Several polyethylene and fabric row cover materials, and clear and black polyethylene mulch, were evaluated in a 2-year field study. For cucumbers [Cucumis sativus (L.)], visible wilting and slowed growth rates of young transplants exposed to cold nights were minimized when grown under row covers that maintained high humidities and higher air and soil temperatures than in the exposed controls. Early cucumber yields were increased 2- to 6-fold by the use of covers. In contrast, tomatoes [Lycopersicon esculentum (Mill.)] showed no significant early yield increases, but a 63% reduction in early yield in 1985 under a perforated clear polyethylene cover. The frequency and duration of daytime air temperatures exceeding 35C had a negative impact on tomato fruit size, quality, and percentage marketable. For cucumber, the relationship between cumulative degree days (during the covered interval) and biomass, early, and total yields was linear (r2 between 0.70 and 0.82) with positive slope. Tomato yields could not be accurately predicted using this approach, but correlations were improved (for the 1985 data set) by using modified degree-day formulas incorporating a negative high-temperature factor.