Day Wilters May Not Need Watering

Squash plants often wilt on hot summer days, but probably don’t actually need watering. Photo: www.groworganic.com

Question: My squash plants seemed quite wilted in the midday sun yesterday, yet when I went to water them, I discovered the soil was already moist, so I didn’t. Then this morning, they looked fine again. What’s going on? 

Fernanda di Luce

Answer: There are plenty of causes for wilting leaves, but lack of water is only one of them. It could be caused by root rot or stem-boring or root-eating insects, but since your plants recuperated overnight, that’s not the problem either. Instead, the main cause is something much simpler.

Many large-leaved plants wilt in the afternoon heat—not only squash, but melons, morning glories (Ipomoea spp.), ligularias, Japanese butterburs (Petasites japonicus), rhubarb and others—yet don’t necessarily need watering and aren’t suffering from insects or rot. If the soil is still moist (insert a finger into it and feel), watering isn’t required. You’ll note such plants recuperate perfectly overnight even if you don’t water and will be in fine form the next morning.

So, what’s happening?

These “day wilters” are in a situation where they lose water to transpiration more quickly than their roots are able to absorb more. All plants transpire (dispel moisture into the air) and indeed, in most species, more water is lost to transpiration than goes into growth. Plant transpiration is in part a self-protective mechanism plants use to survive excessive heat. As with people, the hotter and sunnier it is, the more plants transpire, as transpiring cools them off. Some just go a bit too far and start to wilt, that’s all. And big leaves offer more surface area for transpiration, so are more likely to wilt

Day wilting is not particularly harmful to the plant, but shows it’s really not happy with its growing conditions. Of course, you can’t do much about the heat, but you can modify the plant’s environment so it won’t transpire at such a rapid rate.

Simply mulching with a good, thick mulch (3 to 4 inches/7 to 10 cm), which keeps the soil cooler, is sometimes all you need to make the plant happy, but some day-wilters, like ligularias and butterburs, are actually better off in partial shade or even fairly deep shade. So, you might want to consider eventually moving them to a shadier spot. Even squashes, reputedly full-sun plants, will do best in spots where they get most of their sun in the morning and will appreciate light shade in the heat of the afternoon.

Certainly, don’t waste water by watering a plant when its soil is still moist. Save that activity for times when the soil really does start to dry out!