We are having a mini heatwave here in the uk 😱
Many of my plants are due a watering . I noticed tomorrow's night time temps are 71 f
I am sure thijs and others have said that water can be harmful if the night temps are too high . Shall I water or just wait till after this heatwave
. I am tempted to do the latter but I am interested in what those of you in hotter climates think . Am I just worrying over nothing 🤔
Steve

That sounds cool to me Steve . My overnight temps have stayed around 80F beginning in May with brief reprieves down into the 70sF. I think you could water, but some of that depends on the plants. There are plants that HAVE to get the water regardless of the temperature. My problem when watering during long heat spells with warm overnight temps have always been with succulents. The crassulas, echeverias and sedums have been the ones that have reacted negatively - and very quickly. Apparently not everyone has had that happen and I frequently see advice saying to be sure and give water during the heat of summer. My bad experiences have made me cautious with all the plants that store water, but not with other types of plants. I know you grow a lot of aloes and my experience with those is they tolerate going without water better than almost any other plant, including many of the cacti. In their case, I'm sure you'd be fine to skip a watering or two if you want to play safe. Too much water and freezing temps are the only way I can recall where an aloe got killed. They tend to be really durable plants once established. A newly planted speciman of one is the most likely to be water sensitive in combination with the heat. IMO, of course. There's plenty of alternative ideas on it floating around. It was 82F before the sun was up and I have to go out and water some plants before we hit those triple digit temps this afternoon, but due to a small shower mid-week I'll likely skip the water storing plants. I saw where it may reach 108F today. Not good.

Yeah low 70s should be fine. I think we are talking mid 80s before things can get gnarly. Right now we are barely getting into the 80s at night last night, it was 100F at 10.30 PM... with a low of like 91...

Thanks both
Thought it would be ok but wanted to check . I think I will leave it now until weds night . Nothing is desparate
Steve

Any logic that we should not water if night temps are too high?
My simple logic is that when the temps are high, the soil dry fast and need immediate watering. Maybe this is why I failed in leaf succulents and jade plants. I think I will never have any below 70 on my rooftop even if it rains.

I am a bit puzzled reading this thread.
Only if the temperature is still chilly as it happens in early Spring when I just had moved everything outside, do I watch the timing and amount of water. Wet and chilly is not a good combo for many succulents, like Stapeliads as example. Once temps are warming up, everything dries quickly around here and needs to be watered, either faith -based or by me! Summer temps in NJ are eighties and more or less nineties F.

Ursula said: I am a bit puzzled reading this thread.
Only if the temperature is still chilly as it happens in early Spring when I just had moved everything outside, do I watch the timing and amount of water. Wet and chilly is not a good combo for many succulents, like Stapeliads as example. Once temps are warming up, everything dries quickly around here and needs to be watered, either faith -based or by me! Summer temps in NJ are eighties and more or less nineties F.

I can understand that! It puzzles me as to why, I just know it can happen via my own experience. The one I remember with the most regret was a large Crassula ovata which looked great after growing here for more than 20 years. It would show stress in the heat and I would water and it would perk up. Then we had a spell where the nighttime temps were warm to hot and the day temps were reaching 112-115F - really hot for this area, or used to be. The jade was stressed from the heat, the leaves all crinkled. I watered as the sun was setting as is often my habit here since the night temps are still cooler than those in the day. The next morning the plant had simply collapsed into a blackish muck - like sewer debris. Subsequent to that experience, it's occurred with other plants. It's a quick death, at least. No time time is spent trying to salvage them. But it's really difficult not to give clearly stressed container plants water when you know they are dehydrated and that means from time to time I do give them some and occasionally lose the gamble. My best resort is to move plants into shade and try to set them up on damp earth and just try to mitigate the heat. I bought some little cheap misters that screw onto the end of a water hose and hang under the oak tree where I have a lot of hanging baskets including some gesneriads which don't love excessive heat. They will lower the temps in their immediate vicinity 5-7F. That's significant when you are dealing with triple digit temperatures. I can't identify the line where you are better off skipping the water. Probably it varies from plant to plant. Other people live in hot climates and their advice is usually to keep plants watered and that is certainly true of most plants, but not always those that efficiently keep a store of water in their leaves and stems.

Donald, I learned something new here! I would never have thought!!

Yes agree.. Thanks a lot for sharing this.
Logically seems strange, but experience had proven that. When Donald mentioned "blackish muck" remind me of how my leafy succulents died, I will say more than 50% of them died that way. I always can not understand why that happened, 2 days earlier they looked ok and healthy. Now we get the answer.
In fact now I have 2 crasullas that look so thirsty, I will follow your recommendation, instead of watering them, I moved them to a cooler place and wait for a day or two before I give them some drink. Thanks again for the info.

@Kaktus
Good luck, TJOE. Beware though. I don't give advice, just recount first hand experience and observation. It's been obvious to me for many years that good advice from successful folk doesn't always work out for me. I believe micro climates are real. My yard is large, sloping, with variable soil and there are spots where different plants perform and react to that variation - sometimes to their benefit and sometimes lethal.

Its alright Donald @needrain, I have been struggling to keep leaf succulents for years, still try and error , any information will give me another option or hope. This is the first time I get explanation of what may have caused them to turn blackish mud.

You know, maybe that's what happened some years ago to a Phalaenopsis of mine. I move everything outside in the Spring, my Orchids go into a 30 foot cage in our backyard. ( Safe from Squirrels) This plant was hanging high up in the ceiling, somewhat shaded. One day it was perfectly fine and healthy, the next morning it was a smelly black mess. I never understood what happened, I figured one of the numerous Squirrels did its business up on the cage.
Perhaps this one got cooked too??

The reason why it may not be good to water many succulents and cacti when it stays hot at night - and by hot we are talking night time lows ~85F or higher is that most of the plants we are talking about have CAM respiration, which effectively means they breathe at night. However, at high temperatures they cannot reverse their circulation to breathe.
Having water around the roots is bad in that case for two reasons: it might still be taken up, without a way for the plant to actually deal with it, this can lead to quite sudden cellular deterioration and the plant turning to mush. In the slightly longer term the plants will be sitting in wet soil when in a state of what can be called dormancy just making it an inviting environment for rot to set in.

Every year here, when we start hitting summer, it is always interesting to see how the big box store garden departments deal with any succulent displays they may have sitting in their outside areas. Those that keep watering them as normal usually have trays of smelly black mush, whereas those that either stop watering them or bring them inside still have some decent looking succulents to sell.
It would appear that the night time low Ts above which these issues start to happen are not the same for all plants or at least depend in some part on the local conditions for the plant. It appears to be less of an issue with in the ground plants (soil probably stays cooler), but it is not unheard of for it to affect in the ground plants.

Here is a picture of an aloe, that suffered a sudden overnight deterioration. Wet soil unexpected few nights of >83-85F. The brown one was all gooey... I think the little ones may survive - they are sitting in my shade structure, I hope they make it through the summer.

Thanks Thijs, with the situation where we only concern on the temps at night, will it be ok if I water them early in the morning? Not until the media totally wet, just give them something to drink and some moisture, I believe the media will dry again in the afternoon.
Now I started to think that I may have been saved by my ignorance on the way I water my succulents, I always water them at night, but I am not really a fan of water until the media in complete saturation, 90% of the time there were no water dripping out from the drain holes. As far as I can remember through the years of my succulents live, I have less than 5 times where I gave them full drink until the water dripping out from the bottom. Only once that I pour them with really a lot of water, the amount of water equal to the volume of the pot, which is more or less 6 times of my normal water volume. Except for the leaf succulents, they had rooughly one tenth of the water volume.
Btw, it is 2.45 am here, and the outside temperature is 82 to 83 F. More or less at the border line...
And sorry Steve, I hijacked your thread.

There is some discussion about temperature tolerance in Park Nobel's book on the environmental biology of agaves and cacti.

The nighttime respiration of CAM plants relies on their stomata opening at night, which is when they fix their CO2, and this process is temperature dependent. The optimum temperature for these plants to breathe is about 20°C (68°F), assuming they are spending their days in greater warmth. Above about 25°C this process becomes dramatically less efficient for agaves.

So that is one effect of nighttime temperatures on plant growth. It does not necessarily explain the crisis "meltdown" response but rather a breakdown in normal metabolic activity, or a loss of efficiency in the tasks of everyday growing and energy capture. Which may sensitize plants to meltdown if they are subject to additional environmental stress related to water.

Temperature has all kinds of crazy effects on plant cells other than stomata, but one thing that stands out is that sensitivity to high temperature can be reduced by chronic exposure. So just like with cold, the harshest temperatures are the ones that are the furthest away (quantitatively) from the running average a plant may be experiencing. A baseline of moderate heat results in plants that are more tolerant of extreme heat, if that makes sense.

Another interesting thing he brings up is how overwatering (or presumably more frequent water delivered by mother nature) can override the usual switch to nighttime (CAM) respiration. In other words, a plant which would otherwise breathe at night (the example given is Agave deserti) may switch to normal (C3) metabolism and daytime respiration if it is given lots of water, regardless of other conditions. This may not be healthy for the plant, but it is a predictable and measurable response to excess water in the soil.

Higher temperatures will promote nighttime respiration, but even plants that mostly follow the CAM pattern may switch over to a daytime mode given very cool temperatures (like what we experience here). The simplest case of plants being C3 or CAM (but not both) is typically not true for many of the succulents that have been studied, which can be coaxed one way or the other, depending on conditions. I suspect that many or most of the plants which are fully CAM at Thijs' place right now would be partly or mostly C3 on our patio here given the mild summer temps.

Finally, just a word to the wise, the point of greatest vulnerability with respect to heat tolerance is usually the roots, when a plant is growing in a pot and not in the ground. Especially if the sun hits that pot and warms it above the ambient temperature. To the extent you can shield your pots (the actual containers) from direct sun for some or all of the day, you may be able to coax greater heat tolerance out of your succulents. Or to put it another way, even in our mild climate, I can put plants in a position where they get uncomfortably warm and start complaining loudly, because I left their pot in the sun and it turned into a casserole.

This reminds me of what i
I read some time ago
https://easysucculentcare.com/...

Just an FYI: Baja said it all! I live in Phoenix AZ where the temps for the last 15 days have averaged 110º during the day and 92º overnight. It don't water until the temps drop below 85º overnight. What I do is mist the plants at around 6 PM to help lower the temps at least for awhile at night. It does help them at least some and makes me feel better at least! The temps you have described in England at night should not be a problem to water. Just let the soil dry out before watering again.

nmumpton said: Just an FYI: Baja said it all! I live in Phoenix AZ where the temps for the last 15 days have averaged 110º during the day and 92º overnight. It don't water until the temps drop below 85º overnight. What I do is mist the plants at around 6 PM to help lower the temps at least for awhile at night. It does help them at least some and makes me feel better at least! The temps you have described in England at night should not be a problem to water. Just let the soil dry out before watering again.

Thanks Nancy
I do not think we would hit overnight Highs of 85 in rhe UK
We just had a record heatwave of with a peak temp of 104.5 f in rhe day so overnight we always dropped below 85 . We are not set up for those temps here in the Uk. We don't have air con in our houses I would not like the temp to get that hot every year . What was unusual was the whole uk for hot all at once
Steve

Well, I took Baja Costera's advice and bought a Dudleya Brittonii back in April. It is doing extremely well so far! I keep it in my greenhouse now because I just don't want to take a chance it will get rained on. If I see that there will be several days without rain I will take it out and place it where it gets limited sunshine. Where I live (USDA Zone 5b) we have very humid summers. I water sparingly along the sides of the pot once a month, but only if the moisture meter says "dry". I do have one question, though...the leaves are more parallel to the pot as opposed to upright. Is this normal? Or is it asking for more direct sun?

Is there any way you can share a photo? I'm curious to see what you're talking about.