Answer: How disappointing to have your petunias become victims of the heat! In general, most tender annuals and some perennials react to high temperatures (90F and above) by slowing down and sometimes even stopping their respiration and transpiration in an effort to conserve moisture. This results in wilting and can eventually cause cell collapse in both stems and leaves. It's not a lack of water that's causing the wilting but the inability of the water around the roots to be transported up through the plant's vascular system because the stomata (natural openings in the leaves) have shut down in a defense against the heat. Usually heat stressed plants resume transpiration and respiration when the temperatures drop below 90F, and by the next morning they're fully recovered. Apparently your petunias are becoming so over-heated that they can't recover even when the temperature drops. If the containers or baskets are hanging out in the afternoon sunshine, the roots may be baking, as well. Have you tried putting the plants where they receive only morning sun? This may give them just the protection they need to endure and then recover from high temperature stress. Hope you have better success with your petunias in the future!
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