Replicated treatments show several fall production benefits The desert valleys of Coachella and Imperial produce a diverse array of vegetables. These are frequently grown in rotations of two crops per year that receive extensive inputs of fertilizers and pesticides.

Soil temperature in early fall is usually very high and can delay planting. If soil temperature were reduced, it may be possible to extend the growing season and allow produce to reach the market when costs are higher. The extended season would also give growers additional flexibility in crop selection and timing of field operations.

Our research into this problem had two objectives: (1) determine if cowpea cover crop grown in the summer and incorporated into the soil or used as mulch could provide non-chemical weed control in fall planted vegetables; (2) evaluate the potential of cowpea mulch to improve soil temperature regime in early fall.

Field experiments were conducted at Coachella Valley, Calif., during summer and fall 1999. Cowpea (variety: Iron Clay) cover crop was grown in the summer, and lettuce (variety: Shining Star) was grown in the fall.

The experiment had three treatments replicated four times. Treatments included:

1.Cowpea mulch (CM) - cowpea residues were left on the soil surface as mulch prior to lettuce transplanting.

2.Cowpea incorporated (CI) - cowpea residues were incorporated into the soil prior to lettuce transplanting.

3.Bare ground (BG) - the soil was fallow during the summer prior to lettuce transplanting.

Effects on temperature To study the effect of cowpea mulch on soil temperature, a data logger was installed in the field to monitor the soil temperature in the CM and BG treatments. Thermocouples were placed at 6.5 centimeters in the soil and temperature was recorded every hour for the first three weeks of the growing season.

The data collected included the density of each weed species, soil temperature, and lettuce marketable yield.

Results - Summer cowpea cover crop decreased the number of weeds in fall planted lettuce throughout the entire growing season. The weed control effect of summer cowpea cover crop was more pronounced when cowpea residues were used as mulch rather than when the residues were incorporated into the soil.

- Summer cowpea cover crop decreased the number of weed species in fall planted lettuce. Reduction of weed species was greater in cowpea mulch plots.

- Soil temperature regime was improved when summer cowpea residues were used as mulch in the fall compared to when the ground was bare in the summer. Cowpea mulch acted as a temperature buffer.

- Night temperatures were increased in cowpea mulch plots.

- Temperature rise in the morning was delayed by cowpea mulch.

- Maximum temperature was slightly decreased and the duration of the daily peak was shorter in cowpea mulch plots.

- Summer cowpea cover crop increased fall planted lettuce yield when the residues were incorporated into the soil.

Reduction of weed population and diversity by summer cowpea cover crop will reduce the need for herbicide and labor inputs for vegetable production in the fall. In the Coachella Valley, the buffering effect of cowpea mulch on soil temperature may have the added advantage of allowing growers to plant earlier. Early planting will ensure supply of the produce at a time when there is less competition on the market and better prices.

In our study, we are also investigating the combined effect of summer cowpea cover crop and crop management system on weed population, insect population, soil organic matter content and lettuce growth parameters. These data will be analyzed and available later this year.