Spring is just around the corner, and that means it’s onion planting time. Onions are one of the earliest crops we plant in the garden. They’ll tolerate the spring frosts, and they need a chance to put on some growth before the days get long. Onions planted too late often look fine but form small bulbs or none at all.  

For most gardeners, onion slips offer the best combination of ease and reliable large harvests. If you’ve struggled to grow large onions, or aren’t sure what the difference is between onion slips and sets, we’ve got some tips below to help out.

What is an onion slip?

Onion terminology can get confusing. Onions can be grown from slips, seeds, or sets. Choosing the right method will help you achieve the results you want in your onion patch.

Onion slips or onion plants are young onions started from seed during the current year, and typically sold in bunches for transplanting into your garden. You can tell onion slips apart from onion sets because onion slips have green tops. They actually look like the slender green onions we buy at the grocery store. They’ll also have little roots on the bottom. Onion slips are the most reliable way to harvest large-bulbed onions from your garden, provided you give them proper care. 

Onion seeds are just what they sound like. Growing onions from seed is very inexpensive, but takes a bit more time and labor. Onion seedlings grow rather slowly, so it’s common to sow onion seeds indoors about 10 weeks before your transplant date. Onion seeds lose viability quickly, so buy new onion seeds each year. 

Onion sets look like miniature onions. They are grown from seed the previous year, harvested, and stored for planting the following spring. Onion sets are easy to plant and a great way to grow green onions for harvest, but they often won’t produce large bulbs. Since it is their second year and onions are biennials, many gardeners have trouble with onion sets bolting and flowering rather than forming big onions.

Long-day varieties in the North, Short-day onions in the South

When choosing onion varieties, you’ll see descriptions such as long day, short day, and intermediate (sometimes called day-neutral). These terms refer to the amount of daylight those onions need to trigger bulb formation. 

• Long-day onions won’t start to form bulbs until they receive 14 hours or more of daylight.

• Short-day onions will bulb when they receive 10-12 hours of daylight.

• Intermediate or day-neutral onions bulb when they receive 12-14 hours of daylight. 

Standard advice is for Southern gardeners to grow short-day onions, and those in the North to grow long-day onions, and that’s true. Summertime day length, or simply the time between sunrise and sunset, is longer in the north and shorter in the south. For onions, the daylength or photoperiod is what triggers them to shift from vegetative growth (more and larger leaves) to storing energy in the bulb and growing a bigger bulb. 

If you see an onion described as able to grow anywhere, it’s probably an intermediate day onion, meaning a compromise between long and short day varieties. 

The amount of daylight up North is far in excess of what a short-day variety needs, so why can’t a northern gardener grow short-day varieties? The answer lies in how early in the season the 10-12 hour threshold is reached. 

Onions will start to bulb when the length of daylight triggers the change, regardless of how long they’ve been growing. For short-day varieties in the North, the daylight triggers bulbing while the plant is still quite small, often with only a couple of leaves. Without sufficient leaf structure to support growth, the bulbs end up small and disappointing. In much of the South, day length doesn’t reach the 14–16 hours that long-day onions need, so they rarely form full-size bulbs.

Choosing a good site for onions

All onions like full sun, and for the best and largest bulbs, finding a spot with at least 8 hours of sunshine per day is important. If you consider the shape and size of an onion plant, there isn’t a lot of leaf surface, but we want it to make a large onion bulb. If the plant is going to harvest enough energy from the sun to make the firm, big onion we hope for, it’s going to need a lot of sunshine time.

Drainage is also important for onions. A good onion site is well-drained and fertile. A sandy loam or loam soil is perfect. If your soil is heavy clay, you may need to grow onions in a raised bed. Heavy soils can restrict the growth of onion bulbs, as well as cause problems with onion root rot and soil-borne diseases.

Planting guidelines for onion slips

Onion slips are fast and easy to transplant. Since they are frost-tolerant, plant them as soon as the soil can be worked in spring. Normally, that’s several weeks to a month before your last frost date. In the warmer regions of the South, gardeners can also plant a fall crop of onions.

• Prepare the bed. Remove rocks, weeds, and loosen the soil with a garden fork. Incorporate a layer of compost, and smooth the surface with a garden rake.

• Lay out the spacing. Follow the recommendations for spacing between plants and rows for the variety you’ve chosen. It’s easy to count out how many slips you have and make a plan based on your space. 

• Fifty onion slips could be planted in four rows of 12-13 plants each, or three rows of 16-17 plants each. 

• Most onions do well when spaced 4-6 inches between plants, with rows 12-18 inches apart. 

• A few sticks and some string to mark the rows can help keep things organized.

• Onion slips should be planted about 1 to 1.5 inches deep. The white or red part should be in the soil, but not the neck or leaves. If your soil is loose, you can probably make a hole with your finger, drop in the onion slip, and firm up the soil around it.  For a quick reference, most people’s fingers are around one inch to the first joint, and near two inches to the middle knuckle. 

• Gently but thoroughly water in your newly planted onion slips. 

• Mulch around the slips to keep weed pressure down and prevent the soil from drying out too quickly. Onions like well-drained soil; they have shallow roots and need adequate soil moisture in the upper layers.

Growing onion slips into great onions

The best onion slips in the world won’t make a fat, tasty onion without proper care. Besides sunshine, pay attention to soil moisture, weeds, and fertilizer.

Soil Moisture

Keeping up with irrigation is important. Onions have shallow root systems, and in hot, sunny weather, the top layer of soil can dry out quickly. While other plants can draw from water farther down, onions can’t. Check the soil with your finger. If it’s cool and damp, the onions are happy. If the soil feels warm and dry, it’s time to water.

While onions can be watered with a hose and watering head, or a watering can, you may find it easiest to set up a drip line with a timer. When the onions need a drink, hit the button and let the irrigation water run for an hour, or until moisture penetrates as deep as your finger can reach.

As the onions mature and harvest approaches, you’ll see the tops begin to tip over. When half of the plants have bent-over tops, stop watering the onions. 

Weed Pressure

Keep weed pressure low. This is where a good layer of mulch is worth its weight in gold. Onions don’t do well with competition from weeds. Newly created beds can spring forth in a carpet of annual weeds in late spring, and if those get too established, removing them can be a chore. It’s also hard to pull larger weeds without disturbing the onion roots. 

If you haven’t mulched, use a stirrup hoe or a wire hoe to cut those weeds off just below the soil surface. Take care not to bump the onions or break the leaves. 

For mulch, a two-inch thick layer of clean grass clippings works wonderfully. Don’t use grass mulch from your yard if you spray weed killers, or if your neighbor does and it drifts onto your yard. Also, avoid grass clippings from yards treated with Weed N Feed-type products. Shredded leaves and pine straw also work well as onion mulch.

Fertilize

Onions are heavy feeders, so you may need to fertilize to get the best yields. After transplanting, when the onions have started to perk up and look healthy, it’s time for their first fertilizer. A balanced product, like a 5-5-5 or a 10-10-10 (check this article for a refresher on fertilizers), is fine. What we’re looking for is some nitrogen to encourage vegetative growth. More leaves and larger leaves equal bigger onions. 

As the plants grow, continue monthly fertilizer applications. It’s okay to use a balanced fertilizer for these feedings as well (who wants to have a bunch of different products on the shelf?), but many gardeners prefer to use a low nitrogen product for later feedings. When tops begin to bend over, stop fertilizing.