I've brought in most of my orchids as it got hotter in the last two weeks. I'm bringing in most of the rest: Cymbidium seedlings, Ansellia, Dendrobium speciosum. Our nights will exceed 85 F/30C from last night through at least next week. It will be like this until our summer monsoon starts.
Many orchids use a form of metabolism in which they open their pores at night to absorb carbon dioxide. It's called crassulacean acid metabolism because the whole plant family Crassulaceae uses it: Adromischus, Aeonium, Crassula, Dudleya, Echeveria, Sedum, Sempervivum. Tillandsias use this metabolism, too. These are all difficult or impossible to grow outside here in summer. When nights are hot the pores don't open, and the plants suffocate. This is a big reason many orchids tolerate warmer days than you might expect if they can have cool nights.
The only orchid I know that easily tolerates our heat is Eulophia petersii, an African semi-desert terrestrial. But it won't take full sun all day. I haven't tried Catasetums outdoors in our heat, because humidity this time of year is in the 5%-15% range, and spider mites are established in our environment. Den. speciosum might tolerate this, but I'm not testing it this year.
Here is my Eulophia:
The empty pot is 3.5"/9cm diameter at the top, for scale.
You can see the weather forecast for my neighborhood at the link in my signature block.
It's hot in Phoenix
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