The New York Times
CUTTINGS; When House Plants Are Growing Too Large
February 3, 2002
»ROOT pruning» sounds like an awful way to treat a plant. Nonetheless, it’s a periodic necessity for any houseplant once it has grown as large as you want it. Unrestricted, many houseplants could grow quite large, but while a 30-foot-tall weeping fig is a glorious sight on a Caribbean island, my living room ceiling can’t accommodate such growth. In addition to slowing stem growth, root pruning makes room for new soil and new root growth without having to shift a plant to a larger pot.
How often I root prune any potted plant depends on how fast it grows, or tries to, and any quirks it has about being root pruned. My angel’s trumpet (Brugmansia spp.), for example, gets one severe root pruning and one stem pruning every year. But after 19 years I still don’t need to root prune my ponytail palm (Beaucarnea recurvata), which slowly each year fills more and more of its clay pot with its bulbous stem.
Plants generally tell us when they need root pruning. If a plant is more than two or three times taller than the height of its pot, ready to topple, root (and stem) pruning — or a larger pot — is in order. Exceptions exist, of course, like clivia, which thrives, cramped for years, in the same pot.
Plants also have more subtle ways of indicating that their roots need new soil: They will dry out very rapidly once their roots have filled a pot. Cramped roots might even begin to creep out the drainage holes.
The most foolproof way of telling whether a plant needs root pruning is to slide the root ball out of the pot and examine it. This is most easily done when the potting soil is slightly moist. For smaller plants, I place my hand flat on the soil at the base of the stem, the stem between my middle and index fingers, turn the pot upside down and rap its rim on the edge of my potting bench. For a large houseplant, like my six-foot-tall weeping fig, I lay the plant on the ground on its side, hold the pot in place with my feet, then give a tug to the stem. Thick roots pressed right to the the edge of a root ball or circling its outer layers indicate that the time has come to root prune.
Depending on the size of the root ball and thickness of the roots, I use either a sharp knife or a saw to prune. This is the time to grit your teeth and remove a layer of soil and roots from all around and below the root ball. How much to remove depends mostly on the size of the ball: I might cut away anything from a half-inch all around and underneath a small ball to two inches from the root ball of a plant growing in a large tub.
Next, I go over the whole root ball again, this time with a pronged hand cultivator or a stick, loosening soil and roots all over its surface. This coaxes young roots to quickly grow out into new soil. I keep pruning shears handy to cut back any lanky or damaged roots.
With roots pruned, the plant is ready to go back into its pot. Potting soil that is dry or only slightly moist sifts most readily in among the roots. First I put enough soil in the bottom of the pot that the base of the plant’s stem will sit a half to one inch below the level of the pot’s rim. With plant height adjusted, I gradually start filling the space between the root ball and the pot with potting soil, packing it in with my fingertips or and a blunt stick as it’s added. Once the soil is up to the level that I want, I give it one last firming, then add more if necessary.
Watering is very important during the couple of weeks or more that a plant is recovering from root pruning. Right after pruning, I give the whole pot a thorough soaking. As new growth begins, I make sure to thoroughly wet all the soil at each watering. (Succulents are an exception: let their soil thoroughly dry between waterings, even after root pruning.)
Finally, the stems need pruning so that the reduced root system has less leaves to support, and, of course, because the purpose has been to keep the plant from growing larger. It’s like mowing the lawn: cut the tops (and the roots, when you root prune a houseplant) when they get a bit larger than desired to slightly less than desired, let the plant grow again, cut it back again, and so on.
Don’t worry that the plants won’t tolerate this treatment. Think of bonsai trees whose height can still be measured in inches after 100 or hundreds of years, and kept that way with regular root and shoot pruning.
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