Some hydrangeas are referred to as trees since some varieties, like the peegee variety, can grow to be 25-feet-tall and 10-feet-wide. However, all hydrangeas are technically shrubs, even if they look like trees. No matter which kind of hydrangea you have, they all can be planted in a similar manner and most can tolerate USDA Hardiness Zones 3 to 9.
Plant hydrangeas in the spring after the last frost or in the fall.
Select a planting site. Hydrangeas prefer an area that is partially sunny. Morning sun is best, if possible. While hydrangeas can tolerate more sun, especially larger varieties and those planted further north, they cannot tolerate full shade. Ensure the planting site is large enough for the hydrangea to grow to full height. The average size of a hydrangea is 4-foot by 4-foot, but if you have one that resembles a tree, you need much more room than that.
Prepare the soil so that it drains well, if necessary. To determine if your soil is well-draining, dig a hole that is 2-feet-deep and fill it with water. Check on it after 24 hours; if there is still water in the hole your soil does not drain well and you need to add organic matter like compost or peat moss to the soil. To do this effectively, till the soil about 18-inches-deep, then add 3 to 6 inches of organic matter. Mix together to create a planting site with well-draining soil.
Dig a hole that is just as deep as the hydrangea's current container, but twice as wide. Take the hydrangea out of its container and set it in the hole (soil and all). Backfill the soil you dug out and push down with your hands or feet to be sure the soil is firm without any possible air pockets.
Water your hydrangea well after planting. About once a week, if it has not rained, water your hydrangea. Do not over water; this can lead to root rot.