why not save myself the work and leave well enough alone. I box up and wrap with leaves small potted plants and bonsai. But in general things don't need winter protection here.
For a mulch, sawdust could be used plain, depending on your goals. As a compost or soil amendment it should be mixed into the compost pile first and allowed to decompose. As a weed barrier it's fine by itself.
Thanks to both for detailed answers! I'm in S. Everett (PNW).
I have about 60 gal. of sawdust, and I'm just trying to do some $ saving things with it.
I have a common hardy fuschia to protect (sometimes not so hardy).
Also I planted an acca (pineapple guava) this year that's about 2.5 x 2.5 and looking beautiful, and I don't want to lose that. Off Topic, can you comment on that? It's on a SE corner, full sun, good soil, drains well.
Mostly, though, I have some areas that need a weed barrier, so the two comments here took care of that.