How long to leave rootstocks before grafting apples?

If you bud-graft in the summer you'll be able to re-graft any failures next winter - and vice-versa.

It might be worth trying a few grafts now (say about half of your rootstocks) if the rootstock hasn't sprouted and if you use only scion wood that hasn't begun to swell its buds (maybe take a piece of scion wood from dormant wood further back down the stem). The worst that will happen is that the grafts fail and you can re-graft with buds in the summer.

If the graft fails the rootstock will sprout a new stem. In fact, rootstocks often try to grow-out from just below the graft in their first year and those shoots will need to be pinched-out/removed at an early stage to prevent them smothering the slower-growing grafted shoots which grow slower because the graft takes a few months to fully heal and for full sapflow to be restored where the two pieces were joined.

When you dormant-graft MM106 it is worth sticking the part you cut off into the soil because MM106 will grow from cuttings, especially if the shoot used for the cutting is a few years old and has small lumps on it (these lumps will send out roots and are sometimes known as burrknots).

That's very helpful, many thanks FB . Very handy tip about the MM106 cutting rooting too - should never need to buy any more!

I agree with FB.

The first year I tried grafting, like you, I received my batch of MM106 stocks in late February. I'd read in Garner that it is better to let them establish stonking great roots for a year before grafting. However, being much too impatient, I did 10 saddle grafts and 8 took (the other two I regrafted the next year.). I have to say that the first year's growth of the scions on these stocks was significantly weaker compared to later experiences with stocks that had been established for a year before grafting.
This could be an issue with very weak growing varieties.

Very useful too, many thanks boundtothesoil. I think I'll try a couple now on the rootstocks, as the only thing I have to lose is scion wood anyway