Red Onion

Quick Reference

Transplant/Direct Sow: Direct sow or transplant.

TIP: If planting onion sets, the smaller bulbs are often the last to go to flower. For that reason, we tend to plant the smaller bulbs first rather than those over an inch wide.

TIP: Traditionally, warmer areas plant onions and garlic in November and allow them to settle in before the colder winter weather.

When to Plant - Start seeds indoors 4-6 weeks before your average last frost date - transplant when the soil is workable.  Transplant or direct sow seeds when the soil temperature is 55-70°F and the soil is workable. Plant in Oct/Nov in warmer climates and allow the onions to overwinter.

Days to Germinate: 4-10 days

Days to Maturity: Onions need 90 plus days for most varieties.

Planting Depth: 1/4" deep for onion seeds and 1" for onion sets.

Germination Temp: 55-70°F soil.

Seed Spacing: Direct Sow onion seeds at one seed every 1-3".  Space wider for wider varieties. For onion sets, plant each bulb 2-3" apart. When transplanting seedlings, space them 2-3" apart.

Row Spacing - Space rows 1.5' apart.

Light: Full sun

Soil ph: 6.2-6.8 pH

Water:  Water seeds often to keep the soil moist. Adult plants like soil that is consistently moist to 1-2". Onions have very shallow roots.  Soak the onion row rather than just sprinkling it.

TIP: Mulch is an excellent tool to help prevent water evaporation and to fight weeds.

Fertilizer:  Start with nutrient-rich soil. Amend it with aged compost to bring up the nutrient level.  Onions need a lot of nitrogen. If fertilizing, use one cup for every 20 feet of row space. A 10-10-10 N-P-K ratio fertilizer is good.  For poor soil, use a 12-10-10 to give your onions a nitrogen boost.

Companion Plants

Strawberries, lettuces, carrots are all excellent choices for companion plants for onions. Peas and beans do not do well with onions.

Growing conditions

Onions are a cool-weather-loving crop. They need well-draining soil that holds moisture to 2-3 inches. They will tolerate cold weather and frosts but may need a row cover in very cold regions. A cold frame is a good tool for growing onions in cold climates. Soil should be high in nitrogen.


Onions are heavy nitrogen users, so rotate to a new spot each year or amend the soil.

Anticipated yields

TIP: You don't have to plant onions in a straight line and can cram more onion sets into a one-square-foot space alternating them. They need only about three square inches of space per plant.

Plan your garden yield at 12-20 plants/sets per person. You should get 4-5 sets per foot of row space.

Fall & Winter Garden 30
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