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Pruning Hydrangea

You’ve got to know who’s who before the how-to

Four different species of hydrangea grow in Maryland, and while now is the time to prune them all, each is pruned differently. So you’ve got to know who you’re pruning to know how to prune.
    Hills-of-snow hydrangea grows entirely from a crown close to the ground. It produces stiff upright stems in the spring, two to four feet tall and, starting in July, large creamy white flowers. This variety can be pruned with a sturdy hedge clipper in the same way you prune ornamental grasses in the spring. Remove each stem as close to the crown as possible. Thinning the new shoots three to four inches apart results in larger blooms. Allowing all of the stems to flower results in smaller blooms.
    House hydrangeas flower pink, blue and purple. Some of the cultivars such as Niko Blue or Niko Pink are true to color regardless of the pH of the soil. Some of the older cultivars produce pink flowers if the soils are alkaline and blue if the soils are acid. When pruning these cultivars, remove all canes that produced flowers last year and all that are less than a pencil in diameter. Thinning the stems to three- to four-inch spacing results in larger blooms.
    Peegee is the tree form of hydrangea. Some plants have but one stem while others have two or three stems. For these species, leave the stems alone and remove only the canes growing at the top of the main stems.
    Oak-leaved hydrangeas are pruned only to maintain form. These have a branching habit similar to viburnums often with flaky or loose bark. Removing last year’s spent flowers is often the only pruning needed other than removing dead branches that detract from the appearance of the plant.

Pink Asian Dogwood? Not Yet

    An excited reader told me he had a one-of-kind Asian dogwood producing pink flowers.
    By the way, what we tend to call flowers are actually bracts, similar to those of poinsettia.
    Were the emerging flowers white? I asked.
    After a week or so, did the base of the petals turn purplish-pink?
    Drought stress appears to be partly responsible when Asian dogwoods develop pink discoloration, which starts at the base of the bracts. The discoloration does not occur when there is adequate rainfall or when the plants are irrigated appropriately. I have never seen discoloration of the bracts during years when there has been excessive moisture.
    There are major advantages in growing Asian dogwoods. They flower after our native dogwood, and they are immune to discular anthracnose, which is responsible for the demise of our native dogwoods.
    But to my knowledge there are no true pink-flowering Asian dogwoods in the trade. There have been attempts to breed pink-flowering cultivars, and in time, through genetic engineering, there will likely be one. But none so far.

Ask Dr. Gouin your questions at [email protected]. All questions will appear in Bay Weekly. Please include your name and address.