Growing Onions

 Growing onions in your garden

A variety of onions from the garden

Introduction: From my experience, growing Onions (Allium cepa) has been pretty straightforward once you understand a few nuances - alliums in general need full sunlight, loamy soil rich in compost, moist soil but no standing water ... best of all very few pests affect alliums making them easy to grow. If you need more details keep reading ...

For beginners instead of starting from seed, use onion sets of your preferred variety from a nursery. Onion sets are tiny onions that mature in about 14 weeks but for best results, make sure the sets are not dried or stressed. But growing from seeds is fairly easy, remember it is a cool-season crop that requires the soil to be at least 50°F to germinate. You can also start seedlings indoors about 6 weeks before transplanting to the garden. 

All you need to know when growing onions

1. In NorCal you can plant intermediate-day and long-day cultivars. Growing onions requires abundant sun (they do not tolerate shade) and good drainage, and they grow best when the soil pH ranges between 6.0 and 6.8. Improve your native soil by mixing in several inches of aged compost or other rich organic matter.

2. Some people plant onion seedlings in Fall, which is doable in NorCal but remember the fall-planted crop of onions needs at least 4 to 6 weeks of warm temperatures to become established in the ground. They will remain dormant during the cool season; as the temperatures and soil warm again in early spring, the onions come back to life. Onions are a cool-season crop and tolerate moderate freezes, but when cold snaps arrive, cover beds if temperatures reach 20°F or below. 

3. Set plants, growing tip pointing up, 1 inch deep, so that their roots are well covered with soil but the top of the plant's neck is not buried too deeply as this will adversely impact bulb development. Space onion plants 6 inches apart in rows that are 12 inches apart. Grow them in a sunny spot that has fertile, well-drained soil with a pH of 6.0 to 6.8. Fertilize regularly with a nitrogen-rich amendment every few weeks since onions are heavy feeders. Use ammonium sulfate (21-0-0) in alkaline soils or calcium nitrate (15.5-0-0) in acidic soils, applying at a rate of 1 cup per 20 feet of row. If you want larger bulbs you can use phosphorus-rich (the middle number in N-P-K) amendments like bone meal or blood meal. Repeat this application every 2 to 3 weeks until bulbs begin to form, making sure to water the onions after each application. Stop fertilizing once the onions reach the desired size.

4. With very shallow roots, Onions aren't great at taking up water, so it's important to keep the soil moist. Water about 1 inch per square foot per week, ideally delivered through drip irrigation or furrowing, whenever the top inch of soil becomes dry. You may mulch with a light layer of weed-free and herbicide-free grass clippings or another fine mulch. Remove mulch that might keep the expanding bulbs excessively moist. Additional watering makes the onions sweeter.

5. Onions naturally push toward the surface as they form bulbs, and it's best if the tops of the bulbs are allowed to bask in dry sun. Do not mound or hill over emerging onion bulbs.

6. You can harvest young onions just a few weeks after planting if you want to use them as "spring onions" or scallions.

Onions starting to bolt

7. In any case, pull any onions that start to bolt i.e. if the centre stalk of the onion becomes thick and tall, and then develops a flower stalk. This means that the bulbs have stopped growing and energy is diverted to seed production. At this point harvest the onions and use the green sprigs in the kitchen. Check to see if the onion bulb is soft or mushy as this will not store well but can be used in recipes within a few days.

Onions ready to harvest

8. For full-sized bulbs, let onions grow and mature. They are ready to harvest when the bulbs are big and the tops begin to turn yellow and fall over. At this point, bend the tops down or even stomp on the foliage to speed up the final ripening process.

9. Pull them up in dry weather because moist or wet onions tend to quickly rot, shake off the soil, and lay them out to cure with the tops still attached. Set onions on dry ground for a few days to cure, weather permitting, or in a protected place such as a garage or barn. Check periodically for sprouting or rotting onions and remove them.

10. Once cured, hang onions in a mesh bag or nylon stocking; spread up to two layers deep in a box; or braid and hang them in a cool, dry, well-ventilated area. The ideal temperature range for storage is 40 to 60°F (4 to 15°C). Do not store in a refrigerator, as conditions will be too damp.
After 7 to 10 days, clip off the tops of the onions and the roots with pruning shears, remove as much dry dirt as possible without taking off the papery outer skins, and store your onions in a cool place. Very sweet, juicy onions may be stored, wrapped in newspaper or paper towels, in the fridge.
A pungent onion will store longer than a sweet onion. Sweet onions have a high water content and do not keep well. Eat the sweet varieties first and save the more pungent onions for later.

11. Don’t store onions and potatoes, with bananas, apples or pears, as the ethylene gas produced by the fruits will interrupt the onions’ dormancy.

Common onion pests

Thrips and onion maggots are the two most common pests. 

Thrips feed on foliage, causing damaged leaves to take on a silvery color, with specks visible on the leaves. Thrips overwinter in weedy areas and on onion bulbs not removed from the garden. Good sanitation—keeping the planting area free of debris—can help manage this pest.

Onion maggots feed on the roots and bulb, which can lead to bulbs rotting during storage. Crop rotation will help with maggots, as will good sanitation. Onion maggots are attracted to un-decomposed organic matter such as manure and rotting foliage. Do not amend soils with fresh manures, use only well-composted organic matter. 

Type of Onions

1. Chives, walking onions, leeks, and Welsh onions belong to different species of Allium.
2.  Green onions (sometimes called scallions) are simply onions harvested at an immature stage. Harvesting yellow, white, or red onions for “green use” is a useful way to thin onion rows and allow adequate space for bulb development in the remaining plants.  
3. Shallots and potato onions belong to the Aggregatum group, (Allium cepa var. aggregatum) and produce clumps of smaller bulbs rather than one large bulb.

FAQ: Can you plant a store bought sprouted onion?  Yes, you can plant a spring onion after using the leafy parts, though you won’t get more onions from it. You will get lots of tasty green sprouts, 

Here’s how:

1. Choose a sunny spot and place the onion in the shallow hole and cover with soil.
2. Onions are super forgiving as far transplanting goes and in a few weeks you should be able to harvest the green sprouts as needed for cooking - allowing more sprouts to spring up

References

1. Growing onions (By Bonnie Plants). (2017, January 2). Retrieved February 7, 2024, from https://bonnieplants.com/blogs/how-to-grow/growing-onions
2. Onion sets. (2023, December 12). Almanac.com. Retrieved February 7, 2024, from https://www.almanac.com/plant/onion-sets
3. Toscano, K. (2023, March 21). How to plant and grow onions. Southern Living. Retrieved February 7, 2024, from https://www.southernliving.com/how-to-grow-onions-7369245