It's time for pot-bound plants to graduate to new containers

(Illustration by Kenneth Harrison)
on September 12, 2013 at 9:55 AM, updated September 12, 2013 at 10:03 AM

September and early October are ideal times to repot tropical container plants that have summered outdoors. It's amazing how well our indoor plants do when placed outside all summer.

Excellent outdoor growing conditions -- abundant light, good air circulation and high humidity -- encourage the plants to grow vigorously during the season. As a result, they may have outgrown their pots.

The term pot bound is used to indicate plants that have filled their containers with roots. Most plants will tolerate being somewhat pot bound. In the case of some plants, it will encourage blooming. This is true for bougainvilleas, for example. Many bromeliads, succulents and cactuses grow better in relatively small pots.

But a pot-bound condition also can cause problems. Once they fill the container, the roots are limited in how much more they can expand. Even though they've run out of room for growth, the roots must support the upper part of the plant with water and nutrients as it continues to grow.

Potbound.JPGThe roots of this plant are pot bound. Notice how packed they are and how many of the roots are brown and unhealthy.

As long as adequate water and mineral nutrients are provided, some plants may remain happy in this condition for quite a while.

Eventually, however, the roots can become so packed they begin to suffer. As a result, the upper portions of the plant will suffer as well.

Common symptoms of an excessively pot-bound condition include frequent wilting, stunted growth, smaller new leaves, poor-quality flowers or lack of flowers, yellowing and dropping of older leaves and signs of nutrient deficiencies.

Many other problems can cause similar symptoms, so how do you determine if a plant is actually pot bound?

Potbound-Not.JPGThe roots of this plant are not pot bound. The roots are far less packed together and are white and healthy.

There are several ways. First, you'll often see a dense growth of roots on the soil surface (some surface roots are normal). In extreme cases, the soil may be so full of tightly packed roots that there is resistance when you try to push your finger in the soil.

Also tilt the pot over. Look for roots growing out of the drainage holes.

To be absolutely sure, take a look at the root ball. Lay the pot on its side and hit the sides with your hand forcefully a few times to loosen the root ball. Place one hand over the soil with the stem of the plant between your fingers, and tilt the pot so the plant slides out supported by your hand.

If the root ball does not slide out, run a knife around the inside between the root ball and the pot, and then gently pull on the plant.

Avoid pulling hard on the plant. It's easy to damage the roots that way.

Once the root ball is out of the pot, you will be able to clearly see the plant's condition. If the roots do not look like they have filled up the pot, you can slip the root ball back into the pot and do nothing.

If all you see is a dense network of roots with little potting soil showing though, this indicates the need to repot. If the plant is still growing well and looks healthy, you may put off repotting until a later date. But a plant that is struggling should be repotted immediately.

When choosing a larger container, try not to get carried away. The new pot should not be tremendously larger than the container it has outgrown. The new pot should allow only about two to four inches of new space between the root ball and the sides.

Over potting a plant - potting it in a container that is too large for the plant - can lead to root rot from over watering. And aesthetically, the size of the plant needs to be in pleasing proportion to the size of the pot. In other words, a relatively small plant looks out of place in a relatively large pot.

By the way, that brings up another reason to repot: when a plant has grown so large it no longer looks proportional in its current pot or is top heavy and prone to falling over.

Don't cut corners in obtaining the best possible potting mix. The health of the plant is influenced by the quality of that mix.

Most nurseries and garden centers offer quality prepared potting mixes primarily made up of peat moss. Make sure the mix is light weight and drains well. Avoid dark, powdery, heavy potting mixes.

Some plants, such as cactuses and orchids, are grown in specialty mixes.

When you're ready to get to work, place a layer of potting mix in the bottom of the new pot. Adjust the soil depth so that the top of the root ball will be somewhat below the rim of the pot. Place the plant in the new pot. Generally, we do not pull apart or rough up the root ball the way we might do when planting landscape plants.

Use potting mix to fill in the side spaces. Do not push on the mix with your hands. Instead, settle the soil by lifting the pot an inch or two off the table and dropping it. In the case of large pots where this is impractical, gently firm the potting soil with your hands but don't pack it. Add enough soil to reach the top of the root ball. Finally, water the plant to finish settling the soil, then you are done.

Some root damage may occur in this process, and plants may undergo some shock after repotting. So place them where environmental conditions do not put great demands on them. Shady porches, patios and areas beneath shade trees usually are best for getting plants established in their new containers.

Allow about four weeks for plants to become reestablished before moving them back where they were originally growing outdoors. Or you may choose to move them indoors for the winter.

Plants generally do not require frequent repotting. This is especially true if the potting mix drains well and contains a high amount of organic matter. For plants in larger containers, it's generally not necessary to repot more than every two to three years. Frequency depends on the plant, condition of the potting mix, container size and the growth rate of the plant.

One last point: I mentioned that a pot-bound plant can be stunted. Generally that's not a good thing. However, if a plant has grown as large as you want it to be, you may decide to leave it pot bound. As long as it stays reasonably healthy, keeping the plant pot bound to reduce its rate of growth can be advantageous.