Choose a site with at least 6 hours of direct sunlight.
Onions can be started 3 ways: by Sets (tiny bulbs), Transplants and Direct Seeding. Though planting onion sets is the most popular way to grow them, you'll have better results transplanting seedlings you start indoors ahead of time.
SETS: Sets are just tiny bulbs that were started from seed the year before. Although they are the easiest way to plant onions, transplants actually offer better success and store better.
Onion sets should be about the size of a marble. Larger sets don’t always adjust well and could bolt or split. For similar reasons, don’t buy sets that have already sprouted. And as with all bulbs, onion sets should be firm and healthy looking.
Sets can be planted early in the season, before the last frost, but after the soil has dried and warmed a bit. Plant onion sets pointed end up and cover with about 2" of soil. Depending on the mature size of your variety of onion, space about 3-4" apart.
TRANSPLANTS: Transplants generally result in larger onions than sets. You can buy transplants or start your own indoors from seed. Start onion seed about 8 - 12 weeks before your transplant date. Plant onion seed about 1/4 - ½" deep. You can plant thickly and thin at transplant time. Keep the soil moist. As the tops grow, keep them trimmed to about 4".
Transplants or onion seedlings will need to be hardened off before planting outdoors. Wait until all danger of frost has passed before planting.
Don’t bury transplants too deeply. Plant them right on the surface of the soil, spaced about 4" apart. Keep onions well watered throughout the season. The bulbs need regular water to swell in size. Transplant grown onions are the type you see pushing up out of the ground.
DIRECT SEEDING: Direct seeding onions require a longer season, so warmer climates will have better luck. To direct seed, follow the directions above for seeding transplants.
Pests & Problems:
- Rot: In damp soils you may encounter neck or stem rot or bulb rot. Avoid with good soil drainage and air circulation.
- Splitting: Bulbs will split or double if the soil is allowed to remain dry while the bulbs are forming.
- Thrips: Small yellowish-brown flying insects that feed on leaves and can cause twisting and curling. Repeated attacks cause the plant to stop growing, so bulbs don’t mature. Plant resistant varieties. Don’t plant near grain crops. Neem and insecticidal soaps provide temporary control.
- Onion Root Maggots: Eggs are laid near the base of onion plants. The larvae hatch and burrow into the stems, feeding on the plants below the soil and eventually killing the plants. Rotate plants yearly to avoid infestation. Covering new seedlings will prevent eggs being laid. Diatomaceous earth is also effective.
More Onion Growing Tips:
- Growing Larger Onions: Onion bulb size is related to the size and number of the leaves. Each leaf translates to a ring of onion. Larger leaves make larger rings. So choosing the right type of onion for your day length will give your onion tops time to form before the onion bulb begins developing: more leaves, more bulb.
- Water stressed onions are stronger in flavor and more pungent.
- Onion seed doesn’t store well, so only buy what you plan to use the current year.