How to make hydrangeas bloom
Q: I have a large hydrangea shrub in my yard, but it never blooms. I usually cut this back every year in the fall, when it loses it leaves. Do you think this could be the reason why it does not bloom?
A: There are more than 75 species of hydrangea. Most are native to China and Japan, but there are also several from North America. The hydrangea macrophylla is the most common hydrangea grown, so I am assuming that your shrub is this species.
Hydrangea macrophylla is the largest group and comes in two forms: Mophead and lacecap. The mophead varieties produce big, round domes of flowers in bright pink, blush, cream, white and violet or blue. Your hydrangea macrophylla’s lack of flowering is most likely due to pruning issues: Either pruning at the wrong time or pruning the wrong branches. The mopheads and lacecaps bloom most commonly on previous years of growth or older wood. Pruning in the fall or early winter will eliminate the flower buds for the next spring, leaving you with no blooms.
It’s best to prune hydrangeas in summer after the flowers start to fade; that’s if it’s even necessary to prune at all. Hydrangeas only need to be pruned if the shrub has grown too big. Cut it back after the spring bloom but before late summer.
Older shrubs can be thinned by cutting out branches from the base or by cutting branches back by about one third of the overall mass of the shrub. Pruning in this way will rejuvenate the shrub, stimulating new growth from the base of the shrub.
In the Redding area, Hydrangea macrophylla do best with afternoon shade, rich soil and regular irrigation.
If you live in a colder climate, like Siskiyou County, where the hydrangea shrub will freeze back to the root system, try a variety of hydrangea that blooms on new wood. These can freeze to the ground, or be pruned hard, every winter; and they will still flower if the season is long enough.
Endless Summer is a mophad variety that produces flowers on new wood. A variety with lacecap flowers is Blue Wave, with blue to pink sterile flowers and darker fertile flowers in the center.
Hydrangea flowers are unique in that they indicate soil pH. Pink flower color often indicates neutral or slightly basic soil pH. Blue flowers indicate more acid soil.
To change the color of your hydrangea flowers to blue, apply aluminum sulfate. Follow the directions on the container and apply it as the plants leaf out in spring, and again every couple of weeks until flower buds are visible.
A more drought friendly variety of hydrangea is the white flowered oak leaf hydrangea, This is an American species of hydrangea with large lobed leaves that are reminiscent of oak leaves. Oak leaf hydrangeas are less particular about soil and water conditions, but still prefer afternoon shade. They produce long panicles of white flowers in spring that age to pink in summer and then turn dark brown in winter. Then in the fall the large, rough leaves turn beautiful shades of bronze and crimson.
The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or firstname.lastname@example.org. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.