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Winter and Summer Soybean Growth in Southern California¹

Soybean growth under mildly low temperatures and short photoperiods from mid–December and mid–February plantings was compared to the normal summer growth from May to June plantings. Seed germination time increased and plant height decreased, depending mainly on the temperature. Low temperatures delayed flowering approximately 30 days beyond the established 25–day limit of flowering of all varieties grown under optimum temperatures and short photoperiods. Extending the photoperiod above the critical for flowering with artificial light delayed flowering time a few days and essentially doubled the per–hectare yield to 1,260 kg of dry stalk and 460 kg of seed from the December planting. Extending the photoperiod for the February planting decreased seed yield and seed quality because of competition with the reinducted vegetative stage resulting from the normal 14–hr photoperiods that occurred later in the growth cycle. These yields from winter plantings compare to 7,907 kg/ha of dry stalk and 2,347 kg/ha of seed from normal summer growth.

Key Words: Winter soybeans • Photoperiod • Temperature • Competition

² Research Agronomist now located at Mesa, Arizona, P.O. Box 858, 85201.

Received for publication June 19, 1969.