If Everyone Says It, It Must Be True
As you get deeper and deeper into the plant rabbit hole, a common piece of advice we’ve all received (or thought we’ve learned) is that you can’t water your plants when they’ll be exposed to direct sunlight. The water droplets refract the light, and it’ll cause scorch marks or sunburn on your plants.
In addition to getting this advice, there’s also probably people who will say that no, it’s not true, the scorching or burns are correlation, not causation. In case you’re not sure what the difference is:
Correlation: A mutual relationship or connection between two or more things (or: this stuff happened together)
Causation: The action of causing something (or: doing this causes that)
Just because two things happened at the same time does not mean that one thing caused the other thing. Such is the case with watering your plants during the day, and then seeing scorch marks later. Turns out, watering your plants midday is probably just fine. They’ll be fine.
You may be saying “but JEN. I did this! I watered during the day and then my plants got scorched and DIED!11!!”
Fun fact: some smart folks actually wrote a research paper on the phenomenon, and have some answers as to why this might seem to be the case. So rather than arguing like peasants over whether or not a thing happened, we can look at science to tell us what’s real.
The TL;DR version?
It’s only hairy leaves that shouldn’t be watered during midday sun. Otherwise, the water droplets do not stay in place long enough or don’t have the right angle to actually cause any burning.
So am I just imagining the sunburn my plants get after I water during the day, or what?
What it boils down to is how long water stays in the roundest droplet form possible, which is what refracts the sunlight and causes scorching or burns on the plant. On smooth leaf plants, the water droplets don’t usually form a round enough shape to refract light in a way to cause burns. For plants with some fuzz, however, the fuzz or hair can hold the droplets far enough away that the angle is there for the sun to burn the plant – similar to a magnifying glass at just the right angle.
The catch is that most of the time, the plants are jostled or shaken and the droplets just roll off. They evolved in a way that water won’t readily stick to their leaves; instead, it falls to the ground. The research paper DID note that when water droplets were held above the leaf, sunburn was possible.
That small caveat is where our beloved cacti and succulents come in.
In pots or in the ground in our gardens, our plants aren’t in conditions like they would be in the wild. The jostling or breeze or even the shade the plants might have evolved to be adapted to isn’t likely to be there. As a result, you encounter conditions that the study would describe as unique or improbable.
Plants that have rosettes, farina coating on the skin, or other growing shapes that hold water in pools or puddles on the leaf are much more prone to sunburn spots as a result of water on them. Your echeverias, dudleyas, and similar succulents are actually prone to sunburn from water sitting on them during the brightest part of the day. This is especially the case if they are new or it’s their first season of hot weather.
What do I do???
The obvious answer is to not water midday. I highly recommend watering at the end of the day to allow your plants enough time to absorb the water before a hot day. This is especially the case for cacti, who also open up their stomata at night to absorb more moisture.
If it’s early enough in the day, watering in the morning can also be just as good, but I have found that the water is often not evaporated or absorbed enough for my morning-sun-exposure plants by the time the sun hits them. What ends up happening in that case (as I learned from sad experience) is that the water sitting in the center of the rosettes, particularly on my echeveria hybrids, refracted light just enough to severely burn the center portion of the leaves. In that case, I couldn’t even try propogating the leaves that came off – the burn had damaged the part that puts out the roots!
When the stem remains intact for succulents affected this way, the plants will eventually grow back. They’ll just look a bit ugly in the process, like my little Lilacina here:
This type of burn is much less common on cacti, although it is definitely still possible. It is most likely to occur on cacti with small, dense spines that hold the water up and away from their skin. It’s also mostly likely on the delicate top or apical meristem, where the growth is tender and new. Smaller cacti, those that grow in a slightly convex habit (many of the notocactus, for example), and again, those with really dense spines are most at risk.
For both the cacti at risk, and potted succulents, the way to prevent this issue if you notice water pooled in the center of the plant is to pick up the pot and just tilt the plant so the water rolls off. In the ground, this is obviously not so easy, and for your high value plants you may need to be that crazy person outside with a paper towel, dabbing excess water off of your prized plants!