Growing onion in zone 3b

Zone 3b's last spring frost typically falls in late May or early June, and the first fall frost arrives in early September, compressing the outdoor season to roughly 90 to 100 days. Onions benefit more than almost any other vegetable from an indoor head start in this zone. Seeds sown 10 to 12 weeks before the last frost date produce transplants ready to go out in mid to late May, gaining several weeks over direct seeding. Bulb development accelerates through June and July as day length peaks above 16 hours at northern latitudes. Tops typically fall by late August; harvest should be completed before the first hard frost in early September. Curing onions promptly after digging, in a warm and well-ventilated space, is critical given the narrow gap between harvest and the onset of cold, damp fall conditions.

Starting onions from transplants rather than direct seed is the baseline adjustment for zone 3b; direct-seeded onions rarely reach full size in a 100-day season. Seeds started indoors by late February or early March produce transplants ready for the garden as soon as the ground is workable in May. Onion White Rot (Sclerotium cepivorum) is the primary disease concern; the pathogen can persist in soil for decades, so rotating allium crops to a new bed each season is the single most effective preventive measure. Zone 3b summers are short and cool, and slow-drying conditions after rain can favor fungal development near the soil line. Wide in-row spacing of at least 4 to 6 inches and drip or furrow irrigation rather than overhead watering reduce that risk meaningfully.