Minimum Winter Temperatures - References

[Iain]

With temperatures dropping quite rapidly I've already suffered 2
casualities; a young but much prized Larryleachia cactiformis (victim
of black rot) and a Lithops julii (a bit odd, all my other Lithops fine):(

So does anyone know of a comprehensive species-by-species reference
of winter minimum temperatures for plants in cultivation?
I have tried 'Desert-Tropicals' but would ideally like a second source(s)
to verify.
Some parts of my house are warmer than others so I can use the
information to do some re-locating.
Oh, and my Pachypodium horombense has two stems looking suspiciously
wrinkly and flacid, any recouperative advice welcome. It's lost most of
its leaves so I'm not keen on watering it but if these weak stems get
weaker is it ok to prune? - if it's rot I'd hate to lose anything else.

[Geoff Lovell]

Ian...horombense is a Madagascan one and requires heat, I evacuate mine to the conservatory as with all my 'tropical' plants.
I usually keep mine in leaf but it will only do that with good heat!.
Prune when its growing and only then unless you want infection to set in.

[Iain]

Thanks for the advice Geoff. On further inspection it's certain
that the plant's a goner. All stems are soft lower down unfortunately.
I think I maybe had too much organic material in the soil mix,
especially around the roots. Next time I'll clean the root system and
pot in mostly pumice.
It was warm enough, just too damp. Plus I probably used too larger a
pot that would have retained moisture for longer.
I've removed two stem tips in the vain hope I'll get them to root,
but not holding my breath!
Just one question Geoff, if you presumably keep your Pachypodiums in
leaf by continued watering, how do you minimise the inherent risk of
winter rot? Heat alone? How warm? Thanks again, I'm learning the
hard way!

[iann]

Mesa Garden tags many of its seeds with hardiness codes. Not always accurate but its another source of information.

No reference is ever going to be perfect, for some very good reasons and some very bad ones. The Desert Tropicals site shows one of the bad reasons: it quotes both minimum survivable temperatures and recommended winter average temperatures (also a USDA zone which is perhaps not so helpful in our very different climate) See Echinocactus grusonii as an example. It is shown as able to survive -10C but ideally should be kept around 12C for much of the time in winter. Many references will quote a temperature like 12C as the "cold hardiness" with no further explanation, which is clearly misleading for a cactus which can survive a night that would turn you into a lump of ice.

It was discussed on another thread that most books provide ridiculously high temperature figures for the hardiness of cacti, possibly to be cautious, possibly just repeating commonly heard information. Either way, I find myself highly sceptical about all information in those books when they can't get some very simple and relatively objective information right.

Your Lithops didn't die from cold unless it was very thoroughly frozen. L. julii, like all Lithops, will certainly survive below freezing. I will suggest it is reasonably safe down to -5C for brief periods. On the other hand, Lithops don't seem to need such cold over the winter, so most of mine are stored in an unheated room inside the house where they are safer.

[iann]

You can always use The Edgintons sales list Anything on there will have been grown in an unheated greenhouse in Norfolk, unless otherwise noted.