Issue: July 26, 2003

When to repot houseplants

How can I tell when it is time to repot my houseplants? Alison K.

There are several reasons to repot houseplants. When plants become pot-bound (when the roots completely fill the pot), the plant may begin to grow less and may flower less. Some plants actually flower better when their roots are pot-bound. A plant which has outgrown the pot may need repotting; you will need to determine the need based on the plants condition.

Many potting soils contain compost, peat, or other organic matter that decomposes over time. This may result in the production of chemicals toxic to the plants or a change in the drainage characteristics of the soil. When this happens, the plant may wilt or develop root rot. Repotting should be done before the plant dies. To catch this problem early, periodically check the roots by gently removing the root ball from the pot and looking for fresh white roots. If all the roots are dark brown and mushy, repotting is necessary. In doing this, you will also be able to determine if the plant is pot-bound.

In New Mexico and many other parts of the country, the tap water used to irrigate houseplants contains dissolved salts (calcium salt, sodium salt, and others). These salts accumulate in the potting soil as the water is used by the plants and as the water evaporates. Salt accumulation in the soil results in damage to roots. In this case, it is important to remove much of the old soil and the salt it contains when repotting. Root rot can develop as roots injured by salt are easily infected by fungus and other disease organisms.

The accumulation of salt can be delayed but not prevented by proper irrigation. The plant should be irrigated sufficiently to completely moisten the soil. From this moist soil, excess water should drain from the pot carrying some excess soil away. Do not let this salt-laden soil be reabsorbed into the pot. Pour the leachate water from dishes under the pot or keep the base of the pot from sitting in the leachate.

These are common reasons for repotting, but there are also other reasons - perhaps the pot is cracked or salts have accumulated on the outside of clay pots. Maybe you want to put the plant in a more decorative pot. These reasons don't always require repotting. You can place a cracked or ugly pot inside a decorative jardiniere (a pretty, often undrained pot). If the jardiniere is unglazed, be careful to keep the base of the pot raised above any leachate water that accumulates after watering.

A final thought! You may want to repot to divide a plant so you will have additional plants that you can share with friends and new gardeners.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!