USDA-Agricultural Research Service (ARS) horticulturist Dr. James Altland is exploring an alternative method for controlling weeds in greenhouse propagation systems by using steam or hot water to kill weed seeds.
Altland and his colleagues at ARS in Wooster, OH, with partial funding from the Floriculture and Nursery Research Initiative (FNRI), have started documenting the critical temperature and exposure time needed for killing weed seeds with hot water or steam.
Weed control in propagation is difficult because it primarily occurs in greenhouses or other covered structures where herbicides are neither labeled nor safe for use. Reuse of plastic propagation trays and containers can exacerbate the weed problem. Seeds of many weeds, especially bittercress (Cardamine flexuosa) and creeping woodsorrel (Oxalis corniculata), stick to plastic containers and trays and are reintroduced into the production system when these containers and trays are reused.
Heat from steam or hot water will kill weed seeds. There is an abundance of research on the use of soil solarization to reduce weed seed populations and other pests in soil. Soil solarization, however, is a slow process that involves covering the ground with plastic to heat soil over several days or weeks.
Research on soil solarization has established thermal killing temperatures in the range of 40°C to 70°C with exposure times of hours to days. Use of hot water or steam to kill weed seed would ideally use higher temperatures for shorter periods of time (seconds or minutes).
There is also evidence from some nursery producers that heat or steam will provide effective weed control. In Oregon, some nurseries have adopted the Grower Assisted Inspection Program (GAIP) to prevent the spread of the pathogen Phytophthora ramorum. Part of this program includes the use of hot water for killing P. ramorum on reused plastic by dipping it in a water tank at 80°C for 15 minutes. While this control point was intended to prevent the spread of P. ramorum, it has also proven very effective in reducing bittercress populations.
So far, Altland’s research has found that creeping woodsorrel required exposure to 90°C for at least five minutes for 100% control. Bittercress was completely controlled with 90°C water at just one minute.
Altland’s research is updating an old tool with specific information on which temperatures and exposure times can be used to eliminate weeds seeds between successive crops in propagation.
The Horticultural Research Institute (HRI) is working closely with FNRI to steer research funds to projects like this one addressing propagation challenges. For more information, contact Jennifer Gray, HRI Administrator, at 614-884-1155 or [email protected].