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Onions

Allium

Onion (Allium cepa)

Onions are "photoperiodic" plants, meaning they decide when to start making a bulb based on how many hours of daylight they receive. This is the single most important secret to onion success: matching the variety to your latitude.


The Three "Day-Length" Categories

Before you pick a variety, you must know your "Day Length."

  1. Short-Day: Best for the South. They start bulbing when days are 10–12 hours long.

  2. Intermediate-Day: The "Day-Neutral" crowd. They work almost anywhere, bulbing at 12–14 hours.

  3. Long-Day: Best for the North. They need 14–16 hours of sun to trigger bulb growth.


Setting the Stage for Success

Soil & Fertilizer

Onions are "leaf first, bulb second." The more leaves you grow in the spring, the bigger the bulb will be in the summer.

  • The Soil: Use a loose, "fluffy" potting mix. If the soil is too packed, the bulb can't expand.

  • Feeding: Onions are Nitrogen-obsessed. Feed them every 2 weeks with a high-nitrogen liquid fertilizer until the ground starts to bulge—then stop! Adding nitrogen too late can prevent the onion from curing properly.


The "Shoulder" Depth

When planting your sets, do not bury them deep like a tulip bulb. Only bury them about 1 inch deep. As they grow, the "shoulders" of the onion should actually be visible above the soil line.


Maintenance, Trimming & Troubleshooting

The "Scallion" Harvest

If you are impatient, you can pull any onion variety early (when the base is about the thickness of your thumb). At this stage, they are "spring onions"—tender, sweet, and perfect for the grill.


Troubleshooting

  • Bolting: If your onion sends up a tall, stiff flower stalk with a round bulb at the top, it has "bolted." This usually happens after a sudden cold snap. Pull it and eat it immediately; a bolted onion will not store well and the bulb will stop growing.

  • Onion Maggots: These tiny larvae can tunnel into the bulbs. Using fresh, bagged potting soil in your containers almost entirely eliminates this risk!


Harvesting & The "Flopping" Signal

How do you know when an onion is done? It will tell you.

  1. The Faint: When the onion is mature, the green neck will weaken and the tops will flop over onto the soil while still green.

  2. The Cure: Once 50% of the tops have flopped, pull them out. Let them sit in a dry, shady, breezy spot for 2 weeks until the outer skins are "papery" and the roots are brittle.

  3. Storage: Cut the dried tops off (leaving 1 inch) and store in a cool, dark place. Sweet onions (like Walla Walla) only last a few weeks; pungent onions (like Stuttgarter) can last all winter.


Pro-Tip: The "Leaf Count" Rule

Every leaf on an onion plant represents one "ring" inside the onion. If you want a massive, jumbo onion, focus on getting at least 10 to 12 healthy green leaves onto the plant before the summer solstice. More leaves = more rings = bigger bulbs!

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