Air Layering Timing (Hackberry)

I am unsure of when I should start my air layers. I have a hackberry with large exposed roots that I would like to layer individually. The roots are wide and very interesting. The tree is in a wash, so the base of the tree is airborne with three long roots holding it up. It is impossible to dig up the tree. Should I start when the buds start popping or should I wait 'till the leaves harden off? And would it be prudent to only layer two roots and leave the third root untouched or would that not be essential?
Also I have several upright junipers I would like to air layer. Again, should I wait until the needles are really growing, or just 'till after last frost? Would it be better to try and dig the junipers?
-Preston

I'd guess that in the Verde Valley you'd be OK to start the layers now -- unless you think there still is a chance of a hard freeze.

Great, Thanks!
Off to soak my moss!
-Preston