A "humidity box" is a useful method for houseplant hobbyists to rehabilitate stressed plants and propagate cuttings. It is not intended to be a long-term solution for humidity-loving plants, but instead a helpful step toward healthy, happy, acclimated plants that are adjusted to your home. This method is not appropriate for succulents and cacti.
I have an entire wall/shelf dedicated to propagating my hoya, rhipsalis, and aroid collection in my home!
Humidity boxes 101
Plants expend energy trying to reduce water loss through transpiration, so providing a high-humidity environment allows a plant to direct it's energy elsewhere, such as root development or foliage growth. Humidity boxes aim to provide as close to 100% relative humidity as possible in a closed-air chamber, with adequate light and heat to encourage healthy development.
Humidity boxes checklist and considerations
A transparent, sealing container: plastic tote, sealing jar, aquarium with lid, etc.
Light source: T5 or LED equivalent artificial light or sunlight 12+ hours daily; the distance between artificial lights and the humidity box depends on the intensity of the lights, but generally somewhere between 6 and 24 inches.
Container temperature: between 60 and 85 degrees (F); any cooler and plants are not likely to grow; any warmer and plants are at risk of overheating and may die
Optional heat mat: you can also use a seedling hat mat underneath the container to stimulate root growth and to keep the box warm
Optional outlet timer: for your artificial lighting (to automate)
Maintenance
Keep the lid on your humidity boxes 24/7 unless you are purposefully reducing humidity or it is extremely warm. Plants inside one of these boxes are recycling their own air, so there isn't much reason to open the boxes to fan or vent if moisture levels are sufficient. You will need to open the boxes and check moisture level in the pots/potting medium, inspect for pests, and mist if the humidity levels dropped. In the winter I do this once a week. In the summer (with the heat and very active growth) I am checking the boxes every other day.
Try not to let water sit on the leaves in the boxes- without sufficient airflow stagnant water on leaves can invite a host of problems including bacterial and fungal growth.
Keep an eye out for algae growth and clean the boxes as needed. Every couple of months I'll scrub and bleach out the boxes as I am putting new plants in them.
How long do plants remain in a humidity box
This really depends on what you are doing with the humidity box.
Propagated plants
I place cuttings in damp sphagnum moss inside of a humidity box until they are rooted enough for soil. I then put the potted plants back inside a humidity box for a couple of weeks to ensure they are adjusting to the new potting medium happily, and I'll begin to reduce humidity slowly (as described above). Eventually all of my propagated plants are adjusted to my home, which decreases transition stress when I gift them or sell them to someone locally.
Rehabilitation
Plants should stay in a humidity box until you see substantial healthy roots and new growth. I've made the mistake of taking plants out too soon, eager to get them up on the living room shelf, and the plant quickly suffers when it is removed from the humidity box. So now I practice patience and allow the plant to regain it's strength and begin to grow before I decide to begin to slowly reduce the humidity and get it ready for my regularly home environment.
You'll want to reduce the humidity slowly, when the plant is ready to come out of the box. The plant needs to slowly adjust to expending energy on water retention again, so a quick transition from high humidity to typical indoor humidity will drastically shock the plant.
Lid on (100% relative humidity)
Lid ajar (90-95% relative humidity)
Lid off but sides intact and regular misting (70%)- if a plant doesn't do well in this stage I go back to high humidity
Plant out of the box but closely watched and placed in the stream of a cool mist humidifier (50-70% humidity)
Regular home humidity with ambient humid zones for specific plants (40-70% humidity)
My humidity boxes set-up
I have three shelves along a wall in my house dedicated to rehabbing and propagating plants. I know it ain't pretty, but it works very well and after plants have been rooted they generally go outside into the greenhouse. There are four linking workshop lights on a timer for 14 hours every day. Every morning when I'm brushing my teeth I walk through and check out the boxes, but really I don't need to do this- I just like to!
Transparent totes with transparent lids from IKEA in various sizes
Light option #1: GE Balanced Spectrum plant growth bulb 24" away from the box in a desk lamp
Light option #2: GE Full Spectrum 9W plant growth bulb 6" away from the box in a desk lamp
Light option #3: T5 equivalent workshop light similar to this one (but I purchased mine from Costco on sale)
Seedling mat to keep specific "rooting tubs" warm
Surge protector with timer to automate lights turning on and off
Final thoughts
Propagating plants is always an experiment for the houseplant hobbyist. Some plants do totally fine propagated in a jar of water (e.g., pothos, philodendron hederaceum, peperomia), but the humidity box method can be extremely fast and has greater success rates from my experience. It may take some troubleshooting and trial/error to get the humidity box to work for you. It's sometimes helpful to ask other hobbyists which propagation methods have worked the best for them with a specific species of plant.
Common mistakes I've observed from reading countless propagation failures in a humidity box:
The moss is too wet/soggy/dripping
The jar of LECA is full of water to the brim of a jar
No drainage in the propagation pot/cup
Low humidity
Not letting a cutting callous before placing in moss or LECA
Letting the substrate dry out completely
Humidity boxes get too hot and the plants "cook"
Not treating for pests, even if visible pests aren't present- if you see pest damage automatically treat for them!
Moving to soil too soon
Moving to inappropriate potting medium after propagation
Moving to low humidity too soon or too quickly
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