summer dormancy and watering

[RichR]

Came across something on the web (I know, there's a lot of bad info out there) to the effect that when summer greenhouse temps (or outside temps for that matter) exceed 110F that cacti go dormant and should not be watered. Is there any truth to this? Doesn't seem to me that cacti should ever be allowed to go dry in that kind of heat.

[peterb]

It depends more on overnight lows. CAM cycles in highly thermoperiodic cacti rely on cooler nights for those stomata to open and gas exchange to occur. If overnight low temps stay up around 80F-90F, many cacti from thermoperiodic habitats just sit there and do indeed go dormant and really will rot and die if watered. I have lost almost all of the fatalities here in Phoenix from summer water for plants like Escobaria missouriensis, Pediocactus, etc. who need cooler nights.

Now, I check the habitat. Tropical or even subtropical cacti grow like weeds with hot nights and will take all the water you throw at them. All Chihuauan, Great Basin and generally montane cacti need to be relatively dry over the summer if the overnight temps are too hot, basically anything over about 70-80F. Sonorn and Mojavean cacti are more forgiving of hot nights and thunderstorms, an adaptation to their habitat conditions.

Sadly, I have found it really is this complicated.

However, if your nights cool off, no worries at all. Everyone will be happy, dormant or not. I have found that cacti watered during summer dormancy are just fine, as long as nights are cool.

just my .02.

peterb

[RichR]

Thanks for your .02, Peter. It could be worth many times that if it saves me a few cacti this summer. Our night temps remain stuck in the 70s from around late May 'til the first cold front in late Sept. We often exceed 80F for lows in August. I guess I had better exercise caution this summer, although the only thing I lost to rot last summer were a few rebutias and a couple of Notocacti (graessneri and haselbergii).

My daytime temps were regularly exceeding 110F in the greenhouse even with lots of air circulation.

[Harriet]

I fight with this all the time. Except for winter, we do not have cool nights here and the nights also stay exceptionally humid. So I'm finding that there are some cacti that just won't do well here at all, no matter what I do. Some boil in the heat and humidity, and some just suffer from the humidity.

Someday I will be able to hone in on what I can grow well and what I cannot and stop torturing the unknowns to death.