terrible heat!!

[sundanz]

Here in east Texas the heat is getting opressive!! 100 degrees now and up to 104 next week. Humidity up and down. I have been watering all my cacti and succulents every Saturday morning while the temps are still in the 70's, before it gets hot. So far they are doing good but the heat will continue all through august into september, maybe not 100 but close to it. All my plants are in claypots. My question: do I need to water more or less? My plants are in a GH with a fan and shade cloth.
Thanks

Karin

[cactushobbyman]

Those temps are normal for me in the summer. The humidity most of the time is below 15%. In the greenhouse I water every two weeks, but heavy, and some I water weekly. My clay get very hot to the touch. Over all my plants thrive. I do use a fan and the greenhouse is covered.

[jmoney]

I would think that once a week would be ok as long as the soil isn't perpetually wet. I would let soil dry out some between watering. I always tend to water less than risk over watering and having issues with rot.

[K.W.]

Good evening Karin,

I would my plants through spray late in the evening with water.
This cools the plant something and corresponds to the natural temperature differences.
The cacti need the coolness at night the stomata open.
If it is not cool enough, they go into a summer rest. No growth, no flowers.

If the plants do not grow, less watering. Even if it is very hot.
The plants protect themselves in this way the heat.


Best wishes

K.W.

[DaveW]

I have always wondered if cacti really can use transpiration cooling (rather like humans sweating) during the day like ordinary plants as supposedly being CAM plants their stomata are closed and also do not open much at night if nights are really warm. Therefore will extra watering help cool them as the plant may simply swell up rather being able to use the extra water for transpiration cooling, therefore excessive extra watering might simply cause the plant to rot rather than protect the plant from heat.

"Desert plants have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis, in which the stomata are closed during the day and open at night when transpiration will be lower."

The only information I can find on evaporative cooling in cacti is :-

TRANSPIRATION.jpg (80.54 KiB) Viewed 1543 times

Obviously from the above, unlike ordinary plants evaporative cooling does not happen much for cacti since they have adapted to reduce water loss in dry areas, therefore simply providing extra water does not overcome the normal daytime stomata shut down due to higher temperatures and increase evaporative cooling? As KW says spraying the plants to cool them may increase the transpiration rate at night, but will not be much use during the heat of the day when the stomata will remain shut?

http://feedthedatamonster.com/home/2015 ... the-heat-1" onclick="window.open(this.href);return false;

[K.W.]

Dave, I think you're right.
More water makes for CAM plants no sense.
I sprinkle the plants with a fine water mist. Then they will get colder, by evaporative cooling.
If it's not too hot, at night, the stomata open. The plants make normal metabolism, do not fall into a summer rest.
I think the heat in the day does not matter, for this the plants are made.

Oh. my English is so bad. Hopefully I can express myself naturally.


Best wishes

K.W.

[jp29]

Here in southern Arizona we are pretty much in the middle of our summer "Monsoon" thunderstorm season with frequent (but sporadic) heavy rains usually (but not always) in the late afternoon or early evening. In the Tucson area high daytime temperatures are usually in the 100°F (approx. 38°C) range (often much higher) with night time temperatures dropping into the 75°F (approx. 24°C) range. This differential allows CAM to come into play -- the plant stomata are open during the night and they can uptake water and nutrients.

Not so in Phoenix, which is generally much hotter than Tucson -- high daytime temperatures are frequently in the 110°F (approx. 43°C) range and sometimes in the 115°F (approx. 46°C) range, with night time temperatures remaining very high -- temperatures at midnight have been known to be close to 100°F (approx. 38°C) on occasion. The all-time high temperature recorded in Phoenix was 125°F (approx. 52°C) -- in Tucson it was 117°F (approx. 47°C),

At this time of the year Cacti go into a summertime dormancy, particularly in the Phoenix area -- no matter how much you water and feed them they simply sit there awaiting the cooler night time temperatures of autumn.

Typically "monsoon" season commences with "stops and starts" days -- heavy cloud build-up in the early afternoon with thunder and lightning followed by strong gusting winds and blowing dust -- but little rain. Pretty soon that pattern gives way to frequently long lasting downpours replacing the blowing dust. The thunderstorms inevitably dissipate by nightfall. This "monsoon" season usually lasts from early July until early September with thunderstorm days frequently interspersed with several "dry days". We may have a string of thunderstorm days followed by days of sunshine and nary a cloud in sight. Thunderstorm occurances are very erratic throughout the Tucson area. For instance, We can be experiencing a torrential downpour at our house while at Bach's greenhouse - which is only a twenty minute drive away it will be bone dry. That is so unlike where I grew up in Lancashire, northern England.

For me, the "monsoon" season means a change in my watering regimen. The frequent heavy rains (often deluges) together with the lower afternoon and early evening temperatures reduces my watering frequency (all of my plants are under 30% shade cloth) considerably. On "non thunderstorm" evenings I simply mist spray them. An added benefit of thunderstorm days for me is that the heavy rains flush out fertilizer salt build up from the pots.

James

[jp29]

Today here in Tucson at 8:30am we are experiencing an unusual and un-forecast early morning light rain with the temperature at 85°F (approx. 29°C) on my rear patio thermometer. A thunderstorm is forecast for this afternoon.

James

[jp29]

Yesterday's predicted thunderstorm did not materialize but we had overcast skies with occasional rain showers throughout the morning and early afternoon at our location.

Today is starting off with the same atypical weather pattern as yesterday -- at 7:30am at our location we are experiencing light rain with the temperature at 85°F (approx. 29°C) on my rear patio thermometer. The forecast for today is mostly broken cloudy skies with occasional light rain and a possible thunderstorm in the afternoon.

James

[jp29]

Well, we did get that thunderstorm in the afternoon with a vengence. At about 3pm the skies darkened considerably with hevay clouds accompanied by thunder and lightening ..........



.......... the temperature dropped to 75°F (approx. 24°C) on my rear patio thermometer -- then came the deluge. It has been raining very hard for over an hour now but is beginning to let up. My cacti on shelving under 30% shade cloth on my rear patio revelled in the rain -- as did the native desert cacti in my front garden such as Mammillaria tetrancistra.


Brazilians enjoying a soaking during the thunderstorm

James

PS Edit -- 4:30pm -- I spoke too soon, the rain is coming down in torrents again!

[iann]

I'm just starting my Mojave species on some water now. Strange how they hate it in June and want it in August, even though it can be hotter in August.

[Pushrestart]

James, your plants are loving that rain! Im always happy to see/hear monsoon rain posts , reminds me of my time in Az.

[jp29]

We traveled to Phoenix today thereby missing the afternoon thunderstorm downpour at our house. We just got a little rain on our car windshield from the remnants of the storm as we approached Tucson during our return this evening. We have had a LOT of rain here during the past three days.

[jp29]

I'm just starting my Mojave species on some water now. Strange how they hate it in June and want it in August, even though it can be hotter in August.

I lived in the high Mojave desert in the early 1960s and I do seem to recall the summer thunderstorms occurring mostly in late summer.
It was a long time ago, but I still remember how the spines on the huge clumps of Echinocactus polycephalus (which grew in great profusion among the rocks of the foothills between Victorville and Apple Valley) would turn a vivid blood red after a rain storm.

James

[jp29]

James, your plants are loving that rain! Im always happy to see/hear monsoon rain posts , reminds me of my time in Az.

Yes, I do enjoy our summer monsoon season so very much.

James