How to Plant Onion Sets, Seeds, and Transplants

The short answer

If you want onions this year with the least fuss, buy sets. A set is a small immature bulb grown last season and held dry over winter. It is already alive and ready to go, so it beats seed by weeks.

• Plant sets pointy end up, root end down. Getting this backwards stunts the plant.
• Plant about 1 inch deep, just barely covered. Deeper makes misshapen bulbs.
• Space 4 inches apart in the row, with 12 inches between rows.
• One set makes one onion. Plant 40 sets if you want 40 onions.
• Buy the right day-length type for your region or the bulbs stay small no matter what else you do.

Sets vs seeds vs transplants

All three grow the same onion in the end. They trade speed for control, and each one has a real downside worth knowing before you spend money.

Sets are the fastest and the most forgiving, and they are also the most likely to bolt. Bolting means the plant sends up a flower stalk instead of finishing the bulb. It happens because the set already lived through one cold season, and a second cold spell can convince the plant it is time to make seed. A bolted onion still tastes fine but it will not store. Eat those first.

Seed is the cheapest per plant and gives the widest variety choice, including the best storage onions. The cost is time. Direct sown seed needs the full 110 days to maturity, and in most regions that means starting seed indoors 8 to 10 weeks before you plant out so the crop bulbs before the days get too long.

Transplants are pencil-thin seedlings sold in bunches. They are the middle path: more variety choice than sets, less bolting than sets, and no indoor seed starting. They cost more per plant than either other option.

• Sets: easiest and fastest, fewest varieties, most likely to bolt.
• Seeds: cheapest and widest variety choice, but need the full 110 days plus an indoor head start.
• Transplants: middle path, no seed starting and less bolting, highest cost per plant.
• First time growing onions? Buy sets. Learn the crop for one season, then try seed.

One set makes one onion

This is the question people ask most and the answer is simple. One set grows one bulb. It does not divide into a cluster the way garlic splits into cloves or the way shallots multiply. Plant 40 sets, harvest about 40 onions.

Plan the count that way. Onion yield runs about 0.7 onions per planting spot once you allow for a few that bolt or rot, and a full-size bulb weighs roughly half a pound. So a 10-foot row at 4-inch spacing holds 30 sets and gives you somewhere around 21 keeper onions, about 10 pounds.

If you want green onions instead of bulbs, use the same sets but change the plan. Plant them about 1 inch apart and pull them young, while the tops are still slim and the base has barely swelled. You are trading one big bulb for one scallion, so plant extra.

• One set equals one bulb. There is no multiplying.
• Figure about 0.7 keeper onions per spot, roughly half a pound each.
• A 10-foot row at 4 inches apart holds 30 sets.
• For green onions, plant 1 inch apart and pull young instead.

How to plant a set, step by step

Pick sets that are firm and dry, about the size of a marble. Sets bigger than a quarter bolt more often, so if the bag has a mix, plant the big ones for green onions and save the small firm ones for bulbs.

• Work loose soil in a spot that gets 6 or more hours of full sun.
• Find the pointy end. That is the top, where the leaves come out. The flat scarred end is the root plate and goes down.
• Push the set in about 1 inch deep so the tip sits just at or barely under the soil surface. Sets go deeper than seed does, since onion seed is sown about half an inch down.
• Firm the soil around it with your fingers. Do not bury the shoulders of the bulb.
• Space sets 4 inches apart down the row, with 12 inches between rows.
• Water in once, then keep the bed evenly moist while the leaves grow.

Long-day vs short-day, the mistake that costs a whole crop

This is the highest-value thing on this page. Onions decide when to start forming a bulb based on day length, not on the calendar and not on how big the plant is. Every variety has a trigger, and if your daylight never reaches that trigger, the plant just keeps making leaves and you dig up something the size of a golf ball.

Long-day onions need about 14 to 16 hours of daylight, so they are for northern gardeners. Short-day onions bulb at about 10 to 12 hours, so they are for the South, roughly Atlanta and below. Day-neutral, sometimes labeled intermediate, bulb at 12 to 14 hours and work through the middle of the country. Day-neutral is the safe pick if you are not sure.

The bag or the seed packet says which type it is. If it does not say, do not buy it. This is the single fact that decides whether you harvest onions or scallions.

Red onions are not a separate case. Red, yellow, and white are just color. Reds come in long-day, short-day, and day-neutral versions exactly like the others, and you plant a red set the same way: pointy end up, 1 inch deep, 4 inches apart. Check the day-length label on reds the same as any other onion.

• North: buy long-day, bulbs at 14 to 16 hours of daylight.
• South: buy short-day, bulbs at 10 to 12 hours.
• Middle, or unsure: buy day-neutral, bulbs at 12 to 14 hours.
• Red onions follow the same rule and the same planting depth as yellow and white.

When to plant and get your date

Onions want to be in the ground early, while nights are still cool, because the plant needs to build as many leaves as it can before day length triggers bulbing. Every leaf becomes a ring. More leaves before the trigger means a bigger onion.

Count backward from maturity. A bulb onion takes about 110 days, so the planting window is tied to your last frost date and your region, not to a fixed week in March. Sets handle a light frost fine, which is why they can go out early.

The planner takes your ZIP code and gives you the actual planting date for onions in your yard, along with the row feet you need at 4-inch spacing. Once they are in the ground, the growing side of the crop is covered in the full onion guide below.