Sempervivum and temperature
Sempervivum can survive in a broad range of temperatures, they are cold hardy to USDA Zone 4, which is an average minimum temparature of -30F, and some have successfully kept Sempervivum in Zone 3 which is 10 degrees colder! Experiments show they can tolerate short term temperature bursts all the way to 143F and still survive. In one experiment Semps were kept at 122F and it took 6 days before the plant was damaged.
Survival is one thing, but as gardeners, we want to know what makes them thrive! As alpine plants they have evolved to deal with little water and wide temperature swings from day to night, in fact the best conditions are warm days and cool nights. The metabolism of Semps affects how they can handle temperature. Semps in the summer growing period were damaged by a drop to 23F, but if the Semps were kept at 32F for 3 days, they werent damaged until the temp dropped to 14F. If the period they were kept at 32F was extended longer, they could withstand temperatures colder than -13F with no harm. This means that if the Semps metabolism is slowed down by cold, they are able to withstand extreme cold with no harm.
An extensive study was done to see how temperature affected photosynthesis in Sempervivum. Remember that if they arent processing CO2, then they are not growing, or even dormant. Semps show CO2 uptake in the light, in temperatures from 28F to 113F. They show CAM fixation of CO2 into malic acid in temperatures from 28F to 95F, although at night temperatures above 68F, they quit taking in CO2 and instead only convert CO2 from cell respiration into malic acid. High night temperatures seem to slow growth considerably. Even when well watered, Sempervivum still use CAM photosynthesis in most conditions, to availability of water does not change the type of photosynthesis they use. Temperature can make them change the mode of photosynthesis they use. At daytime temperatures colder than 50F, or nighttime temperatures warmer than 95F, Semps to act like C3 plants. At 50F or below this is not usually a problem, but in a heatwave, if they dont get cool nights, and go into C3 mode full time, they will probably experience great waterloss.
So what does all this data mean? One thing to keep in mind is that Semps in direct sun can be 15F warmer than outside air temperatures, so keep that in mind for the daytime figures.
Summarizing all that scientific mumbo jumbo:
-30F to 27F Sempervivum not processing CO2, no growth, if temperature stays below this, dormancy. Not much need for water, but snowcover will help stop water loss from cold winds.
27F to 50F(daytime) Sempervivum using C3 photosynthesis but growth is slow, meaning they use some water and take in CO2 during the light instead of at night.
50F to 113F(daytime) Sempervivum in ideal growth mode using CAM photosynthesis as long as nights are cooler than daytimes and are 28F to 68F. If nights are warmer than 68, growth slows down because not as much CO2 is fixed at night, and it may also allow an early start to phase 4 CAM causing greater water loss in the late afternoon. Too much sun in temperatures warmer than 85F may cause the plant to use up its stored CO2 too quickly and start phase 4 CAM earlier in the afternoon as well. This is why Semps seem to melt in high heat and high light. More watering is needed to compensate for this water loss. If nighttime temperatures stay above 95F, the sempervivum will go into C3 photosynthesis and at such high temperatures, lose even more water and probably die off.
113F to 122F Sempervivum cant process CO2 at these temperatures and will go dormant.
122+ More than 5 days at these temperatures causes permanent damage/death.
It seems that Sempervivum definitely do best spring/early summer, and fall, even winter plants for those of us who dont get snow up to their eyeballs, and that the most trying time are hot summers, especially if the nights dont cool.
