I would go ahead and plant them and maybe consider covering them with a breathable cover for insulation. You could also keep them in an unheated garage. You do not want to take them inside a heated building.

Hydrangea are extremely hardy, though the garden Hortensia is less so ( Zone 5 would be the limit I think). They are one of the few flowering plants that will grow happily in both central Southern Canada and the tropics. My worry is that these plants are still very young. Wait until they are bigger. Keep them overwinter with only light frost then plant them deep early next summer. Good luck. Ian.

Planting woody plants "deep" is something almost universally advised against. I'm a little surprised to see someone recommend it here.

A plant with only 5 or 6 leaves has to be planted near the surface and will only produce minimal root growth between now and fall. If you leave it until next year you will be able to get the roots in deeper and increase their chances of survival. I didn't mean bury them! If you are at all worried about this you can apply a thick mulch of leaves around the crown as soon as severe frost threatens in the middle of next fall, this will reduce the effects of wind chill. Another useful tip I picked up in Canada was to plant in lily pots. You know, the tall thin ones. That way the roots will spread more downwards. You want deep roots. Good luck. Ian

I have over 1000 hedges in a propagation box with the white greenhouse plastic on the top for a cover, They are rooting in sand, I pulled 2 out on Sept 20th, 2013 and they both have a bush of roots. Should I leave them in the propagation box for the winter or plant them out in my garden now. Our first frost should be in 2 to 4 weeks. I would like them to survive. what would be the best thing to do so they would survive the winter? Any help would be appreciated. Thanks Darrell

I have over 1000 hedges in a propagation box with the white greenhouse plastic on the top for a cover, They are rooting in sand, I pulled 2 out on Sept 20th, 2013 and they both have a bush of roots. Should I leave them in the propagation box for the winter or plant them out in my garden now. Our first frost should be in 2 to 4 weeks. I would like them to survive. what would be the best thing to do so they would survive the winter? Any help would be appreciated. Thanks Darrell