Maximum temperature?

[stanislas]

I'm building a small greenhouse for Edithcolea grandis plants. As it's little, it can quickly change temperature in the sun. I therefore include a ventilation system that starts venting once a certain treshold is reached. But I wonder what is an acceptable maximum temperature in a greenhouse for hardy, heat loving succulents? And how do you measure the temperature (air temperature?).

Thankyou!

[CoronaCactus]

Our small 8x16 greenhouse is usually on the hot side, 85-110F. Thats with all vents open and a fan going for 10 hrs per day and the whole greenhouse is covered in 30% shadecloth.

There are universal automatic vent openers that have adjustments from 60-80F.

The fan is important, as even if the air is hot, having it circulating will reduce any chance of scorching. I have the fan on just a regular timer from 8am to 6pm. I know that here in California it's going to be hot, so no need to have any fancy temp timer thingy

Interestingly enough, you mention Edithcolea grandis. I just noticed that 2 buds became very shriveled and will probably abort. The temp reached 122F. The day before, they looked great. Other buds on the plant where not affected. I'm just guessing the reason was the heat...? I bumped the fan speed up to max, but I think it's time to remove some panels for extra venting!

I guess to answer your question directly, i'd shoot for a nice comfortable 85F during the day.

[Peterthecactusguy]

Stan,
good luck growing your cacti!
Darryl. Interesting. I would think that a greenhouse in AZ would get hotter tho.. hahaha It's been 117 at my house(in the shade). I think the outside temp in the sun was around 120ish.

I suppose where you live would account for different settings for greenhouses.

BTW from living in Arizona and not having A/C in my car that works I can attest to having the vents on beats opening the windows.

[iann]

The maximum survivable temperature will depend on the plants obviously, but also on how much shading you have. No shading and you'll probably struggle with anything above 40C. Even below that and with ventilation you could get scorching on very clear days. You can even get scorching on plants completely in the open with air temperatures of below 30C. With a good amount of shading then most really xeric plants will be able to stand 50C or more although I don't recommend you let it get that hot.

There are certainly plants that can take both the high temperatures and the full strength of the Belgian sun through glass, but you'll struggle to adapt them to those conditions that only occur on a few days a year.

I don't know exactly what your greenhouse is like, but do watch out for those plants right next to the glass on the south and west sides. They will be stuck in trapped heat and with less ventilation and they will be the first to scorch. Any on high shelves are also at right but people usually use them for really tough plants. Softies tend to go under the staging but that can still mean full on sun next to the glass in late afternoon.

I keep my main thermometer under a wooden bench where the sun can never strike it directly. If the sun shines on your thermometer then it is useless. Another suggestion is to place it right in front of a fan if you run one continuously. Although it may be in the sun, the strong air flow will keep the thermometer near the true air temperature. When the sun is shining, the plants will get a lot hotter than the air. I've been using an infrared thermometer to measure them this year. Even on quite cool days, the plants and pots can reach 60C in continuous strong sun. With just a little haze or some fluffy clouds, they stay cooler even if the air is warmer.

I aim to keep the air temperature well below 40C on days with hazy or occasional sun, but occasionally it might get hotter. By about August, the plants are well adapted and the sun is a little weaker, they can take some surprising heat. On completely clear days, especially in spring when the plants are not adapted, I try to keep it as cool as possible and also run a fan. This can mean removing some of the glazing to keep the air temperature below 35C.

[Loph]

ours has no fan and little shaded area, can get up to 60C tops, but tend to stay lower than 40C most days. nights are 20s C.

[Peterthecactusguy]

Loph, that sounds hot just thinking about it!

And btw I might mention that I didn't have that bad a guess. I thought that 60c was about 135f. I was close!
(btw) since I am the lousy American I do not remember how to factor Celsius to Fahrenheit. I should have paid attention in school, I suppose. However there are handy websites that do it for you hahahaha, plus I can guess and am pretty close most of the time!