Although my blog is full of articles about “How to Grow” different types of fruits, flowers, herbs, and vegetables, I had not written a “How to Grow Onions” blog post . . . until now!
Over the past 10 years, I’ve had varying levels of success growing onions. Some would grow well, but an overwhelming amount seemed to bolt, or “go to flower”. After seeking expert advice along with more trial and error, I finally had consistently good harvests and now feel confident writing this “How to Grow Onions” blog post.
10 Tips for How to Grow Onions
1. Start onions from seed or onion plants (not onion sets)
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.
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 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? This map from Dixondale Farms will help you decide.
3. Plant onions at the correct time
Planting onions at the right time is a critical element of how to grow onions 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 a month before this.
In the low desert of Arizona, the best time to plant onions from seed is September. The best time to plant onion plants in the low desert of Arizona is in November. Onion plants can be planted through the middle of February, but the earlier you plant, the larger the onions will become.
4. How to plant onions correctly
- Plant onions in an area that gets direct sun (at least 6-8 hours).
- The top foot of the soil must be loose and have good drainage. Add compost to compacted soil before planting to improve drainage and friability.
- Onions grow best in soil with a pH between 6.2 and 6.8. Consider having your soil tested. (I use this soil testing kit). If your soil is too alkaline, amend the bed with peat moss. If your soil is too acidic, add limestone.
- Plant the onion plants 1” deep. If planted deeper, the onions may not bulb.
- Space most onion plants 4” apart in rows 8” apart.
- If using square foot gardening, plant 9-16 onions per square 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 a high nitrogen organic fertilizer (blood meal, manure, etc.)
Continue to feed your onions about every 3 weeks during 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 application of fertilizer.
- 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 onion plant will begin to wither and dry up, and then the stems will fall over. This means the onion bulbs have finished developing and are almost ready to harvest.
Once the tops of most of the onions 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).
8. Cure onions after harvesting
Let harvested onions cure in the field or a dry place for at least 2 days to dry out completely. Cover onions with leaves from other onions to prevent sunscald.
Onions are dry when the area where the leaves connect to the bulb are dry. Once they are dry, trim the roots and cut the stem (leaving about 1” of stem), and bring indoors.
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 as a reaction 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.
10. Store harvested onions correctly
Throw away decaying onions before storing. 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 mesh bags from Amazon) hung up in a cool place.
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.