If your garden is anything like mine, right about now insects and disease start rearing their ugly heads signaling time for hydrangea care. In most cases, they show up as a result of cultural conditions.
We had a very wet and cool spring and things have finally begun to dry out. But all that rain and lack of ultraviolet light have left their mark. Here’s the low down on what I am seeing right now which may match what’s happening in your garden as well.
HYDRANGEA CARE: INTERVEINAL CHLOROSIS
I am starting to see something called interveinal chlorosis on both my bigleaf hydrangeas (macrophylla) and oakleaf hydrangeas (quercifolia). This is what the oakleaf tissue looks like.
Oak leaf hydrangeas with interveinal chlorosis have yellowish leaves. Exactly what is interveinal chlorosis? It’s what happens when the plant can get enough chlorophyll (the stuff that makes it green) to the leaves. How does that happen? It goes back to the plant’s need for iron to make chlorophyll, which is why most people think it’s a simple lack of iron. They apply a foliar spray to their plant to “green it up.” But you already know that things are usually not simple with this plant. Be aware that this immediate result doesn’t solve the chlorosis. It just treats the symptom of whatever is happening in your soil. It’s up to you to figure out the issue in the soil chemistry, maybe with a little help from your local cooperative extension office or a reputable garden center.
There are several causes of chlorosis so you need to do some investigative work. Your first step is to do a soil test since an out-of-balance pH can cause this condition. If it’s your pink-flowered bigleaf hydrangea, your soil might be too alkaline. When that happens, the iron is less available to the plant. Your ideal pH for a pink flowering hydrangea should be in the range of 6.0-6.2. An amendment of aluminum sulfate can bring that pH down. The label on your product will give you all the info you need to do the job correctly. Once the pH is adjusted, the iron in the soil will be more available to the plant.
But if pH isn’t your problem, then what is causing this chlorosis? It could be that the ground is too wet. Excessive moisture in the soil inhibits root development and that leads to the plant not being able to properly grow and again, use what’s in the soil for its development. Try and check the roots of your affected plant to be sure it’s not suffering from the excess rain and irrigation. Anything wetter than damp isn’t a good situation.
Other causes can be traced to deficiencies of manganese or zinc. How do you know which is the culprit? Inspect the leaves to determine what foliage turned chlorotic first. Iron chlorosis starts on the younger or terminal leaves and later works inward to the older leaves. That’s what I have on my oak leaf. Manganese and zinc deficiencies on the other hand develop on the inner/older leaves first. Plus your soil test will tell you what’s what so you can take the proper action.
HYDRANGEA CARE: PANICLE HYDRANGEA WEB WORMS
The other issue I’m seeing is on my panicle hydrangeas (paniculata): web worms. They have begun to form their webs and rear their young within those webs. This is an easy one to address, but you must pay attention to your plants as this webworm activity will continue as the season moves along. This is what some foliage on one of my plants looked like when I discovered the webs:
You can snip off the damaged leaves and preserve your flower at the tip of the stem for later enjoyment.
HYDRANGEA CARE: INSECTICIDES
If you must apply an insecticide, know that the one to use is bacillus thuringiensis subspecies kurstaki (Btk), not Spinosad. Btk is the one that is effective on members of the moth family. Keep in mind that although Btk is an organic treatment, it is lethal to the caterpillars in the webworm nest and that other caterpillars can be impacted, including caterpillars that turn into butterflies. Just another reason to use a mechanical approach.
Like all organic products, Btk is ineffective when it dries and only works on contact with the caterpillar: it must be ingested. You can’t spray preventively; you must target the caterpillar.
These are minor fixable issues that you can easily scout and treat now that you know more about them.
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