Studies in the Northeast United States and Canada have shown that sweetpotato yield can be comparable to the national average when grown with black plastic mulch. Western Washington has a diversity of microclimates, from the Pacific Coast to the Cascade Mountains, and assessing yield in response to growing degree days (GDD) will help growers determine if soil warming strategies are needed to achieve acceptable yield. This experiment was carried out in Mount Vernon, WA in 2024 and is being repeated in 2025. The experimental design was a split-split plot: the main plot treatment was bare soil and black polyethylene mulch (25.4 µm); the split plot treatment was cultivar, Covington and Cascade; and the split-split plot treatment was harvest date, every two weeks from 8 weeks after transplanting (WAT) to 18 WAT to assess root growth in response to accumulated GDDs. The experiment included 4 replications with 10 plants per plot, and raised beds with drip irrigation. Soil temperature was measured at 15-min intervals with sensors installed in both the mulch and bare soil plots. GDDs were calculated at two base temperatures, 10°C and 15.5°C, as both are reported in the literature. In 2024, for bare soil and plastic mulch treatments at base temperature 10°C, GDDs were 528 and 667 at 8 WAT (29 July), respectively, and 1080 and 1300 at 18 WAT (7 Oct.), respectively. At base temperature 15.5°C, GDDs for bare soil and plastic mulch treatments were 238 and 375 at 8 WAT and 428 and 635 at 18 WAT, respectively. At 8 and 18 WAT, GDDs for AgWeatherNet air temperature located 410 m from the field site were 629 and 1308 for 10°C base temperature and 138 and 275 for 15.5°C. Overall, black plastic mulch increased soil temperature by 1.78°C on average, and soil GDDs were 44% and 21% higher with plastic mulch compared to bare soil for a base temperature of 15.5°C and 10°C, respectively. At 18 WAT, root yield (US No. 1 and US No. 2) without accounting for wireworm damage was 11.7 t/ha for ‘Covington’ and 9.9 t/ha for ‘Cascade’ with plastic mulch and 7.7 t/ha for ‘Covington’ and 8.4 t/ha for ‘Cascade’ with bare soil. Sweetpotato yield in Mount Vernon, WA, was 35% greater on average with plastic mulch, but plastic mulch may not be necessary in warmer regions of the Pacific Northwest.
