Effects of Weed-Free Periods on Organic Romaine Lettuce Production

ABSTRACT

Weed management in Romaine lettuce (Lactuca sativa L. var. longifolia) is an economic challenge for organic farmers. This study evaluated the effect of weed-free durations on crop yield and quality, weed densities and biomass, and weeding time in two growing seasons. Treatments included weedy entire-season; weed-free for 7 days after transplanting (DAT), 14 DAT, 21 DAT, 28 DAT, 35 DAT, 42 DAT; and weed-free entire season (49 DAT). Weeds were removed by hand-hoes and the time taken to weed each plot was recorded. Weed densities were recorded prior to each weeding and weed biomass was recorded at crop harvest. Total and marketable lettuce heads were recorded and crop quality was estimated. Weed interference up to 21 DAT resulted in approximately 45% total and 58% marketable yield loss. Weed control beyond this period did not result in significant (p < 0.05) increases in crop yield and quality. However, weed densities at the end of the season were 4- to 15-fold and weed biomass approximately 18-fold greater in plots kept weed-free only for 21 DAT compared with plots kept weed-free season-long. This suggested that weed seed-return may be a concern if late-season weeding is not conducted. To minimize weed seed-return, the plots would have to be kept weed-free for about 35 DAT. However, labor costs must be taken into consideration because the time required to hand-hoe was approximately 87 hr ha⁻¹ person⁻¹ greater in plots kept weed-free for 35 DAT compared with plots kept weed-free for 21 DAT.

KEYWORDS:

• Critical period for weed control
• critical weed free period
• chlorophyll concentration
• hand weeding costs
• Lactuca sativa
• transplanted vegetables

Acknowledgements

We also acknowledge the assistance of Fresno State students Upasana Bansal, Ryan Cox, Larissa deSouza, Isaac Giron, Armando Guzman, Jeremy Mora, Elizabeth Mosqueda, Eduardo Padilla, Sonia Rios, Alfredo Rojas, and Mala To in the project. We also thank Growers Transplanting Inc., Gustine, CA for donating the lettuce plants and California Organic Fertilizers, Fresno, CA for donating the fertilizers. Finally, we thank Calliope Correia of the Horticulture Unit of California State University, Fresno, CA for assistance with all the field operations.

Funding

Support for this project from the Weed Science Society of America and the California Weed Science Society to S. Parry as an undergraduate research grant is highly appreciated.

Additional information

Funding

Support for this project from the Weed Science Society of America and the California Weed Science Society to S. Parry as an undergraduate research grant is highly appreciated.