Light-colored containers recommended for production of heat-sensitive plants

Does the color of the container in which they are produced actually affect the health and growth of plants? Researchers at Kansas State University say it does. John Markham III, Dale Bremer, Cheryl Boyer, and Kenneth Schroeder from the school’s Department of Horticulture, Forestry and Recreation Resources published a report in HortScience that could change the nursery industry’s allegiance to standard black containers.

According to the authors, heat stress on the roots of container-grown plants is a critical problem in the nursery industry. "In a number of nursery grown species, substrate temperatures over 30°C may cause root growth to slow considerably, and in the case of some woody species, root growth stops completely at temperatures above 39°C," they said. High root-zone temperatures in container-grown nursery plants can cause leaf wilting, reduce flower numbers and quality, interfere with normal physiological and biochemical processes such as photosynthesis and respiration, and can increase the incidence of disease and cause plant injury or death.

The researchers designed experiments to test whether growing plants in containers made of colors lighter than the industry standard black could improve root growth by helping to moderate the effects of solar radiation on substrate temperatures. They investigated the effects of container color on growth of roots and shoots in bush beans, red maple, and eastern redbud. Flat and gloss white, silver, and black containers were used with all three plant varieties; additionally, a green container was used in the tree studies.

Plants were grown in bark-based soilless substrate and temperatures were measured at 5-cm depths in the south sides and centers. After 4 months, plant variables were measured again. The data indicated that heat-sensitive plants benefit from being grown in white containers or from painting the outer surfaces of green and black containers white. "Our results showed that red maple was relatively more heat-sensitive than redbud and would benefit more from production in flat or gloss white-painted containers. Redbud growth did not respond substantially to container color, although there appeared to be a slight advantage to using flat white rather than black containers," the authors observed. Growth of both redbud and red maple were greater in flat or gloss white-painted containers, while silver containers were only marginally effective in cooling root substrate and did not provide a significant improvement over black containers.

The researchers recommended that development and use of an economical, durable, white-colored container could offer important benefits for the nursery industry. Producing plants in white containers could increase profits and plant quality, especially in heat sensitive nursery crop species. "Because black containers are the industry standard and are more widely available than white containers, it may be more cost effective to focus production on white containers to use for heat-sensitive crops rather than heat-tolerant crops," they noted.

The complete study and abstract are available on the ASHS HortScience electronic journal web site: http://hortsci.ashspublications.org/cgi/content/abstract/46/5/721