Onions - Short Day/Long Day & Bunching

Short Day vs Long Day vs Bunching

Short Day onions - Bulb creation begins when days reach 10-12 hours per day, it’s important to note that if you have a short day variety and are starting indoors, you MUST keep your grow lights on no more than 8 hours per day to prevent premature bulbing.
Long Day onions - Bulb creation occurs at 14-16 hour days, which only happens above the 35th parallel. If you grow Long Day onions below this line, your onion will flower before ever trying to bulb. Bunching onions - Similar to garlic chives or leeks, these are designed to have no true bulb, so location is unimportant.

There are also day-neutral (intermediate) varieties that require 12-14 hours of daylight to set a bulb. These are a bit harder to find, but are well suited to the area right around the 35th parallel.

Planting seeds vs buying sets

You will always have larger onions by starting from seed. Sets are stunted bulbs that have been pulled and have gone dormant. Because of the stunting, they will waste energy re-establishing roots instead of growing large leaves and forming a proper bulb.

Starting your onion seeds

If you are in zones 8a or lower, start your onions in the ground once temperatures start to cool off - onion seeds take weeks to establish, so you want to be early enough that a freeze won’t kill them (4-6 weeks before first frost date is recommended). If you’re above 8a, your seeds may not have enough time to properly establish before winter, but you can try planting fairly immediately with a few seeds to let them establish. Plant your seeds 2-3” apart, and once the seedlings are about 4” tall, thin them to 6” apart. Onions can have mulch added to them, but don’t need much tending over winter, except to ensure that the soil is not overly dry if there is no regular rainfall.
If you’re choosing to hang on to your seeds, you will essentially establish your own sets and transplant them once your soil is workable in spring. To do this, you can either start them inside under grow lights (again, be sure to keep your grow lights on for LESS than the bulbing hour requirements, so 8 hours for short day and no more than 12 for long day onions), or you can start them with winter sowing.
For indoor planting, set up a growing tray with sterilized potting soil (or you’ll have soil gnats all winter) and ideally, have your grow lights on an adjustable hanger and a timer. As your plants grow, you can lift the light higher to help them grow stronger. If your tray has cells, plant 1-2 seeds per cell and thin when the sprouts (they look a bit like grass) are at least 2” tall. If your tray does not have cells, you can scatter the seeds or place them an inch or 2 apart (the further apart, the less thinning you will need to do later). For winter sowing, you will need a plastic lidded container and potting soil. You’ll start your winter sowing between December and February, so that you will have established plants by the time the soil is workable in spring. Ensure that your plastic container is not full of holes like a strawberry container, but you will need to poke a few holes in the bottom for drainage and a few in the top for aeration. Plant your seeds 1-2” apart and water. Place your container in a shady area outdoors off the ground, such as on a table or bench. You do not need to worry about your outdoor container freezing until you see sprouts. Once there are sprouts, you will need to ensure that the container soil is moist, but not saturated, and that the seedlings are protected. If it’s warm, open the lid, and close it at night. If it freezes, cover the container overnight with a blanket.
Once the soil in your area is workable, you can move your seedlings out to the garden. Find a sunny area with good drainage! Give them a good 6” of space and ensure that your soil is free of stones and other debris. Be sure to fertilize while planting, and again once bulb development starts.
Onion leaves can be used in cooking, but it’s usually best to leave bulbing onions alone. Bunching onions are better harvested for their greens, and will regrow after being cut. For regular onions, it can be quite awhile before the onion is ready to harvest. You can technically pull any bulb that is of a usable size, but for the best flavor, pull onions that have had their stalks fall over. Usually they have turned brown and this happens in summer. Onions and garlic need time to dry, so once you harvest, if the next few days will be sunny and warm, you can make a braid of the stalks and hang the onions out to dry on a porch. If there will be rain after the harvest, hang them inside and it may take some extra time to dry. Do not wash your onions before they’ve had a chance to dry out!