CAN PEBBLE TRAYS HELP PLANTS DURING LONG WINTER?

They don’t work well as humidifiers, but they are great decorative features

HITS AND MYTHS

“Pebble trays are great for increasing the humidity around plants during our dry winter months.”

Pebble trays filled with water are often touted as being the salvation for houseplants that struggle in our desert-dry winter homes. But are pebble trays all they’re cracked up to be or are they more like a spit in the ocean of parched, prairie air?

A BIT OF SCIENCE

A pebble tray is marvellously unsophisticated. It consists of nothing more than a catch tray that holds water and small pebbles. The pretty pebbles not only conceal the surface of dull plastic or clay catch trays but they also prop up the pots, separating them from direct contact with standing water that could move up into the pot’s drainage holes and drown roots.

But pretty pebbles and drowning roots aside, the raison d’etre of a pebble tray is really to provide a canopy of humid air around our plants, thereby preventing leaves and flowers from becoming brown and crispy.

The principle of a water-filled drip tray acting as a rudimentary plant humidifier sounds like simple science at its best. After all, water evaporates and it has to go somewhere, so a pebble tray humidifier seems like the perfect plant moisturizer. Still, I’ve talked to many people over the years — some who use them and some who don’t — and the anecdotal evidence no difference in plant quality between the two groups.

Now, while I haven’t lost any sleep at night thinking about the efficacy of pebble trays, curiosity finally got the best of me a few weeks ago, and I decided to put the trays to the test with a simple experiment.

When I say simple, I suspect any Grade 2 student could easily replicate my experimental design without much problem. I simply took a typical houseplant drip tray, filled it three quarters full of water, dropped in some pebbles and then measured the relative humidity (relative because the warmer the temperature, the more water the air can hold) just a couple of centimetres above the pebbles. From there, I relied on a pretty sophisticated humidity sensor that I use in our greenhouses to provide the results.

And I must say the results were interesting. While I wasn’t expecting to see a large increase in humidity from the pebble tray, I was surprised that there was barely even a slight blip on the computer screen from the time I placed the humidity sensor above the pebbles until the time the water had completely evaporated.

The large volume of dry, household winter air acts like a giant sponge and quickly absorbs any water evaporating from pebble trays and it quickly dissipates into the air leaving little humid air for the plants.

WHAT SHOULD YOU DO?

As humidifiers, pebble trays don’t work well, but they are great decorative features for pots and excellent at separating roots from standing water. Grouping a large number of plants together on pebble trays may provide a little additional relative humidity because of the shear numbers of plants transpiring water, but don’t expect a dramatic increase in humidity.

The real solution to avoiding brown foliage and flowers has less to do with humid air and a lot to do with well functioning root systems. Plants with healthy roots can readily draw water from potting soil and distribute it throughout the plant canopy. Roots that are growing in poor quality potting soil can easily become damaged and unable to transport water, which leads to brown foliage and flowers.

Another simple way to keep your houseplants vibrant and healthy during the winter is to use grow lights. Yes, I know what you are thinking. What do grow lights and humidity have in common? Well, light is plant food and this plant food helps to build healthy roots, foliage and flowers. We often forget just how bleak interior lighting is in the winter thanks to our short days and low sun angle, but too little light rather than low humidity has a greater impact on plant health.

Now, despite the tenuous link between pebble trays and humidification, the one feature of a pebble tray that I really like is that because it needs to be filled regularly, it forces me to check out my plants more often. And the more often I inspect my plants, the greater the odds of me checking soil moisture levels and keeping my plants properly hydrated.

And if pebble trays do little to moisten the air, but remind us to water our plants, then I think they’ve done their job exceptionally well.