How to Grow Onions: 10 Tips to Stop Bolting and Grow Big Bulbs

For the first few years I grew onions, my harvests were hit-or-miss. Some bulbed up fine, but an overwhelming number bolted and went to flower before they ever sized up. It was frustrating, and for a long time I couldn’t figure out what I was doing wrong. After a lot of trial and error, and finally understanding why onions bolt in the first place, I started getting consistent harvests of big, storable onions. These are the ten things that made the difference.

1. Start onions from seed or onion plants (not onion sets)

Tired of onions bolting? This first tip for growing onions will help you grow them more successfully than any other tip I can share. Remember to start onions from seed or onion plants (not onion sets) if you remember nothing else in this article. 

Do not plant from onion sets. An onion set (looks like a miniature onion) has dried out and is already experiencing stress – it believes it needs to produce seeds or bolt. Once it begins growing, it often sends up a flowering stalk rather than bulbing and becoming a big beautiful onion. 

Start onion seeds in the ground or in containers, densely sown. Once the onions have grown several inches, separate and plant them in the garden. 

Alliums like onions can help repel common cucumber pests. See what else you can plant near cucumbers in this comprehensive cucumber companion planting guide.

Planting onion seeds or small onion plants (they look like miniature scallions) keep the onion from drying out and bolting before it is even planted. This gives the onion the best chance of success. Whether it gets stressed during the rest of the season is up to you.

2. Plant the correct type of onion for your location

Onion varieties come in 3 different types; the best one to use depends on where you live. 

• Short-Day Onions: Begin bulbing when day-length is 10-12 hours; grow best in Southern States, including the low desert of Arizona. Good types to try: Yellow Granex, Texas Sweet, and Red Creole.
• Intermediate-Day Onions: Begin bulbing when day length is 12-14 hours; grow best in the middle regions of the United States. Good types to try: Yellow Candy and Super Star.
• Long-Day Onions: Begin bulbing when day length is 14-16 hours; grow best in Northern States. Good types to try: Walla Walla, Red River, and Highlander.

Not sure which day-length type to use? If you live in the continental United States, this map from Dixondale Farms may help you decide. 

3. Plant onions at the correct time

Planting onions at the right time is critical to growing them successfully. In most areas, the best time to plant onions is 4-6 weeks before the last frost date.

Seeds should be started indoors about 8-10 weeks before planting outside.

In the low desert of Arizona, the best time to plant onions from seed outside is in October. The best time to plant onion plants in the low desert of Arizona is in November. Onion plants can be planted through January, but the earlier you plant, the larger the onions will become. 

• Start seeds indoors: August – November
• Plant seeds outside: October – November
• Plant transplants outside: November – January

Onions and garlic go in around the same time here, so if you’re planting one, it’s worth planting both. Here’s how to grow garlic in the low desert.

4. How to plant onions correctly

1. Plant onions in an area that gets direct sun (at least 6-8 hours). 
2. The top foot of soil must be loose and well-drained. Add compost to compacted soil before planting to improve drainage and friability.
3. Plant the onion plants 1” deep. If planted deeper, the onions may not bulb.
4. Space most onion plants 4” apart in rows 8” apart. 
5. If using square foot gardening, plant 5-9 onions per square foot, depending on the variety.

5. Feed onions regularly during the growing season

Onions are heavy feeders and benefit from regular additional fertilizer. Feed your onions right after planting with an organic fertilizer high in phosphorus (the middle number on a fertilizer). 

Feed onions again 3 weeks after planting with an organic nitrogen fertilizer (blood meal, manure, etc.) 

Continue feeding your onions about once a month at the beginning of the growing season to encourage large leaves and onion tops. Once onions begin bulbing, do not fertilize. 

Water well after each fertilizer application. 

6. Water onions correctly

• After planting, water onions well.
• Water deeply after each fertilizer application.
• Between fertilizing, water onions once top inch or so of dirt is dry.
• Yellow-tinged leaves are a sign of overwatering; cut back on water. 
• When the onion tops begin to fall over, stop watering. 

7. Harvest onions at the right time

As the onion nears harvest time, the lower leaves on the plant will begin to wither and dry up, and the stems will then fall over. This means the onion bulbs have finished developing and are almost ready to harvest. 

Once most of the onion tops have fallen over, stop watering and leave the onions in the ground for another week or so. To harvest onions, pull gently on stems (but if they don’t come out easily, use a shovel to dig them out). 

In the low desert of Arizona, onions are usually harvested in early May. Watch for signs of readiness to harvest as early as late April and as late as the end of May. 

8. Cure onions after harvesting

It’s important to cure onions before storing for longer storage life. 

• Choose a shady, protected location that is around 75-80°F.  Provide a slight breeze with a fan if possible, if it is indoors. (I cure mine on a wire rack in my laundry room).
• Lay the onions out in a single layer on a rack or floor
• Allow the onion stems to wither, and the papery skins will tighten around the onion. 
• Trim roots and stems to about 1″ when the necks are moisture-free and completely tight and dry. Don’t trim green stems.
• Discard (or use right away) damaged, bruised, or still green onions.

9. Use bolted onions right away

If the center stalk of the onion becomes thick and tall, and then develops a flower stalk, the onion is bolting. 

During the growing season, onions will occasionally bolt in response to stress (cold, heat, lack of water, etc.). Bolted onions must be eaten right away; they will not store well. If onions bolt or flower during the season, harvest and use them as soon as possible. Use fresh onions right away in my favorite garden salsa recipe. Find other ideas here for what to do when onions bolt.

10. Store and preserve harvested onions

Store onions in a dry, cool place, ideally not touching each other. A great way to store onions is in mesh net bags (I use these from Amazon) hung in a cool place. I store my onions in a hall closet.

Store sweet onions for up to 3 months. Other types of onions store longer depending on storage conditions. Check stored onions occasionally for soft spots or decay, removing affected onions before it spreads to others.

Preserve harvested onions by dehydrating and freeze-drying.