When helping people with plant selection, the No. 1 request I get is “Can I have hydrangeas.” Something about the blue mophead hydrangeas gets the horticulture juices flowing. These southern gems are called Hydrangea macrophylla or commonly known as bigleaf, French or snowball hydrangea.
Over 500 known cultivars of bigleaf hydrangeas are divided into two main groups: hortensias with large snowball-like flower clusters, and lacecaps, which have somewhat flat-top (lacecap) flowers, with fertile, non-showy flowers in the center and more showy sterile flowers on the outside. Plant sizes vary from 4 feet to 12 feet, depending on the variety.
My criteria for giving the go-ahead to plant these beauties are morning sun, afternoon shade and moist, well-drained soil. Avoid planting on hot, dry, exposed sites. Hydrangea macrophylla is susceptible to freezing temperatures, so find some protection from the cold. Late spring cold can freeze leaves and occasionally damage flower buds. French hydrangea can easily be grown in containers and is an excellent patio plant.
Bigleaf hydrangea responds to several light applications of fertilizer during the growing season. A general-purpose fertilizer, such as 8-8-8 or 10-10-10 applied at a rate of 1 pound (2 cups) per 100 square feet in March, May and July is suggested. It isn’t necessary to remove the mulch when fertilizing, but do water soon after application to help dissolve the fertilizer.
Hydrangea macrophylla is a water-demanding plant best-suited for areas where moisture is available. Water whenever the plant begins to wilt. Avoid plant stress in the spring when the flowers are forming. Apply 3 to 5 inches of an organic mulch, like pine straw, pine bark or fall leaves to the soil surface to conserve moisture and control weeds.
You can change the color of the flowers on these hydrangeas from blue to pink because of the plant’s ability to absorb aluminum. When the soil pH is lower than 5.5, hydrangeas can better absorb aluminum, which results in a blue flower. When the pH gets more alkaline (6.0 and above), hydrangeas can’t absorb the aluminum as well and the color changes to pink. Since most of our soils are acidic, our hydrangeas naturally are blue.
To gradually change flower color from pink to blue, broadcast ½ cup of wettable sulfur per 10 square feet and water it in. To make the flowers pink, broadcast one cup of dolomitic lime per 10 square feet. It may take a year to see a noticeable change in flower color from this treatment.
Another, quicker way to achieve a change in flower color is through liquid soil drenches. To make the flowers blue, or perhaps more blue during the growing season, dissolve 1 tablespoon of alum (aluminum sulfate) in 1 gallon of water and drench the soil around the plant in March, April and May. To make the flowers pink, dissolve 1 tablespoon of hydrated lime in 1 gallon of water and drench the soil around the plant in March, April and May. Avoid getting the solution on the leaves because foliar damage may result.
Prune bigleaf hydrangea when the flowers begin to fade. Prune out flower heads and leggy branches to encourage fullness. Flower buds will begin forming in late summer for the following season, so avoid pruning after mid-August. Sometimes it is necessary to prune plants after a harsh winter to remove damaged foliage. Although this will encourage vegetative growth at the expense of flowers, it is better for the health of the plant and its aesthetic appearance to remove winter-damaged foliage.
Reach Campbell Vaughn, the UGA Agriculture and Natural Resource agent for Richmond County, at firstname.lastname@example.org.