Humidity: the factor that defeats most tropicals in a flat
Misting achieves almost nothing. What actually raises humidity around an understorey plant, from grouping plants together to a cabinet, and which species will not do without it.
Air humidity is the factor that fails the most tropicals in a flat, and it is also the last one anybody thinks about. Understorey Begonias, Sonerila and Alocasia come from places where the air sits steadily between 70 and 85 per cent. In an Italian house in winter, with the radiators on, we are around 30 to 40. It is like asking a freshwater fish to live in brackish water: some tough species get by, but velvet leaves and iridescent veins — exactly the plants worth having — curl, dry at the edges and stop opening new leaves.
First, let us kill a myth: misting does almost nothing. The spray lifts the humidity for a few minutes, then everything goes back as it was, and meanwhile you have wetted the leaves — which on Begonias and other hairy species is an invitation to spotting and fungus. If misting had a real effect, it would have to be done every twenty minutes all day. Let it go.
What actually raises humidity, in order of increasing effect. Grouping the plants: stood close together, transpiring, they build a damper microclimate between them. It is free and it helps. A tray of wet expanded clay under the pots (without the pot touching the water) adds a little local evaporation. A real humidifier, one of the ultrasonic ones, lifts the humidity of a whole room and holds it there: for anyone serious about tropicals it is the answer, it costs little and it works while you are not thinking about it.
But the definitive answer, for the fussiest species, is to shut the air into a small space: a cabinet, a display case, even just a clear container with a lid. Inside, the humidity the plant itself transpires stays trapped and settles high without any effort. It is why the rarer Sonerila and Begonias, in a terrarium, become a different plant from the one that merely gets by in the open.
Watch out for one thing, though: high humidity plus still air equals mould. A closed environment needs a minimum of air exchange, even just opening the cabinet for a few minutes a day, or a small fan. Damp yes, stagnant no.
Which species will not do without it, and which forgive
The commoner aroids (Philodendron, many adult Alocasia) tolerate household air if everything else is right. Rhizomatous Begonias do better with high humidity but live at 50-55% too. Sonerila, Bertolonia, thin-leaved Begonias and the delicate variegated ones genuinely want the cabinet: outside they survive, inside they thrive. On each plant's care sheet the percentage I give is the threshold below which that species starts to suffer: it is the number your environment has to be able to hold.