Hydrangeas are one of the most popular garden plants, but, as many disappointed gardeners can attest, if you do not choose the appropriate species and cultivar for your location, your plant may not flower.
There are a few species of hydrangea, including four common species that are shrubs: Hydrangea macrophylla or bigleaf hydrangea, Hydrangea paniculata or panicle hydrangea, Hydrangea quercifolia or oakleaf hydrangea, and Hydrangea arborescens or smooth hydrangea. This post focuses on Hydrangea macrophylla, or bigleaf hydrangea. Hydrangea macrophylla is one of the most common species among home gardeners.
According to Dr. Alex Niemiera, a professor in the Horticulture Department and an expert on Woody Plants, there are two main types of Hydrangea macrophylla: mophead and lace cap. Mop head varieties produce big, full orbs of flowers, while lacecap varieties produce flatter blooms of small flowers surrounded by a few larger flowers.
In addition to choosing the flower type you prefer, there are three other major considerations for selecting the right bigleaf hydrangea for your garden: plant hardiness, whether or not it is “reblooming,” and flower color.
As with all plants, be sure to double check that it is appropriate for your USDA hardiness zone before planting. You can check your hardiness zone here: http://planthardiness.ars.usda.gov/PHZMWeb/ and you can search USDA plant database to check the hardiness range of your potential landscape plants: https://plants.usda.gov/.
Non-reblooming varieties of Hydrangea macrophylla flower in the following way: In summer, branches produce new buds for the next spring. These buds stay dormant all winter and bloom in spring. If winter temperatures got too cold, or if there is a spring cold snap that kills the buds, non-reblooming Hydrangea macrophylla will not produce flowers that year.
However, innovation in plant selection/plant breeding has produced cultivars of Hydrangea macrophylla that are “reblooming” (or “remontant”) meaning that, in addition to the buds produced in summer that lie dormant all winter and bloom in spring, “reblooming” varieties produce new buds throughout the summer on old wood (old wood means branches produced in a previous year, e.g. not new growth from the current season). This means that if cold winter temperatures kill the summer-produced buds, your plant will still be able to produce new buds to bloom in the summer.
According to Dr. Niemiera, the exact climatic conditions that kill dormant Hydrangea macrophylla buds depend on a variety of factors, however, in general, Virginia residents who live in zone 6 or colder should plant reblooming varieties of Hydrangea macrophylla. Temperatures in zone 6 or colder are likely to kill dormant blooms. With reblooming varieties of hydrangea, however, Virginia gardeners in zone 6 or colder can still enjoy their hydrangea blooms.
If you live in zone 6 or if you are concerned about cold temperatures affecting your Hydrangea macrophylla blooms, visit your garden center and ask for a reblooming variety of Hydrangea macrophylla.
Another consideration when selecting a bigleaf hydrangea cultivar is flower color. For some Hydrangea macrophylla, soil pH affects flower color. According to Dr. Niemiera, “In a relatively acid soil (about soil pH 5.0 to 5.5) flower color will be blue. At soil pH values higher than 6.0 to 6.5, flower color will be pink. Soil pH in the range of 5.6 to 5.9 will produce flower colors that intermediate between pink and blue.”
Some cultivars of Hydrangea macrophylla do not have the pigment responsible for the blue color and will produce the same color bloom regardless of soil pH. For example, there are some cultivars that produce white or pink flowers, regardless of the soil pH.
Other bigleaf hydrangea tips:
When planting hydrangeas, you also need to carefully consider the sun exposure and moisture your plant will receive. For the best-looking healthy plants, Dr. Niemiera recommends that bigleaf hydrangeas get morning sun and afternoon shade and that gardeners should be sure to keep hydrangeas adequately moist to prevent wilting!
Hydrangeas can make a beautiful and showy addition to your garden, but they require attention and care. Virginia gardeners interested in planting Hydrangea macrophylla should carefully select the right cultivar for their area, plant it in an appropriate location, and ensure it gets enough moisture.
If you are interested in learning more about selecting hydrangeas (or other showy flowering shrubs) for your garden, visit this Extension publication: https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/HORT/HORT-84/HORT-84-PDF.pdf
If you are interested in learning more about bigleaf hydrangeas, please visit this Extension page: http://pubs.ext.vt.edu/3010/3010-1463/3010-1463.html