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Growing Concerns

August is a time when some gardens start to look a little worse for wear. Many of the perennials have stopped growing and the annuals are getting tired if the summer has been a warm one.

Hydrangeas are a great way to add colour to the borders this time of year. Their large white, pink or even blue blooms add interest from now till well into the winter.

The two most common types are Hydrangea macrophylla which is a bigleaf hydrangea that has either mophead or lacecap type flower arrangements. Delicate in appearance, bigleaf hydrangea is particularly well suited for placement at the edge of a wooded area or in shady locations. Varieties consist on Compacta, Glowing Embers, Nikko Blue and Pink Beauty.

Hydrangea paniculata the Panicle hydrangea which is the coldest hardy member of the genus. It can be reliably grown in USDA cold-hardiness zones 4 to 7. Native to Asia, it grows 10 to 15 feet tall. Large creamy-white flowers, which are borne in 6- to 18-inch long panicles, are produced in mid-summer as flowers mature, they may turn pink.

Plants, particularly those of the cultivar Grandiflora (Pee Gee), are sometime pruned into a tree form and grown as a specimen plant. Panicle hydrangea is also suitable for use in a mixed border or as a deciduous hedge. Other popular varieties of panicle are Pee Wee, Pink Diamond, and Kyushu. There are many other varieties of hydrangeas on the market, all equal as beautiful and showy.

The most common question about hydrangeas is ‘Why doesn’t my hydrangea bloom?’

There are three possibilities for lack of flowering among the hydrangea species:

  • Too much shade and improper pruning apply to all hydrangeas
  • Weather-related damage to flower buds applies primarily to the bigleaf hydrangea
  • Too much shade can reduce flowering with bigleaf hydrangea

Most hydrangea species benefit from some shade. This is particularly true of panicle hydrangea, which is the one hydrangea species that grows well in full sun. If you have a hydrangea that used to bloom well but now flowers only sparsely, evaluate whether the growth of nearby trees has reduced the amount of light that reaches the hydrangea. If so, you may want to consider moving the hydrangea to a sunnier location.

Improper pruning can also reduce flowering in hydrangea. Since bigleaf hydrangeas flower on previous year’s growth, potential flowers buds would be removed if the plants were pruned in fall, winter or spring. Panicle hydrangea flower on this year’s growth, so pruning them in early summer would reduce or eliminate flowering for that year.

The most common reason for lack of flowering in the bigleaf hydrangea is unfavourable weather.

Weather conditions that damage above ground parts of the plant can reduce flowering. Damaging weather conditions include early fall freezes that occur before the plant is completely dormant, extremely low winter temperatures, and late spring freezes that occur after the plant has broken dormancy. The most common of these unfavourable weather events is late spring freezes that damage tender new growth. This is particularly true where “see-saw” temperatures are very common.

It may be possible to protect plants from weather-related flower bud damage by covering them during late spring freezes with blankets, sheets, etc. There is also evidence that some bigleaf hydrangea cultivars have the ability to flower on current year’s growth, which means that, even if the plant is killed back to the ground, it should still flower during the subsequent summer.

Established bigleaf, panicle, hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in winter, since the absence of leaves makes it easier to see and reach inside plants.

Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning differs between species. Bigleaf hydrangea, which flower on previous year’s growth, should be pruned shortly after flowering is complete. Panicle hydrangea flower on current year’s growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear. Plants of H. arborescens Annabelle have been known to produce a second flush of flowers if pruned lightly after the first flowering.

Other late summer blooming shrubs are Smoketree, Summersweet Clethra, Rose of Sharon, and Blue Mist Caryopteris.

By Denise Hodgins

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