A humidity tray is a simple device to increase the humidity around orchids or other plants grown indoors. The basic idea is to position plants above a tray filled with water, and let evaporation do the rest. Most types of orchids prefer 50-70% humidity. Some, such as Masdevallia, need much more than that. Low humidity also means you'll have more trouble with spider mites and other orchid pests.
The simplest way to set one up is to take a tray, fill it with aabout 2 inches (5cm) of gravel or coarse sand, and set your orchids on top. Add water to just below the level of the gravel so the plants aren't sitting in it. (Most orchids hate to sit in water. The exceptions are plants that live next to streams such as Disa and Phragmipedium.) Evaporation from the water surface will help add additional humidity for the orchids!
There are also commercially-made models you can buy, which usually have a plastic mesh on the top of the tray to support the plants. They work well, but are often overpriced.
If the tray and gravel that you use are dark in color, the sunlight coming from the window will be absorbed more quickly, warming the water during the day and increasing evaporation. This can be very helpful.
Unfortunately, typical home environments are quite dry, so you might not be able to raise the humidity as high as your orchids might like. But every little bit of humidity you can provide will reduce stress on your plants. Misting the plants regularly with a spray bottle can help, too. Often, the outdoors is more humid than the environment inside your home, so opening the windows can be very beneficial your orchids, as can growing your orchids outdoors when temperatures are suitable. (Make sure not to put them in direct sunlight, as most orchids will get sunburn.)
Orchids like good air circulation, but fans will move your humidity into the rest of the house, away from the plants. Personally, I tend to think humidity is more important than air movement when caring for orchids. But bacteria and fungi disagree with me, unfortunately. So monitor closely for orchid diseases if air circulation is low.
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