This is written from the point of view of the author who resides in Australia, but the tips and tricks might be useful here in our Mediterranean climate.
I know that sometimes I get questioned about how they grow and what constitutes a warm growing Masdevallia. I thought I would just explain how I and others grow them on a hot, dry continent. Firstly please understand that climates vary greatly from person to person in this forum so this won't work for everyone but I think with some experimentation maybe more people will be able too in the future.
Firstly I would say that I don't monitor humidity all that closely, usually in winter and at night the humidity is adequate anyway. During summer humidity does drop as low as 10% at times which would be considered way too low for just about any orchids. The key is creating a micro climate.
Temperatures in southern Australia where I live are around 35F to 60F in winter and 60F to 110F in summer.
When I originally started growing them I used bark and I grew them in the open under the shade of a cherry plum tree. Subsequently I lost a lot of leaves due to the roots getting to warm and not receiving enough moisture when they needed it. It has taken a long time to get the culture right - about 5 years or so.
1. I now plant completely in CHC and Coir. I find this to be a great medium, if this isn't available then I recommend sphagnum moss, I use Tasmanian. I don't recommend potting into bark like I used to do as it breaks down too quickly and doesn't retain moisture evenly enough in my opinion.
2. I put the pots into orchid/plant trays that are already lined with plastic. On the plastic I place Perlite or sphagnum moss. When the pots are in the trays I usually wrap the sphagnum around the pots to keep the black plastic pots cool (which keeps the roots cool) and keeps the humidity a little higher. Note I only do this in summer as they don't need cooling and extra humidity in winter. I grow outside 12 months of the year.
3. I mist regularly during the warmer months and make sure I water so that they never become dry, this takes time and practice to get used to the conditions and the speed at which the pots dry out. You get "a feel" for it.
4. If the temps get to 35C (95F) then the plants are carried into the house on trays into a cool room where they can sit until the real heat subsides. Then I put them back out in the evenings.
5. I water once a week in winter and at least every second day in summer depending on the temps at the time.
I don't agree that Coccinea, Veitchiana and others cannot grow warmer. I have been growing a coccinea for 4 years in temps that reached as much at 100F in a glass house and the plant bloomed better in there than it ever did in a shade house or other structure. This coccinea is beautiful and leafy with two strong spikes growing at the moment.......we shall see how it turns out and I will post pictures. It hasn't flowered before so this is an exciting year for the plant.
The key is keeping the roots cool and providing plenty of air movement, some people I know actually hang their Masdevallia's in trees over summer so that they get a constant strong breeze that keeps them cool whilst also allowing dappled shade.
Another person I know uses perlite alone on the benches and keeps the perlite around the pots moist so they get a cooling effect and humidity whilst also getting beautiful air movement. I asked him today how hot it gets........I won't use the expletive he used to describe it but let's just say it gets very hot......sometimes into the high 30's (Celcius)........that's 90F or more.
All my plants grow fairly warm and probably closer to ideal temps in winter. Obviously it's not a great idea to stress a plant in the warmer temps but I must be doing something right as the plants are flowering regularly now, they no longer lose any leaves during hot periods and they are as healthy as you could expect them to look.
In closing I rang a nursery selling Masdevallia's today and they said that temps above 86F (30C) aren't good for the plants BUT with the right culture they can survive quite well at higher temps. The key is air movement and keeping the root system cooler. I asked how my Veitchiana and Coccinea's are able to do well at higher temps and their answer was short........"they are tough plants and can take it".
Also worth noting is that my Pleuro Roezelli and Dryadella Zebrina along with Chinese paphs all grow side by side with my Masdevallia's in the same conditions.
So that's it......fairly simple and maybe a few more of you can work something out and start growing them. This post doesn't have all the answers and certainly they won't always grow well for you and in fact it may be impossible for you to grow depending on where you live but maybe for some of you they are worth a go. What do you have to lose?
Thanks to Karen O., for sharing this so that others can read about taking care of Masdevallia's