I'm in the mountains of VA (Zone 6b), is now a good time to thin or pinch the water sprouts that have started growing since my dormant pruning in early March? If so, I'm training to an open center (vase shape) so how should I prune - just the interior facing water sprouts? Prune all off or only a portion of them? What about the sprouts growing towards the outside?
Two things I think I know:
Generally, Stone fruits have the potential to set fruit from this years new growth, next year.
And unlike for Pome fruits, heading cuts are a norm when pruning Stone fruits, as well as thinning cuts..
That said, your aim in pruning is to keep the extension growth in check to keep the limbs from getting over extended causing breakage or abrasion damage, and strengthening what remains so it's fruit load will be well supported. You're looking down the road for further shaping and training for good light and air circulation.
Nothing is absolute. There are no mistakes- sometimes just a longer learning curve.
Interior growth is not all bad; if it's well placed or you can cut to a well directed bud on a short stub for a potential fruit site next year, or maybe just a bit of vegetation; that to last only one or two years before it outgrows the space and needs to be dealt with. You don't want to totally denude the interior. You need some canopy or you have the possibility of sunburn. Rubbing off the more vigorous, leaving some of the less dominant new growth now, before it hardens and requires pruners to avoid ragged tears, is a good tact.
Pruning to a pattern ( making similar cuts, throughout) can help to recognize your cause and effects, if you can follow it. ...Well, that didn't work like I'd hoped. What DID it do? Observe, observe observe.
Heading cuts will stimulate the first 3 or 4 buds under the cut to extend new vegetative growth. Dormant and early spring pruning generally will encourage vigorous growth. Summer pruning will tend to decrease vigor. Think on that and use to your advantage.