source: Cutting Edge Plants
Short Answer: Bigleaf hydrangeas wilt because their stomata don't close when under heat stress. It is probably not dying, but read on to make sure.
What are stomata?
Stomata are openings on the undersides of leaves that allow for exchange of gasses - primarily so CO2 can enter a plant and participate in photosynthesis. (Stoma for singular.)
In the case of wilting, the gas being lost is water vapor. Stomata are supposed to open and close to regulate the exchange. To conserve water, they should shut when a plant is under stress (heat or drought stress). In high heat, the stomata on bigleaf and mountain hydrangeas don't shut, so the plants keep losing water and therefore wilt.
You've probably seen the wilting either when in full sun (that raised the temperature) or even in dappled shade when the temperatures soar into the upper 90 degrees. You've certainly seen it in drought - but drought wilting is not the situation we're talking about here - we're specifically talking about wilting in hot weather (or when direct sunlight causes a rise in temperature).
Most plants that you see thriving in the Deep South have stomata that close when under the stress of our baking heat. Most types of plants (random examples: crapemyrtles, coneflowers, gardenias) wilt only under real drought stress.
Under the mild springtime sun, I forget what divas bigleaf and mountain hydrangeas can be during the long, hot, dry summer. It's a roller coaster ride. Leaves are perky at night; wilting during the day. Hydrangea stomata don't cooperate.
Even the flowers wilt! There was full recovery once there was shade again.
Prescription for mitigating hydrangea wilting.
Here are some tips to help ease the situation, but keep in mind that Hydrangeas in the summer are going to wilt in the heat. These are the best horticultural practices for keeping them as happy as possible.
Install drip irrigation - Drip is better than overhead irrigation because it keeps moisture off the leaves and therefore reduces diseases such as black spot and powdery mildew. It's also more economical because it targets just the plants and keeps you from wasting money on watering any surrounding pavement or weeds.
Incorporate organic matter - Organic matter conserves water and reduces the need to water. It also adds nutrients and improves the soil structure to provide a better growing space for roots.
Mulch your plants - Mulch conserves moisture too. Run your drip irrigation under your mulch. You can spread your mulch 2 to 4" think, but keep it 2" away from the base of the plants - there aren't any feeder roots in that area, plus mulch is a cover for insects to get at your plants. Here are instructions on How to Plant a Shrub, including a diagram portraying mulching.
Check out the soil moisture - Bend down and touch the soil, worm your finger into it - Is it moist? Then you've watered enough. No need to overwater.
Sun/Shade balance - Make sure your diva hydrangeas are getting only morning sun or dappled shade. In the Deep South, that's full sun until about 11am.
Plant types with thicker, waxier leaves - the waxy layer seems to conserve moisture better. I have seen Cherry Explosion hydrangea doing well later into the day than other types could possibly fare.
Does temporary wilting in heat and sun hurt a hydrangea?
I've asked other hydrangea growers and they can't see that temporary wilting is hurting or weakening hydrangeas. I can't either. They keep getting bigger and flowering like crazy year after year.
Of course, if you really aren't watering like you should and they are going into drought stress, the shrub is being harmed and could languish or die.
Rule of thumb: If the hydrangea doesn't perk up in the evening, that's true drought stress.
Since you're doing all the right things and they perk up when shade returns or when heat abates in the evening, then don't dwell on the wilt. Go about your business. Rest assured there will be no permanent damage and your hydrangeas will be okay.
You may have noticed how quickly they perk up when shade returns or a summer storm blows in - I've gone around the other side of the house to do a chore, the sky became overcast, and I come back around to find happy hydrangeas. Good grief, what drama.
So, should I do nothing? Correct, do nothing if you're cultivating hydrangeas according to the prescription above.
Do something if you need to make changes because of conditions like these:
A tree has died and you need to move the hydrangea into morning sun or dappled shade.
You purchased a hydrangea that was grown in shade, but you're giving it more sunlight - slowly acclimate it to morning sun by putting it in full sun for increasingly longer durations (start at an hour) or install temporary shade by using an umbrella or something portable.
Your hydrangea isn't wilting, but there's something else going grown . . . it's turning brown. That's sunburn! If it's getting sunburn, move it into more shade. Yes, sunburn can happen even if the hydrangea isn't wilting. Also, sunburn is most likely to happen around high noon or in the afternoon.
If you're practicing good Hydrangea horticulture according to the prescription, your hydrangeas will be fine come the evening. They are tougher than they seem when they're acting out by wilting.
Also, hydrangeas will always make a liar out of me. You may have in mind a hydrangea that's growing great in full sun. I'm envious! Look for the variables that make it happy - perhaps it has an especially waxy leaf that helps conserve moisture or it's in morning sun rather than afternoon sun.
The point of this article is to encourage you to do your best gardening and rest assured, your wilting hydrangea will be okay if it's not suffering from true drought stress.
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