Should you bury the grafting knot while planting a hydrangea tree in zone 5 or plant it at pot level. The knot is 10″ above pot height.
Thank you for contacting the Toronto Master Gardeners regarding you new hydrangea standard.
You likely have a Hydrangea paniculata which is the only hydrangea that grows as a standard or tree and is quite cold tolerant of your zone 5 area. You will want to plant your tree to the level of the pot in full sun (keep moist) or partial shade. Add mulch to your well drained soil and make sure the mulch does not touch the trunk.