Growing onion in zone 7a

For spring production, transplants go into the ground from late February through mid-March, before the average last frost in zone 7a (typically late March to early April). Onions tolerate light frost well once established, so early planting is not a significant risk. Growers starting from seed should sow indoors 8 to 10 weeks before transplant date, placing germination in early to mid-January. As day length passes each variety's bulbing threshold in late May and June, leaf production slows and energy shifts to the bulb. Harvest for most varieties falls between late June and mid-July. Fall planting is also viable: setting out onion sets in October allows overwintering, with harvest of small green onions or modest bulbs the following spring. Zone 7a's frost timing provides a 4 to 6 week fall planting window before hard freezes arrive.

The most significant zone-specific concern for onions in zone 7a is onion white rot (Sclerotium cepivorum), a soil-borne fungal pathogen that produces highly persistent sclerotia capable of surviving in soil for 20 or more years. Once it establishes in a planting bed, recurrence is nearly certain. The only reliable management approach is strict rotation: at least 8 years between allium crops in any affected area. Infected bulbs should be removed and discarded, not composted, and soil should not be moved between affected and unaffected beds. Zone 7a's broader high-humidity disease pressure also promotes botrytis leaf blight and downy mildew during periods of overcast, wet weather in spring. Spacing transplants at 4 to 6 inches and orienting rows to take advantage of prevailing airflow reduces foliar disease incidence. Mulch is useful for moisture retention and weed suppression, but should be kept away from developing bulb shoulders to avoid creating the damp microclimate that accelerates soft rot near harvest.