Full of bubbles and thin porous walls, expanded clay pebbles drink up water when drenched and release it slowly back into the air, so a tray of them under a pot keeps humidity-loving houseplants from browning at the leaf tips. Drain the excess, though: clay only gives up moisture where it meets air.
| Time to set up | 5 minutes |
|---|---|
| Water fill-up | Once/twice daily |
| Cost | A few cents |
| Effectiveness | High |
A layer of expanded clay balls kept moist under your plant will work wonders. It will keep plants from getting brown tips on leaves!
Set a tray filled with wet clay balls near or under your plant pots.
You can rest all your pots atop such a tray. In no time, your plants will turn more lush and lively!
Use rainwater for houseplants. If you haven’t collected rainwater yet, use demineralized water like the one used for ironing clothes.
The bubbles in the pebbles are perfect to absorb water and air. The thin clay walls are porous and let water seep in and out. The consequence? Expanded clay pellets make ideal air moisturizers thanks to their structure and materials.
Water will evaporate from the surface of the clay balls and enter the air around it. Thanks to capillarity, water deeper inside the clay ball will seep to the surface for evaporation to continue.
Clay pellets are always available in garden stores. They’re often used as a cool mineral mulch and as an additive to soil mix since they increase water retention and drainage.
The water you use decides how long the pebbles keep working. Here is what each type does to the porous clay.
| Water type | Effect on the clay | Verdict |
|---|---|---|
| Rainwater | Leaves the pores open, so pebbles keep absorbing and releasing their full load of water | First choice |
| Demineralised or distilled water | Mineral-free, the sort sold for ironing clothes | Good stand-in when no rainwater has been collected |
| Tap water | Minerals build up and block the pores, cutting how much water the balls can hold | Avoid: pebbles end up no better than driveway gravel |
Whatever technique you use to raise air humidity around plants, try to avoid tap water.
After a few hours, most of the water will have evaporated already. At the end of the day, the clay pebbles will usually be totally dry.
For maximum air humidifier effect, replenish water on your clay pellets twice a day, morning and evening.
On hot days, a third top-up might be necessary in the afternoon.
If you notice pebbles are drying up very fast, you can also add a bit more water than necessary to let the pebbles wallow in a little water. For example, a quarter-inch or 5mm water level at the bottom will serve as a “water reserve”. The pebbles, like a wick, will draw moisture up from it during the day.
Another option is to find a glass or bottle that contains just enough volume to douse the pebbles well.
Remember to use distilled water or rainwater for this as much as you can. In fact, whatever technique you use to raise air humidity around plants, try to avoid tap water.
Monthly, replace and/or clean the pebbles to keep pests, algae and insects out.
If they’re very dirty:
Once clean, sterilize them in the following manner:
Plants that benefit most from this technique are those that tend to release lots of water to the air.
Stand several of these moisture-lovers on one large tray, since each plant transpires and the pooled vapour benefits its neighbours.
Desert cacti and most succulents tolerate dry indoor air and generally need no pebble tray, though epiphytic jungle types such as Rhipsalis and Schlumbergera do appreciate higher humidity.
Minerals in tap water slowly clog pores over time. This means they can’t absorb water any more, losing effectiveness. They’ll still do the job, but no better than if you’d used regular gravel picked up from the ground or driveway.
If you don’t clean the pebbles from time to time, they’ll attract pests. These include red spider mite, thrips, scale insects and others.
Soak crusted clay pebbles overnight in a solution of roughly one part white vinegar to three parts water, then rinse them thoroughly under clean water before returning them to the tray.
Stand the pebble tray on a waterproof mat or cork coaster, since a water-filled tray can spill or drip, and prolonged contact with moisture can mark or damage wooden furniture.
Rest your pots atop clay balls to prevent root rot. The bottom of the pot is never in direct contact with wallowing water, protecting roots inside.
