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Why your hydrangeas might have brown spots even if you're watering this summer

"Fielding Questions" columnist Don Kinzler also hears from readers about a plum tree that seems to be dying suddenly and when to cut back suckers on an old lilac.

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A reader wonders why their hydrangeas have brown spots this summer. Special to The Forum
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Q: I'm wondering if you can help with some significant brown areas that just showed up on the leaves of our Vanilla Strawberry and Zinfin Doll panicle hydrangeas. I’ve watered the lawn on a couple of occasions and we just had an inch of rain a couple of days ago. All plants are in very sunny locations for most of the day. — Mike E.

A: Your panicle-type hydrangeas are well-adapted to Northern landscapes, so you’ve chosen wisely, but you’re not alone in your foliage troubles. I’ve received many photos and questions identical to yours about hydrangeas this year.

Although the panicle-type hydrangeas enjoy sunshine, they prefer cool, moist soil. The intense heat, wind and drought of the past weeks are totally opposite of what they need. Even though we can provide supplemental moisture, prolonged heat is beyond our control.

Extreme heat and drying winds easily cause foliage burn on hydrangeas. The foliage problems aren't diseases for which there are treatments. If the weather becomes less extreme, the new growth should be normal.

Heat stress problems can be compounded by the rock mulch that I notice in the photo. Rocks become very hot in full sun, as they store and radiate heat, especially during extreme conditions. This can be mitigated somewhat by removing a 2-foot minimum circle of rocks and replacing with shredded wood product mulch, which would keep the hydrangeas much cooler.

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The word hydrangea means "water-loving," so give your hydrangeas a deep soak several times a week.

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Q: I have two plum trees in my backyard. This spring they both blossomed beautifully but now one is looking like it’s nearly dead with almost no leaves. Any suggestions for how to help it? — Rachel G.

A: The same thing happened to our own plum trees this spring. They blossomed beautifully, and I was hoping for a large crop of plums.

While the trees were flowering, we had near-freezing temperatures. One plum tree dropped its blossoms soon after, and now it’s dead. It didn't leaf out at all, and when I check the branches, there’s no life anywhere on the tree.

It was a difficult spring for trees. In March, we had a near-record warmup followed by multiple days of freezing temperatures. The up-and-down weather apparently injured some trees, including our plums. Many birches were hit hard, along with other types.

I'm afraid there's nothing that can be done at this point. Our own plum tree is totally dead with no life, and I'll be cutting it down and replacing. If your tree has a few sparse leaves, you might wait and see what happens, but by the looks of the photo you sent, the damage seems severe.

Check the twigs throughout the tree for live wood by scratching the outer bark with your thumb nail. If there’s a green layer under the outer bark, the twig is alive. If there is no green cambium layer, the twig is dead. Dead twigs also snap when bent, instead of being pliable.

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I wish I had better news. I'm going to miss what I hoped would be a good plum crop on our own tree.

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Q: Our very old lilac bushes have a lot of suckers. When should I cut them back? Our wild plum trees also have a lot. When is a good time to prune them? — Jim S.

A: The sucker shoots on both lilac and plum can be pruned away now, preferably as close down to their point of origin as possible, which helps reduce how quickly they resprout. In fact, sucker shoots can be pruned out anytime, the smaller the better.

Sucker shoots that arise from trees such as plum can be mitigated somewhat by hormonal products such as Sucker Stopper and Sucker Punch. Label instructions must be followed carefully, and results vary.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.
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