Ten Top Tips for Growing Onions from Seed

9 March 2012, written by Barbara Pleasant

In the past year, both Ben Vanheems and I have written about the virtues of growing onions from seed. We left out many small details, which I've been keeping track of in recent weeks as I've been growing onions from seed myself. Please bear in mind that I am an onion maniac, and I have no explanation for my obsession with growing onions and shallots from seed to table year after year. I am driven to do it, which explains the evolution of these ten guidelines for growing onions from seed.

1. Start with fresh seed and seed starting mix

Onion seeds can be kept for two years in a cool, dry place such as a properly monitored seed storage box, but fresh first-year onion seeds germinate best. Germination rates fall as the seeds age, and though I have had three-year-old onion seeds that sprouted well, germination is always highest with fresh seeds. I also purchase a fresh bag of seed-starting mix each spring as a safety precaution against soil-borne diseases. My onion seedlings stay in pots for up to 10 weeks, so I like to use a soil medium that's unlikely to host diseases.

2. Provide bottom heat

The tops of my fluorescent plant lights are flat, and they give off just the right amount of bottom heat needed to help onion seeds germinate quickly. Onion germination is fastest at 68-77°F (20-25°C), with slight temperature drops at night. Before I started using the top of my plant light, I found that the top of the refrigerator was a sufficiently warm place to germinate onions. I enclose newly seeded containers in a plastic bag to maintain moisture, provide them with bottom heat, and the onion seeds germinate in about 8 days.

3. Clip off seed husks

Soon after germination occurs, my indoor-grown onion seedlings often do a poor job of pulling away from the seed. The stuck seedling looks like a little green loop. Eventually the smaller end pulls out of the soil, weighted down by the black remains of the seed husk. To shorten this drama and encourage the seedling to get on with growing into an upright onion, I often "rescue the chick" by snipping the loop in half, pulling out the smaller end, and discarding it.

4. Keep under lights and trim back

At this point growing onions from seed requires bright supplemental light, which I provide with a two-bulb florescent fixture. I keep the lights on my onions for 12 hours a day, and position them within 1 inch (2.5 cm) of the bulbs. Every day or so I trim the onions back to about 3 inches (7-8 cm) tall, which keeps them from falling over. Use scissors to get a clean cut.

5. Transplant when the third leaf appears

When an onion seedling has three leaves, I gently transplant to containers that are at least 4 inches (10 cm) deep. In my experience, onion seedlings that are given plenty of vertical root space grow much more rapidly than those confined to shallower quarters. They may not be pretty, but containers made from the bottoms of milk cartons or large paper cups house the majority of my adolescent onion seedlings.

6. Trim off seeding leaves

Soon after transplanting, my onions often shed the seedling leaf (sometimes called the flag leaf). I snip it off and remove it to keep the containers tidy. Onion seedlings respond to transplanting by making strong upright growth, which I keep trimmed back to about 5 inches (12 cm) high.

7. Provide maximum light

The best light of all is found outdoors on sunny days, so my adolescent onions get moved outdoors whenever we get warm sun. I shield them from wind by using my cold frame or a plastic-covered tunnel for my onion day care centre. They love it!

8. Harden off, but protect from chilling

Eventually the onion seedlings stay outdoors 24/7, provided temperatures are well above 46°F (8° C) at night. Consistent exposure to temperatures below 50°F (10°C) for more than 10 days can cause onions to bolt rather than producing big bulbs.

9. Band composted fertiliser in the planting trench

Onion roots are concentrated in the area just below the bulbs. As I prepare the planting bed, I make deep planting trenches and line them with rich compost and/or composted organic fertiliser. This technique, called banding fertiliser, places a cache of organic riches where the onions can utilise it early on, during the plants' most active period of growth. Providing bioactive compost is important when growing onions, because onions take up much of the phosphorous they need through partnerships with soil-borne mycorrhizal fungi.

10. Sow leafy greens between rows to suppress weeds

Once my onion seedlings are in the ground, the war with weeds gets into full swing. Skinny onions are notoriously poor competitors with weeds, so they require a certain level of tending. Each year I find myself interplanting more rocket, lettuce and other leafy greens among my little onions grown from seed. Employed as smother crops, salad greens are much more fun to pull compared to weeds, and onions grown from seed seem to like their company.

By Barbara Pleasant