This study evaluated the effects of a nonwoven polypropylene (NPP) covering during overwintering with different mulch types and transplant times on bulb onion growth and yield of intermediate-day onions (Allium cepa L.) during the 2015/2016 and 2016/2017 growing seasons. Field experiments were conducted using a split-split plot design with three replicates. Treatments included main plots containing different mulch types (transparent plastic mulch, black plastic mulch, or bare ground), subplots with different transplanting times (20 Oct., 5 Nov., and 20 Nov.), and sub-subplots containing two onion cultivars (Sunpower and Katamaru). NPP was used to cover all plots on 1 Dec., and it was removed on 28 Feb. Mean daily air temperatures during transplanting and root establishment were 2.6 °C higher during the 2015/2016 growing season compared with the 2016/2017 season. NPP covering on bare ground increased soil temperature by 2.1 °C compared with no treatment. Soil water potential with transparent and black mulches and NPP continued to be more than −10 kPa until early March. Number of leaves, and root and leaf weight were significantly greater during the 2015/2016 growing season than during the 2016/2017 growing season; there were also significantly greater for onion plants grown with transparent plastic mulch than for those grown with black plastic mulch or no mulch on 4 Apr. and 5 May. Marketable bulb yield was lower during 2015/2016 (32.0 Mg·ha−1) than during 2016/2017 (38.5 Mg·ha−1); this was due to the increased unmarketable bulb yield, with 33.2 Mg·ha−1 bolters and 3.9 Mg·ha−1 doubled bulbs during 2015/2016 compared with 3.9 Mg·ha−1 bolters and 0.3 Mg·ha−1 doubled bulbs during 2016/2017. Marketable bulb yield of ‘Katamaru’ (38.9 Mg·ha−1) was greater than that of ‘Sunpower’ (31.6 Mg·ha−1). Marketable bulb yield increased with later transplanting times, and onions grown with black plastic mulch achieved the highest bulb yield (43.0 Mg·ha−1), followed by transparent mulch (34.7 Mg·ha−1) and no mulch (28.0 Mg·ha−1). When the temperatures from early November to early December were similar to the 30-year average temperatures, marketable bulb yield could increase with the NPP covering, especially for onions grown with black plastic mulch or no mulch when transplanted from late October to early November.