Many crab apples layer real easy but not all. I've had a few refuse to root no matter what I did so you'll just have to give this one a go and see what happens.
Theoretically there's no size limit for layers. That's the real beauty about layers - we can start with thicker trunks and sometimes get closer to instant bonsai.
Be sure to look for branches with some twists, bends and/or taper to speed up subsequent development if possible.
Remember that cuts (and therefore roots) do not have to be at right angles to the stem you layer. Cut at an angle for a trunk growing out of the soil at an angle (informal upright, leaning style, etc)
Even up there I suspect that layers can go through winter if there's not sufficient roots come fall but, in any case, you won't have much option if there's not enough roots. Note that layers can survive with very few roots. I've just taken a maple layer off here (early spring) that only had one small root. Looks good after 2 weeks and I fully expect it to survive from previous experience. Layers with no or few roots already have the callus which is precursor to roots so are much closer to roots than any cutting and will usually go on to make more roots after. Separating a layer is a big incentive to grow new roots. Trees with few roots should be able to overwinter in pots with protection similar to any other bonsai as they have fewer needs for water, nutrients, etc while they are dormant. I suggest cross that bridge when and if you need to.
Should be no need to use a board under a layer. The roots are already all laterals and will continue to grow outward with just simple root pruning after it settles into the pot.
Layers should already have mature wood and should be able to flower immediately. Crab flowering depends on fruiting spurs so all pruning is geared toward building more of those and reducing longer vegetative growth. I also feed low N, higher K 'flowering' fertilizer from mid summer through to leaf fall as that's when trees start to form flower buds. That seems to help a little with trees that are ready to flower but very little we can do to force trees that are not ready.
So… I have a beautiful, old (HUGE) crabapple on my property that reliably produces these fantastic little apples. I would like to take an air layer from this tree but I have no idea where (when?) to start and the best strategy for success. This tree branches prolifically so there are numerous places I see that could make a beautiful small tree.
