Answer: Your new hydrangeas are big leaf mopheads (hydrangea macrophylla) and they produce flowers on new shoots which develop on old wood. In your gardening zone it's likely that the plants will freeze down to ground level - therefore you won't have any old wood for new flowering shoots to develop on. You may be able to protect your plants over the winter months so you'll have old wood in the spring. Wait until a frost kills the foliage, then encircle the shrubs with chicken wire and start piling in old leaves or straw. The goal is to protect 12-18 inches of old growth. You can prune away any branches that are higher than about two feet. When you've filled the chicken wire cages with insulating material, the old stems should be sufficiently insulated against winter weather. In the spring, remove the cages, rake the material over the soil surface as a mulch, or rake it up and cart it to your compost pile. You should see new shoots develop from the old wood and these new shoots should develop flowers. Best wishes with your new shrubs!
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