How to Grow Big Onions

Do you want to grow big onions? You need to start early. The key to growing big onions is for the plants to be large when they begin to form bulbs. Bulb formation begins as early as late April to early May in North Dakota.

We want our onion plants to have as many leaves as possible in spring. That’s because every leaf creates a ring of onion. An onion plant with lots of leaves will have lots of rings—and a bigger bulb.

Variety

We grow long-day onions in North Dakota. These onions are sown in spring and form bulbs when days are 14–16 hours long. Ailsa Craig is the most widely available variety that forms giant bulbs; it’s highly recommended. Blush, Sweet Sandwich, Sweet Spanish and Walla Walla are other suitable choices.

Intermediate- and short-day varieties will begin to form bulbs earlier in spring. This is not a good thing when growing big onions. These plants will likely be smaller when they begin to bulb, leading to smaller bulbs.

Seeds, Transplants or Sets

Seeds should be sown in flats in mid-February. Sow seeds thickly and later thin to about ½–inch apart. Clip the tops once they grow 5 inches tall.

If buying transplants, look for those that have a diameter the size of a pencil.

If buying sets, look for sets with diameters of 0.5 inches or less. 

You may think a large set will lead to a large bulb. This is false. Compared to a small set, a large set is more likely to bloom during summer. Flowers draw energy out of the bulb, reducing the size of the bulb.

Planting Dates

Onions tolerate light frosts. Sets and transplants are typically planted outdoors in late April.

Sunlight

Giant onions need full sun. Sunlight is the source of energy for plants. Onions want full sun.

Soil Preparation

The soil should be well drained and loose. A sandy loam is ideal. Raised beds work well. Compacted soils will restrict bulb growth; add an inch or two of peat moss, compost or other organic matter, if needed.

Spacing

Giant bulbs need lots of space. Crowding the bulbs will limit their growth.  Plants are spaced 4–6 inches apart in rows spaced 12–18 inches apart. They may be planted in double rows or multiple rows per bed. If you plan to harvest some of the plants as green onions while young, space the plants 2 inches apart in the row and thin as needed.

Weed Control

Onion is one of the least competitive of all vegetable plants. You must control weeds, which compete for water, nutrients and sunlight. Weeds also attract thrips and other pests that harm onions.

Irrigation

Onions have shallow roots and struggle in dry soil. The planting should receive one inch of water every week either from irrigation or rainfall. The availability of water is especially important while the plant is growing its bulb; some growers increase watering to 1.5 inches every week during this stage. Stop watering when the tops start falling over.

Fertilization

A soil test will tell you exactly what you need. One general guide is to apply ½ cup of 10-20-10 per 10 feet of row before planting. One or two applications of urea (46–0–0) at a rate of 1/3 cup per 10 feet of row may be applied at intervals of 2–3 weeks after planting. 

Fertilizations in late summer should be avoided since they lead to thick stalks and poor storage qualities.

Written by Tom Kalb, Extension Horticulturist, North Dakota State University. Source: Hatterman-Valenti, H. 2015. How to grow large onions. NDSU Spring Fever Garden Forums. Photo courtesy of Chiot's Run. Revised 2024.