Onion Sets or Slips?

How Onions Grow

• Onions can be grown in U.S. Department of Agriculture plant hardiness zones 3 through 9. Botanically, onions are a biennial, producing a leafy stalk and bulb in both of two growing seasons. During the second season, the mature plant blooms and goes to seed, starting the cycle again. To maximize harvest, onions are usually treated in the home garden as an annual, although the occasional "volunteers" left behind can be grown through a second season.

Onion Slips

• "Slip" is a word often used as a shorthand for seedlings, and the thin green shoots characteristic of seed-grown onions look like mere slips of vegetation. Slips can be started indoors 10 to 12 weeks before your last frost date, then transplanted into the garden as early as two weeks before your last frost date. Soil temperature should be 50 degrees Fahrenheit or above. Onions get a good start in cool weather, which is why scallions are often called spring onions. If you plant slips 2 to 3 inches apart in rows 6 to 12 inches apart, you can thin them to 6 inches apart in a few weeks, pulling out scallions for immediate use while leaving some slips to turn into large onions. Plant them 1 to 1-1/2 inches deep so they do not topple as they grow.

Onion Sets

• Onion slips can remain closely spaced throughout the growing season, which means they will form small bulbs. These are dried and stored as onion sets. The following spring, they can be planted at the same time as slips and within a few weeks following. Because sets are beginning the second year of biennial growth, they are less likely to form large bulbs. Instead, when temperatures warm, they are likely to bolt, bloom and go to seed. If you hope for some big onions from sets, choose the smaller bulbs and barely cover them with soil. Deeper-planted sets make great scallions but can be sensitive to diseases carried in wet soil as the season progresses.

Choosing the Right Onions

• In addition to choosing between slips and sets, you also need to choose the right type of onion for your climate conditions. In USDA plant hardiness zone 7 and warmer, you should choose a short-day variety. In a zone cooler than 7, select among long-day varieties. While there are many varieties for your growing conditions, you are likely to find a greater breadth of choices of onion slips. Sets tend to come in limited choices, and some vendors may only offer them by color: red, white or yellow, with no varietal label at all.