Hydrangeas (Hydrangea spp.) sport large balloonlike blossoms whose colors are offset by vibrantly green leaves. They thrive in U.S. Department of Agriculture plant hardiness zones 3 through 9. The roots of these plants can grow as wide on each side as the plant is tall. In the case of a mature hydrangea, this is often between 4 and 12 feet high. While hydrangea plants can be successfully transplanted from a container to a garden bed and vice versa, timing and careful attention are needed. Although they are large, the roots of hydrangea plants do not often interfere with water or gas pipes.
Size of the Root System
Hydrangeas have large root systems that extend out on every side of a hydrangea bush. A hydrangea's root ball extends out from the center about the distance of its height, since the plant spreads through rhizomes. The larger and older the bush, the wider the root system is. Despite their breadth, hydrangeas still have a compact root ball, which makes them easy to transplant.
Digging Up Hydrangea Roots
If you are transplanting container hydrangeas to a garden, you can do so at any time of the year, so long as proper maintenance is given afterward to your plants. The ideal time for transplanting, however, is late fall to early spring, when your hydrangea plants are dormant. Transplanting an in-ground hydrangea requires more work, though. Uproot your hydrangea only in the late fall or very early spring, while the plant is still dormant. Dig up as much of the root ball as you can, to minimize transplant shock. While you can shake loose the dirt around the roots and replant it in entirely new soil, this is difficult to accomplish without damaging your plant.
Other Transplant Tips
When transplanting your hydrangea bush, add a small amount of balanced, slow-release fertilizer, such as a 10-10-10 fertilizer, and ensure the soil is well-amended so there is little risk of compaction. Give your recently transplanted hydrangea a deep watering to help it establish itself. Hydrangeas require a lot of water, so keep the soil evenly moist to prevent wilt. Make sure that they get roughly 1 inch of water -- by rainfall or irrigation -- per week. If you are transplanting your hydrangea to a new in-ground location, dig a hole that is two to three times the size of the root ball. Heavy clay soils that easily become waterlogged are dangerous for hydrangea health. Amend clay soil by mixing in organic matter in a 3-to-1 ratio of clay to organic matter. Add in sand, again in a 3-to-1 ratio of soil to sand. Last, dig a shallower, wider hole to plant your hydrangea in.
Roots and Interference
Hydrangea roots do not have invasive roots. Unlike tree roots, which can swell as they grow or when they take in water, hydrangea roots will grow around obstacles -- rocks, roots of other plants, pipes and house foundations. Hydrangeas can actually enjoy the extra shade provided by houses and other buildings, as they do better in partial shade areas rather than in direct sunlight.