Do I Need a Humidity Tray for my Bonsai?

If you’ve seen pictures of large powerful or graceful, amazing bonsai trees, many of them are pictured in stunning bonsai pots, with a humidity tray, and then placed on a beautiful stand. It’s easy to conclude that your bonsai thus also needs a humidity tray for optimal health.

Humidity is the amount of moisture in the air.

A humidity tray is a shallow dish, frequently filled with rocks or pebbles that the bonsai sits on top of.

Water in the tray shouldn’t touch the bonsai pot. The point is not to have the clay pot touching the bottom water; thereby allowing moisture to bleed back into the pot. Rather the point is to have water from the underlying humidity tray evaporate upward. This slightly increases the level of humidity around the bonsai itself.

 

Target Humidity Levels: The Problem

Most plants do best at a relative humidity somewhere around 70-80%.

Modern homes and other indoor spaces generally have significantly lower levels of humidity than that. The United States Environmental Protection Agency actually defines indoor air quality by using a few factors, one of which is having less than 50% relative humidity.

Even if lower humidity levels weren’t required, higher indoor humidity levels are undesirable because they breed mold and disease. Some indicators that humidity is getting too high indoors are fogging windows or if things have been out of hand too long, a musty smell. Humidity levels somewhere around 35% – 45% are closer to optimal for human comfort and protection of possessions.

Those ballpark numbers (70-80% vs. 35-45%) should give you a good idea of one challenge to growing thriving houseplants.

 

Why Plants Need So Much Humidity

Plants need humidity because they loose most of their moisture through the pores in their leaves. The drier it is, the faster the plant looses its moisture. If the relative humidity in the environment is too dry, plants can loose water through their leaves faster than they can take it in from their roots.

Plants in low humidity may yellow and drop their leaves, or leaf tips will brown, buds and flowers drop, leaves shrivel and wilt or edges curl in. (You may have seen plants in large office buildings, with long leaves that are brown on the sides or tips. Sometimes someone will cut off the brown with scissors. These are prime examples of a plant kept in too dry an environment.

The thicker the leaves the more hearty and exempt they are from this rule. Thick leathery or hairy leaves are better protected from loosing their moisture. (Think desert landscapes. Cacti and succulents have evolved to be better able to protect themselves from moisture loss in dry climates).

Misting your bonsai is absolutely an option, but beware problems work the other way as well. Misting by nature does not improve the health of the plant. Water sitting on leaves can be a home for fungus. When humidity levels are too high they are more likely to rot, mold, and mildew.

Generally speaking, the lower the light requirement for a plant, the higher the humidity requirement. A good example there is to consider mosses, as well as desert cacti and succulents. Once established, they can go for surprisingly long periods without water if the humidity levels are high.

 

What That Means for Your Bonsai

  • Use of a humidity tray can improve the health of the tree, but as with any tool it does so if it’s helping with an existing problem. The inherent use of a tray does not improve the health of the bonsai.
  • Humidity tray use is especially popular indoors because indoor environments tend to be especially dry.
  • Humidity tray use is also popular indoors because humidity trays also serve as drip trays. When watering until water comes out the bottom holes of the pot, a humidity tray catches any drips, allowing the home owner to water the bonsai in place rather than moving it to a sink or kitchen to water and then return to it’s place when it’s done dripping.
  • If your home is really dry, such as if you live in Arizona, or have the heater running constantly in the winter, then you may want to use a humidity tray and also move beyond that to a humidifier.
  • If you live or keep bonsai somewhere really dry, consider species that will do well in that environment, like jade.
  • Air picks up water as it moves. If your bonsai is somewhere dry, make sure to keep it out of drafts inside, and protect from the brunt of winds outside.
  • If you still need humidity beyond what a humidity tray will provide indoors, consider grouping plants together to form a micro-climate. Plants benefit from moisture that evaporates from each other.
  • Keep houseplants in naturally humid areas of the home (if those areas are sunny enough) such as the bathroom or kitchen. I don’t know how many bonsai enthusiasts I see keeping their precious bonsai collection in their bathroom, but hey, it could be a way to go. Terrariums are another option which work really well, especially with a lid that can be partially left open, partially closed.
  • If it is too wet the opposite is true. If you live somewhere quite humid and hot a humidity tray is undesirable as it would increase the chances of rot, fungus, and disease. For those in humid and hot environments, drain excess water after rains or out of drip trays. Use fans indoors and outdoors position bonsai to take advantage of cross breezes. (For any fellow Floridians, remember that standing water is prime ground for breeding mosquitoes. Do not use humidity trays outdoors in wet humid weather!)

 

Sources:

*University of Vermont Extension Department of Plant and Soil Science

*University of Minnesota Extension