Spring Is Repot Time for Most Houseplants 3/16/05

SALINA, Kan. – All plants respond to spring’s longer days – including those living indoors. The resulting growth spurt is why most houseplants need to be repotted annually. And, spring is when that usually occurs.

“If you try to get around this chore, you’re likely to cause problems,” said Chip Miller, Kansas State University Research and Extension horticulturist. “Trying shortcuts rarely works well, either.”

Repot? Or, Not?

SALINA, Kan. – Judging when a houseplant needs a new, larger pot is mostly a matter of roots, according to Kansas State University Research and Extension horticulturist Chip Miller.

Many container-grown plants need a new home every year. But, some grow so slowly that relocating every two or three years can be enough.

“To check whether plants are becoming root-bound, you need to knock them out of their pot. If you water several hours before trying this, you’ll be able to remove the plant more easily,” Miller said.

He recommends two approaches to this dislocation:

* For plants in pots that are 8 inches wide or smaller, place one hand over the top of the pot with the plant’s stem passing between two fingers. Turn pot and plant upside down, and rap the edge of the pot against a table. The root ball should come away from the pot and into your hand.

* For larger combinations, place the pot on its side and rap its edge with a rubber mallet. Roll the pot a few degrees and repeat the rapping. Continue the procedure until the root ball “releases” and you can slide the pot down.

“If you then see a clear network of roots, the plant needs to be moved into a larger pot,” Miller said. “The new pot should be just 1 inch wider if the plant was small enough to remove into your hand. It can be 2 inches wider if the plant’s old container was at least 10 inches wide.”

Ignoring a houseplant’s expansion needs can lead to a gradual decline that can bring a strangling kind of death.

At the other extreme, moving a plant into a significantly larger pot creates a large border of soil that’s unoccupied by roots. This soil will get wet more easily, and it will stay wet much longer than the original soil that’s filled with roots. So, rather than growing into this wet border, the root tips will tend to rot, Miller said.

“On the other hand, if you try to keep the border soil just slightly moist, the original soil ball will be too dry. And, again, the plant will suffer,” he added. “The situation is hard to manage well.

“In fact, even plants just moved up to the next larger pot size need to be managed carefully at first. Overwatering and underwatering both are risks until roots penetrate the new, wetter soil.”

As a rule of thumb, if a current pot is less than 10 inches wide, the new pot should be just an inch wider, Miller said. If the original pot is 10 inches or more in diameter, the new pot can increase in size by 2 inches across.

The horticulturist admitted that “soil” is a misleading word to use, when talking about gardening in containers.

“The worst thing you can do is go out and dig up some dirt in the yard. Even if you’re willing to sterilize it in your oven, it’s always going to drain and handle nutrients poorly, due to its structure and the constrained environment of a pot,” Miller said. “For container gardening, you need a growing medium that’s better than garden soil.”

He recommends using a high-quality potting medium – a recognizable name brand of a soilless mix. Putting gravel or pot shards over the drainage holes in a new pot will keep this soilless mix from washing out. If that gravel/shard layer is more than minimum coverage, however, it also will tend to keep the “soil” wet.

“The plant must sit at the same level it was in the old pot. So, you need to add enough potting mix to the bottom of the new pot to ensure this. Then you have to firm the mix in place, so it doesn’t settle over time, taking the plant deeper,” Miller said.

This firming remains important once a plant is in its new pot and mix goes in to fill around the original root ball.

“You can firm the mix with a slender stick or tap the bottom of the pot on a table. If you don’t get it close to the firmness of the root ball, though, this new ‘soil’ will be so light and airy that water will tend to move straight through it, rather than through the entire pot – root ball and all,” he said.

“But be especially careful not to overwater for about two weeks. More people lose repotted plants to root rot than to an artificially created drought,” he said.

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.