I truly appreciate old-time gardening wisdom. It’s invaluable. Time-honored, solid, quiet gardening techniques are what turns thumbs green, and are far more important than hot new trends and flashy headlines.
Very few old-time gardening ways need updating. But one of the old practices that must stop is the habit of putting rocks, stones, gravel and broken clay pot pieces inside the bottom of houseplant pots. Actually, the practice was shown to be dangerous over 100 years ago, but we haven’t listened. Pick up almost any houseplant literature about repotting, and they prescribe placing a layer of stones or pebbles inside the pot “for drainage” before adding the potting soil. And this is the way most of us learned from parents and grandparents.
Now, to everyone who has pebbles in the bottom of their houseplant pots and whose plants are thriving, it doesn’t mean the plant will automatically die. But overwatering, which keeps soil continually too soggy, is the number one killer of houseplants, and a layer of drainage material actually makes the situation worse, which is counterintuitive. So we need to discontinue the practice.
Let’s investigate. Soil scientists have found that when water moves down through potting soil, when it meets a layer of change (stones, pot chips, etc in the pots bottom), physical science kicks in, and the water accumulates in the soil, saturating the soil before it makes the leap across the layer of change and drains away. Before the water drains away, it has adversely saturated the root zone of the plant, compounding the tendency of many of us to overwater. Plant roots suffer from lack of oxygen, and the overwatered plant declines.
University of Washington says “Nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of finer textured material to layers of more coarse textured. Since then, similar studies have produced the same results. Rather than water freely trickling down across the boundary between the layers, water resists crossing it. That’s because the pull of the water upwards in the finer layer offsets any gravitational pull downwards. Instead, water builds up in the finer level above and is only released into the coarser layer below when it’s saturated, like a full sponge that can’t hold any more water.” University of Illinois, and others, echo these findings.
In summary, the best method is to fill a houseplant’s pot top-to-bottom with high quality potting mix, and skip the layer of pebbles.
(What about large outdoor containers that require huge volumes of soil, if we followed the soil-only guidelines? A common practice is to fill up some of that excess bottom space with Styrofoam packing peanuts, plastic bottles, bricks. etc. Growing plants outdoors in large containers is different than growing houseplants indoors. It does seem to work fine to take up some of the lower space. Planters outdoors in good light with good air movement usually aren’t subject to overwatering, like indoor houseplants are. So although the bottom material probably leads to some temporary soil saturation, the soil will adjust itself more quickly outdoors, through evaporation, etc.)
Also not needed in houseplant pots are coffee filters, screens, diapers, or anything else to cover the drainage hole in the pot “to keep the soil from washing out.” High quality mix doesn’t wash out the pots’ drainage hole, and I can attest to that. Years ago I quit putting drainage pebbles in, and discontinued covering the drain hole with a piece of broken pot, and have never had soil washing.