Day Wilters May Not Need Watering

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

With noon approaching, this Ligularia stenocephala ‘The Rocket’ is starting to wilt. Photo: urban-oasis.blogspot.com

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?

Plants lose much of the water they absorb to transpiration. Photo: http://www.usgs.gov

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.

Well-mulched squash are less likely to wilt. Photo: http://www.thedailygardener.com

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!

3 thoughts on “Day Wilters May Not Need Watering

  1. Kris Sinclair

    Larry, I love your blog. I’ve learned many new gardening ideas. However, and maybe you have nothing to do with this, the ads on your blog are horrible!! Can’t youall find some companies that want to sell garden stuff to advertise with you. The pictures they show are such a turn off.
    Thanks.

    • Sorry, but I have no choice over the ads. In fact, apparently the ads are chosen by some computer program based on your habits, so the ads I see (based on MY habits) aren’t the same ones you see. And it’s the ads that “help pay” (I’d make more working in a garment factory) for the blog.

  2. Rhubarb! That is one of the more reliable day wilters. So are impatiens, but they do not have big leaves. In our arid climate, some plants can get roasted in the dry warmth.

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