Sick hydrangea with webs on flowerheads

Discussion in 'HortForum' started by mamaconnie, Jun 23, 2021.

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  1. mamaconnie

    mamaconnie New Member

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    Hello! I have an outdoor flathead hydrangea in a pot that is at least 15 years old. It has always bloomed well, but this year it seems to be dying. The blooms from the top downward are becoming brown, and each bloom is covered with a fine web. The leaves are also turning brown. This happened very fast, in about a week. The blooms towards the bottom of the plant are still blue, but I think they will succumb as well. I've attached some photos. Is there anything I can do to save this plant? thanks very much for any advice.
     

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  2. Margot

    Margot Generous Contributor 10 Years

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    I can't help but wonder if the webbing is caused by spider mites. Spider mite webbing usually occurs under the leaves however where they live and suck juices from the leaves. Until you get a better diagnosis, it wouldn't do any harm to hose down the entire plant (especially under the leaves) as thoroughly as you can and give it a really good soaking as well. Since the plant is obviously struggling, I'd also cut off the flower heads so it can direct its energy to the roots and leaves. This would not be a good time to fertilize.

    Spider mites (https://www.hgtv.com/outdoors/flowers-and-plants/trees-and-shrubs/a-hydrangea-problems-primer)
    Almost invisible to the naked eye, spider mites cause deformed growth, especially on new stems. While you may not see the mites without a magnifying glass, you’ll recognize their presence by the webs they make between leaves. Since spider mites like hot, dry conditions, you can often control them by keeping your hydrangeas well watered, and by blasting them with water from the hose every couple of days. Many predators eat spider mites, including ladybugs, so use insecticides as a last resort. You don’t want to kill off insects that aren’t doing any harm and that may provide some natural pest control.
     
  3. mamaconnie

    mamaconnie New Member

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    Thanks very much Margot. I thought of spider mites, but none of the photos that I found on line of what they do looked quite like my poor plant. However, I will definitely give the plant a good wash and see if that helps. I also will delay fertilizing, which is something I might have tried otherwise, so thanks for the tip.
     
  4. Luis_pr

    Luis_pr New Member

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    Looks like web worms to me. A moth probably laid some eggs on the hydrangea stems a while back, the eggs hatched into worms that create a baggie-like cocoon-like web to protect from insecticides. I found out some of the same webbing days ago on one of my Possum haw... deciduous holly trees. The area covered by the "baggie" is usually fed off by the worms and if not caught in time, the foliage in the web is consumed or dies.

    Open up the bag with a stick to let the birds feed on the worm/larvae or to cut off the stem ends and dispose in the trash, not the compost pile. If you see worms in the stems outside of the webs, pick them up by hand and kill them. Throw them in the trash.

    If the worms are still active, you can treat with Bacillus thuringiensis var. kurstaki (Btk), aka, as Dipel or Thuricide. But if the caterpillars are gone, forget it. Use Btk instead in future years (apply per label directions, starting a month before you noticed the problem) as the moths are likely to return. Look around to see if you find the webs in other plants in the garden too or in other stems of this hydrangea.

    The plant is stressed now so do not fertilize it but try to maintain the planted in shade by 10-11am and the soil well mulched (5 to 10 cm of organic mulch) and as evenly moist as you can (not moist then very dry then moist then very dry again, etc.). Insert a finger into the soil to a depth of 10 cm and water if the soil feels dry or almost dry. Watch out for heat stress as soon as temperatures typically reach or exceed 29°C.

    Watch for other opportunistic pests that may decide to infect the plant while it is already stressed. I could not "see" anything else on the pics but one normally needs to be up close and personal with a magnifying lens.
     
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  5. mamaconnie

    mamaconnie New Member

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    Thanks very much Luis. Can you tell me how big these worms are? I haven't been able to see anything on the flowers.
     
  6. Georgia Strait

    Georgia Strait Contributor

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    Hello - I have a question about your hydrangea

    Is this “lace cap hydrangea” plant of particular sentimental value to you?

    It puzzles me to see it ailing so

    Usually - aside from killing it with lack of water or some sort of accidental toxicity - these are coast garden workhorses

    If it was me - be forewarned: i have a tough love garden approach - hydrangeas are not expensive ... and easy to make cuttings if THIS ONE is sentimental to you

    For all the time and cost of applying insecticides (“natural” or otherwise ) — I would cut my losses

    That said - it’d be interesting to know what this is and how it arrived on your plant.

    If disposing of the entire plant is not your choice - i suppose you could try cutting it back and burning the trimmings (green waste might not allow diseased plants)

    And see if it comes back healthier next year. I imagine your lace cap blooms on old wood — ie wood twigs grown in 2021 will support the buds and blooms in 2022.
     
  7. Luis_pr

    Luis_pr New Member

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    The larvae can be small and they may get up to 5 cm but it varies with the specific type. They may have left too.

    Note: I would not pull out the plant. At worst, I would cut the affected stems or all stems if it is a bad infestation. The roots will develop new ones but keep the soil moist if you cut them.
     
  8. mamaconnie

    mamaconnie New Member

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    Thanks very much for all your help. Update - it is definitely spider mites. I cut off a blossom and examined it closely, and I could see lots of those tiny pests! I blasted it with water. Tonight I will cut off all the flowers. It is, indeed, a run of the mill lacecap hydrangea, but it does have some sentimental value to me, so I will see if I can save it. Thanks again! This is such a valuable resource!
     
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  9. Luis_pr

    Luis_pr New Member

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    If you blast the hydrangea with water due to spider mites, re-check a few days later to see if they are back. You apparently have a large infestation. After the first blast with water, some of the mites will be killed but, others will remain and fall down. They may return after they climb back up. Be ready to hit the plant with the hose spray a few times after several days. And look for spider mites elsewhere too. I released ladybugs too because they were infecting many of my roses in the summer.
     
  10. Margot

    Margot Generous Contributor 10 Years

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  11. Luis_pr

    Luis_pr New Member

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    Last edited: Jun 28, 2021
    Daniel Mosquin likes this.

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