I’m currently watering the garden of my roommate/landlord. He tells me not to water during the middle of the day as the plants may get burned. Is this true and what is the mechanism? The only thing I can think of is that the water drops on the leaves will form little “magnifying glasses” and burn holes in the underlying leaves.
Your assumption is correct. At least from what I’ve heard.
If this is true, I am amazed that any plantlife at all has managed to survive billions of years of mid-day rain showers.
Here is a page that seems to answer your question.
They generally seem to poo-poo the idea that watering can burn plants, with one exception that might be an issue in areas with particularly hard water. If there are lots of dissolved salts in your water, and a lot of evaporation occurs while watering during the day, the concentration of those salts on the leaves of your plants could get quite high and, I suppose, burn your plants.
Big-time gardener here. Watering your outdoor plants, or plants in front of windows with direct sun, in the heat o’ the day, can lead to scorched leaves. In severe cases, they’ll look burned. In less severe cases, they’ll turn brownish in spots.
I’m not doubting you Shoshana but what mechanism causes that? Is it any water or just water on the leaves? That is, if the water is just put at the base of the plants, will the leaves still get burned?
In some areas around here it’s illegal to water in the middle of the day because it wastes water. Almost all of it evaporates before getting soaked in. That’s the main reason why you shouldn’t water during the day, but like the others say there is a chance of burning (though only under certain conditions).
When I assisted a plant vendor, I believe (it was more than 10 yrs ago so I dont recall exactly if it was HIM that said it but I have heard somewhere,) that being exposed to water makes the plant “think” that it is about to recieve water, and thus will be more “open” (either macrophysiologically or just its stomata, he didnt specify), thereby leading to more actual water loss!
:smack: I was so looking forward to learning what process a plant would use to burn water.
Watering during middle of hot day: Encourages plant to expend energy asborbing water and nutrients, when it should be resting to beat the heat.
Usually is a problem when done habitually, not on ocassionally, and problematic when fertilizer, herbicide or insecticide has been applied recently. All stress a plant under heat, and watering encourages plants to ‘feed’, which mean you encourage them to uptake stress inducers when the heat stress is enough.
So, if you have a plant that gets funky when watering during the day, the water is not ‘cooking’ your plant, although you might be ensouraged to believe that.
In many cases, watering during extreme heat when you have a plant that is growing in the far reaches of its temperate zone could be very life saving. Some grasses and pine trees love a mid day sprinkle to prevent being wind blown dry.
Well, in my experience, there usually isn’t much sun when it’s raining.
Haven’t you heard of sunny showers? 
I too grew up believing this and have always watered my plants in the morning or the evening. I know that these times of the day are better due to it being cooler.
I did find this…
http://doityourself.com/gardenmaint/gardeningfactorfallacy.htm
Well, obviously, they put some broccoli & cauliflower in the steamer (plants being cannibals, after all…) and leave it going while they go watch TV. Lose track of time, and voila! Burned water.
So the anwwer is “no, water drops can’t burn plant leaves”. It’s a good idea to water at other times of the day, but not because of any burning potential.
I know I’ve never felt or recieved a speckled sunburn from walking around with water drops on my skin.