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A dogwood dilemma

Q: I have an 11-year-old dogwood tree (Cornus florida) that has not bloomed well for the past five years. Do you know what the problem may be?

Edda Carballo, Bayonne, NJ

Cornus florida. Cornus florida. Photo/Illustration: courtesy of Missouri Botanical Garden PlantFinder

A: Paul Cappiello, the exectutive director of  Yew Dell Gardens  in Crestwood, Kentucky, responds: Something is going on that is making the tree less than happy. The easy response would be to suggest that the tree might be overfertilized, much the way we see tomato plants push all vege­tative growth when we get a little too happy with the magic blue juice. But that is likely not the problem here. I’ve seen dogwoods from the lushest, most overfertilized landscape to plants on death’s doorstep all produce some flowers. Likewise, flowering dogwoods in full sun to almost broom-closet shade will usually bloom if they are still alive.

The best thing to do here is to inspect the tree for clues. First, try to gauge the growth rate of the tree. How much shoot growth has been produced this year compared to last year and to the years when it was flowering? If the growth rate has been substantially reduced, there has to be a reason.

Second, look at the ground around the tree. Has the grade been changed, altering the depth of soil over the roots or changing drainage patterns around the tree? Remember that the most essential roots of most plants are located in the top 6 to 8 inches of soil. Any significant change in grade can have a major impact on essential carbon dioxide and oxygen exchange at the root level.

Third, closely inspect the base of the trunk for physical damage or signs of borer exit holes. Borers are a big problem for dogwoods, especially those stressed for other reasons. Once a family of borers takes up residence, you should start anew. Dogwoods are also notoriously sensitive to mower blight, and a seemingly insignificant bump into the trunk can spell disaster down the road.

Finally, the dreaded anthracnose could be to blame. This nasty fungal pathogen causes stem dieback and eventually will take the whole tree. The process may take several years. Look for the telltale dead stem tips and suckering shoot growth from the lower portion of the trunk. If the problem turns out to be the anthracnose, it might be best to look to Cornus kousa as a replacement. While it is not identical to Cornus florida in garden features, it tends to be highly resistant to the dreaded fungus.

From Fine Gardening 108, pp. 22