Tips for starting onions from seed indoors

Growing any plant from seed is certainly the most rewarding - watching it grow from practically nothing into an edible harvest - nothing quite beats that feeling! So why is it that one of our most popular kitchen staples, the onion, rarely gets this treatment from gardeners and gets grown from bulbs most of the time instead? Onion-lover and growing fanatic, Everest Fernandez, shows us how to start onions from seed indoors.

Why Start Onions From Seeds Anyway? 

Onion seedsPerhaps the best thing about starting onions from seed (apart from the reduced cost—especially if you saved the seeds yourself!) is the hugely improved range of varieties on offer from the seed companies. But don't get too carried away! Only purchase enough seed for the next growing season. Onion seeds are best used fresh. If they are under a year old they should germinate well. You can keep onion seeds for two or even three years but germination rates really take a dive after that.

Walla walla onion seedlingsWhen Should I Start?

You can get at least an 8 - 12 week headstart on the outdoor onion growing season by starting your onion seeds under a T5 fluorescent grow light. Obviously, starting onions from seed rather than starting from bulbs requires a little extra time anyway. In colder areas in the northern hemisphere you can get started in December (hey it gives you something to do over the winter months) to have seedlings ready to transplant into the garden in early Spring. You can also get Japanese (overwintering) varieties started in August when the intense heat in warmer areas can make starting seedlings difficult. 

What Should I Start Them In?

Any standard seedling potting mix is fine. Avoid anything with too much nitrogen at first - your onions only require lots of this later! For the first few weeks it's all about roots and starting to build a bulb. I like to mix in some vermiculite (1 part vermiculite, 4 parts potting mix) as this aids in nutrient and moisture retention. Fill flats or cell trays with a well moistened mix - it should be quite wet without being sludge - sprinkle seeds generously and top off with a quarter inch of wet vermiculite. The top layer of wet vermiculite is like a duvet for the seeds - it keeps moisture and temperature levels constant and certainly increases germination rates.

What is the best growing environment for onion seeds?

Find a cool spot in your house like a basement or garage. Use a propagator lid on top of your cell tray or flat to help raise relative humidity levels. Lift it once each day for a quick visual inspection. Onion germination is most fast and reliable in the low 70s fahrenheit. A commonly accepted range is 68-75°F (20-24°C). Use a heat mat to keep the bottom of your propagator but discontinue use once the majority of the viable seeds appear to have sprouted.

What about light cycles? Do onions prefer long or short days?

As a general rule of thumb, you should set your T5 fluorescent grow lights on to 12 hours a day to start onion seeds. Keep your T5 fluorescent grow light raised up at least a foot or so above the propagator - monitor temperatures inside the propgator using a min/max thermometer with a remote probe. Don't let it get too warm in there! If temperatures are higher than the mid 70s, try raising your lights or discontinuing use of your heat mat.

How long will it take for the seeds to germinate?

1 - 2 weeks. If you don't see anything after three weeks you're either getting something wrong or your seeds were old bunk. Did you forget the heat mat? That really helps!

My seedlings are up! What now?

Remove the propagator lid. Switch off heat mat. Try lowering your lights just a little, up to a few inches above the plants. It's okay to trim your onions back a little - after all, it's all about the bulb! Don't let them flop about all over the place. Pull away any lingering seed husks - they can weigh down the emerging seedling making it appear like a small green hoop. Be careful to maintain moisture levels in the propagation media as this will dry out quicker now that the propagator lid has been removed.

My flats are starting to look a little crowded. When should I transplant my onion seedlings outdoors or to larger containers?

Wait for three leaves to emerge then move to containers (or "six-pack" transplant trays) that allow 4 inches (10 cm) of depth—that's pretty deep for a small, juvenile onion plant but they will thank you for it!

Don't forget to ask your questions or comment below! Feel free to give shout-outs to your favorite onion seed suppliers too!