The sweet potato (Ipomoea batatas (L.) Lam.) requires a long, hot growing season to attain good yields. In a cool climate, the use of black plastic mulch to heat the soil can improve growth but cultivars, plant spacing, and harvest date must be carefully selected to optimize yields and to attain market quality standards. In this two-year study in Quebec, Canada, two sweet potato cultivars (‘Georgia Jet’ and ‘Beauregard’) were grown at four in-row spacings (15, 30, 45, and 60 cm) and harvested at three dates (mid September, late September, and early October). Cumulative growing degree-days (GDD) with base temperatures of 10°C and 15.5°C were calculated for each harvest date. ‘Georgia Jet’ had higher total and marketable yields than ‘Beauregard’. In-row spacing had no effect on yields per hectare of ‘Beauregard’ and only affected ‘Georgia Jet’ in one year of the study. Average root weight of sweet potatoes, yields per plant, and number of roots per plant increased with wider spacing. Delaying harvest by one or two weeks had little effect on ‘Beauregard’ but increased yields of ‘Georgia Jet’. GDD may be a useful predictor of optimum harvest date but a lower base temperature used to calculate GDD may be desirable with ‘Georgia Jet’ as its yields continued to increase even when growing under cool conditions of late September and early October.