Red onion is a cultivar group within Allium cepa selected for deep purple-red skin and mildly pungent, slightly sweet flesh. It functions as both a fresh-eating and storage crop and is among the most widely grown vegetables in home gardens. Choosing the correct day-length type matters as much as variety: long-day types suit latitudes above roughly 36°N, short-day types thrive in the South, and intermediate types bridge the gap.
Onions are shallow-rooted and cannot compete with weeds. Consistent moisture during bulb formation is essential, but irrigation should taper off once tops begin to yellow and lodge. Raised beds or mounded rows improve drainage and reduce white rot pressure. Bulbs must have fully dry, papery necks before going into storage or they will rot. Curing in a warm, ventilated space for two to four weeks after harvest achieves this.
Red onions are notably rich in quercetin, a flavonoid with documented antioxidant and anti-inflammatory activity. Traditional medicine used raw onion preparations for coughs, earache, and topical infections. Clinical research supports modest cardiovascular and blood-sugar-moderating effects from regular dietary use. The sulfur compounds responsible for eye irritation when cutting also show antimicrobial activity.
These dates are for Zone 6b. Enter your ZIP to time every job to your own frost dates.
- sow · Jan 30Start seeds indoors under grow lights 10–12 weeks before last frost. Sow 1/4 in deep in cell trays and maintain soil temperature at 65–70°F for good germination.
- plant · Mar 13Transplant hardened-off seedlings or plant sets 4–6 weeks before last frost. Space 4–6 in apart in rows 12 in apart, setting transplants about 1 in deep.
- cut · Any time in springRemove any bolting flower stalks at their base the moment they appear. A bolted plant produces a thick, pithy neck and will not store.
- watch · Any time in springCheck foliage weekly for silvery streaking, a sign of thrips feeding. Apply insecticidal soap or spinosad early, directing spray into leaf bases where thrips shelter.
- watch · Any time in summerKeep soil consistently moist during bulb fill - irregular watering leads to split or misshapen bulbs. Side-dress with a balanced fertilizer when foliage reaches 6 in tall.
- cut · Any time in summerStop irrigating once most tops have lodged naturally. Harvest when necks are tight and thin, lifting bulbs carefully with a garden fork to avoid bruising.
- watch · Any time in summerCure harvested bulbs in a single layer in a warm, shaded, well-ventilated spot for 2–4 weeks until outer skins are fully papery before moving to long-term storage.
Problems & troubleshooting
Common problems
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Onion White Rot
Yellowing and dieback of leaves, soft rotting at the base, and a fluffy white fungal growth with small black sclerotia on roots and lower stems.
Remove and destroy infected plants. Do not compost. Avoid replanting alliums in affected soil for several years and rotate crops. There is no chemical cure once established.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.
Fusarium wilt
Sudden wilting, brown streaks in stems, yellowing leaves. Affects sweet basil varieties most.
No cure. Remove and destroy plants. Do not replant in the same spot for 3+ years. Choose resistant varieties.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.
Damping Off
Seedlings collapse at the soil line with thin, water-soaked stems; seeds may fail to emerge or rot before sprouting.
Sow in well-draining mix, avoid overwatering, and ensure good airflow. Remove affected seedlings and let the surface dry between waterings.
Root-Knot Nematode
Swollen galls on roots, poor vigor, wilting in heat, and reduced fruiting. Containers and sandy soils are more prone.
Use clean potting mix in containers. Avoid replanting in infested soil. Maintain plant vigor with mulch and steady watering. Solarize affected garden soil.