Curtis E. Swift, Ph.D., Colorado State University Extension
Contrary to popular belief, re-potting a houseplant is not the simple task of:
- Knocking the plant out of the pot.
- Putting rocks in the bottom of a larger pot.
- Putting the intact root ball into the pot.
- Filling around the original root ball with potting soil.
Let's look at each of these steps with a critical eye to see if improvements on transplanting techniques can be suggested.
Step 1 assumes one can tell if a plant needs a re-potting based on the presence or absence of roots around the perimeter of the root ball. The typical comment is "if many roots are visible and some have begun to coil around the bottom, re-potting is definitely in order." This may not be the case!
Plants tend to form roots around a root ball for reasons other than the pot being too small. Plants develop roots close to the pot any time oxygen starvation of the root system occurs. Over-watering or too compact a soil will force the roots to develop at the interface between the soil ball and pot. Roots also develop near drainage holes and soil surface as a result of oxygen starvation. Changing the watering practices or changing the soil mix to a looser rooting medium may be in order. It may not be necessary to increase pot size.
Step 2 recommends adding drainage for the plant. The concerned gardener is told to add a "one-inch layer of gravel or broken pottery to assure adequate drainage."
Soil/water physics dictates the location of the water table in potting soils. This water "level" is based on the size of the soil particles in the potting mix and the resulting capillary action. In essence, the water table maintains a constant level based on the bottom of the soil mix. Adding a layer of gravel does not lower the water table, but instead raises the water table.
Why is the water table important with houseplants? The location of the water table determines the amount of soil available for root growth. The soil below the water table has a low oxygen content. Most roots are unable to develop in this water-saturated soil. The purpose of repotting is to increase the volume of soil available for root growth.
Using a single pebble, rock or piece of crockery over the drain holes provides for maximum root growth due to a reduced water table level.
Steps 3 and 4 normally recommend something along this idea - "position plant in new pot and add soil to fill." This results in a pot containing two types of soil; one soil type for the root ball and another soil type around the root ball. This creates an interface (physical and chemical barrier) for root growth and development.
Even if the same proportions of ingredients as originally used for the root ball comprise the new soil, a soil difference would exist. Organic matter in the original root ball would have leached out. Some compaction also would have occurred in the original root ball. Watering alone causes compaction of soil.
Some gardeners feel the difference in soil type is of little consequence. This difference, however, can mean success or failure with a plant. If the soil in the original root ball is finer or more compact than the fill soil, water will move into the root ball from the fill soil, often causing a waterlogged condition resulting din root death. If the soil used as a fill is more compact than the root ball, water will move from the root ball into the fill soil. This may result in a dry root ball, again causing the death of the houseplant.
The problem is solved by breaking the root ball up prior to potting. Care needs to be taken to prevent damage to the roots. Some authors recommended washing the dirt off the roots. This seems like a drastic step to take but may be necessary if the root ball is very compact.
Breaking the root ball allows one to check to see if the plant actually is root bound, or if the root-bound look is a result of oxygen starvation of roots within the original root ball.
Step 5 watering the plant after re-potting, is a good idea.
The suggested revised steps are:
Step 1 - Knock the plant out of the pot and check the root system. If the plant does not appear root bound, replace the plant in the same pot or pot of similar size.
If the plant appears root bound, carefully break the root ball to check the
interior of the ball. If the center of the ball is devoid of roots, carefully
Remove as much of the soil as possible. Re-pot in the same size pot. If the root ball appears root bound, and roots are found in the center of the root ball when broken open, re-pot. Use a pot one or two sizes larger than the original pot.
Step 2 - Cover the drainage hole with rocks, broken pottery, or even nylon mesh prior to potting. Use sufficient material to prevent the leaching of soil from the drainage holes, but not enough to build a layer of so-called `drainage material'.
Step 3 - Prepare and moisten the potting mix. Add sufficient mix to the bottom of the pot to raise the plant to its original level. Figure on leaving 1/2 inch to 1 inch at the top of the pot for watering purposes.
Potting the houseplant lower than it originally grew might cause oxygen
starvation of the root system. Planting it higher might cause the roots to
Gradually add soil to the pot, working it into and around the root system. Add soil until the final soil level is reached.
Step 4 - Water. Add soil to compensate for the settling.
Placed on the Internet November 18, 1997; Updated on December 18, 2009