Last winter I thought I'd try my hand at hardwood cuttings. I stuck my cuttings in coarse sand, and left them outside over the winter. Much to my delight, many of the cuttings have leafed out, and appear to be doing well. The easy ones were forsythia, ninebark, viburnum, fothergilla, and sweet spire. I was unsuccessful with longbeard and spirea, but will try softwood cuttings later this Spring.
My question is; when is it appropriate to remove the hardwood cuttings from the sand, and pot them up individually?
The photo below shows how they were overwintered. As you can see, I'll have to empty the entire pot to access individual cuttings.
Thanks in advance!

When you have good roots you can remove them and plant them in small pots or in the ground. Don't pull the stems because that will damage the new roots below. Take a pot and tip it over and pick out the best rooted and leave the rest.

Dave, Just found this older post of yours and thought I'd add a few thoughts not addressed by oneeyeluke or in my post regarding "when to transplant rooted cuttings into individual pots." From your photo it appears you are using different size pots for your cuttings and are sticking each woody species in its own pot(s). These are definitely preferred practices. I would reserve the larger pots for the more reliable species you have some experience with. Use the smaller pots for the species which root more weakly, less reliably, or later in the season. Wait until you see roots in the drain holes in the small pots to transplant those species. If you are attempting to root any species which set buds in the late summer or fall for the next year, don't be fooled by early bud break in those species; they may bud out before any rooting has taken place. Water cuttings which have leafed out with a gentle spray, preferably from a can. Treat these cuttings like softwood and move them into dappled to full shade until rooted. In my opinion, it's best to pot those (early to leaf) species in smaller pots and wait for visible roots in the drain holes before potting up. Don't worry about roots intertwining; roots separate readily in damp (not wet) sand; that's a big advantage of that media. So you can stick many cuttings in a sand filled pot; I often use 3/4" spacings. And don't be afraid of breaking tertiary or secondary roots or even intentionally pruning roots while potting-up your cuttings. In my opinion, root pruning is a better practice than coiling long roots around in the bottom of a pot. Of course, you should reserve severe root pruning for late fall to v. early spring, during dormancy. It's a bit species dependent I've discovered. Many badly damaged plants can be nursed back to health, if necessary; plants are amazingly resilient.
Are you using a rooting hormone? Varying the IBA concentration by type (season) of cutting?
I see that you've successfully rooted Viburnum ssp. I'm attempting some semi-hardwood Viburnum lantana cuttings now (stuck mine on 8/3). My Viburnum cutting stems have very prominent lenticels and I'm wondering if new roots will emerge from them. Have you observed that behaviour?

I very much appreciate the responses, thank you.
This was an older post, and at the time I was perhaps overly optimistic of my success. Many of these hardwood cuttings leafed out, but later died. The cuttings that did survive (roughly 8 months now), I will wait to pot until next Spring.
To answer your question HighDesertNatives, I do use a rooting hormone and vary the concentration based upon the type of cutting. Your insight in both this thread and my most recent thread were very helpful! As you can see in the photo, my outdoor "bed" is comprised of pots. This was my first attempt at hardwood cuttings, which resulted in mixed success.
I'm having a lot more success with my softwood and semi-hardwood cuttings, but the Pro-mix is definitely more prone to creating rot and mold issues in my propagation domes. I think I'll use coarse sand the next time around.
I hadn't noticed the lenticels on my viburnum cuttings, but that may be because I'm sticking them too close to the surface (?). The first real indication of rooting (that I notice) is the development of new leaves. I severely prune the larger leaved cultivars when I stick them, so I cheer the emergence of new growth.
This is my first year and I am learning, clearly some are easier than others. I am amazed what a difference just a few weeks can have on the success of the late Spring/ early Summer cuttings. I did one pot of Pragense Viburnums with zero success, but a few weeks later every one of my Prague cuttings rooted fine.
Again, many thanks for your comments, I appreciate your insight!

@BlueRidge, The prominent lenticels on my Viburnum lantana stems are a characteristic of V. lantana. Other Viburnum ssp. may not even have visible lenticels, I don't know. The geek in me wondered if such visible swellings play any role in root localization. I guess I'll find out If I'm successful and roots initiate at the lenticels. I did look for nodes with increased density of surrounding lenticels when I took my cuttings and I tried to be sure to bury those nodes. I work with a lot of species with little published propagation history so I often try to take advantage of features like abnormal thickening of stems, decreased inter-nodal distance, increased density of hairs and glands, or previously healed wounds. I think cuttings often develop new roots more readily and in greater abundance from such sites. And I think it's always worth looking for such features, especially on your most problematic/challenging species.