I would first ask...
What were the garden beds framed with?
Cedar? Redwood?
Poison "pressure treated" lumber?
How much ash from the houses actually is covering the garden?
And most importantly...
How alkaline was the soil before the fire?
I've been watching the damage on the nightly news too, and recognizing areas where I've visited, even known people...
Climate change is a thing, whether the super wealthy care to acknowledge it or not... And.." a stitch in time"..
I feel bad for everyone affected, and... Worry for the future of the planet...
Having said the above, I would probably scoop off the ash but keep the soil.
You are on site, was the garden contaminated with household ash?
I don't mean ash drifting in on the wind, I mean piles of Chinese drywall, and pleather furniture, chipped wood cabinets and toxic wall to wall carpet...
I could probably provide garden space for a few refugees...
