How to Plant Onion Sets: A Grower's Journey from Bulb to Harvest

Onion sets might just be the most underappreciated shortcut in vegetable gardening. While seed-starting purists debate germination temperatures and transplant timing, those little dormant bulbs sit quietly in garden centers, ready to transform into full-sized onions with minimal fuss. After spending years wrestling with onion seeds that seemed determined to disappoint, I discovered that sets offer something precious: reliability without sacrificing quality.

Understanding What You're Actually Planting

An onion set isn't just a small onion—it's a juvenile bulb that's been interrupted mid-growth, dried, and stored in a state of suspended animation. Picture a teenager frozen in time, ready to resume growing the moment conditions improve. These sets carry within them all the genetic programming and stored energy needed to complete their life cycle, which explains why they're so forgiving compared to seeds.

The beauty lies in their simplicity. Each set has already survived the vulnerable seedling stage, developing enough structure to withstand typical garden challenges. When you plant a set, you're essentially fast-forwarding through the most precarious weeks of an onion's life.

Timing Your Planting Like a Local

Forget the generic advice about "planting after last frost." Real onion success comes from understanding your specific microclimate and working with it, not against it. In my zone 6 garden, I've learned that onion sets tolerate cold far better than most gardening books suggest. I've planted them when morning frost still painted the grass white, and they thrived.

The soil temperature matters more than air temperature. Once your soil consistently stays above 35°F at a two-inch depth, those sets will wake up and start growing. In practical terms, this means most gardeners can plant 2-4 weeks before their average last frost date. Southern gardeners might plant in February, while those in Minnesota might wait until late April.

Here's something rarely mentioned: fall planting works brilliantly in mild winter areas. Plant sets in October or November, and they'll establish roots through winter, then burst into growth come spring. The resulting onions often outsize their spring-planted cousins.

Soil Preparation That Actually Makes a Difference

Onions are shallow feeders with stringy root systems that struggle in heavy clay or compacted soil. But here's where conventional wisdom often goes wrong—you don't need to create some perfect, fluffy bed twelve inches deep. Onion roots rarely penetrate beyond six inches.

What matters is the top layer. Work in compost or aged manure to improve soil structure, but don't go overboard. Too much nitrogen produces lush tops and disappointing bulbs. I learned this the hard way after enthusiastically amending with fresh chicken manure one spring. The resulting onion forest looked impressive until harvest revealed bulbs barely larger than the original sets.

A soil pH between 6.0 and 7.0 works well, though onions tolerate slight variations. If your soil tends acidic, a light dusting of lime mixed into the top few inches helps. But honestly? Unless your pH is wildly off, onions will probably grow fine without adjustment.

The Art of Spacing and Depth

Plant spacing triggers more debate than necessary. Commercial growers space sets 4-6 inches apart because they're optimizing for total yield per acre. Home gardeners can afford to be generous. Give each set 6 inches in all directions, and you'll grow larger individual onions with better air circulation.

Planting depth matters more than spacing. Push sets into prepared soil until just the pointed tips peek above ground—about one inch deep in heavy soil, slightly deeper in sandy soil. Planting too shallow leads to frost heaving and bird theft. Too deep delays emergence and can cause rotting in wet conditions.

Some gardeners insist on planting sets pointy-side up. Others claim it doesn't matter. After conducting my own informal experiments over several seasons, I've concluded that orientation affects emergence timing by a few days at most. Sets planted sideways or even upside-down eventually right themselves underground.

Watering Wisdom Beyond the Basics

Onions need consistent moisture but despise waterlogged soil—a frustrating combination for new gardeners. The solution isn't complicated: water deeply but infrequently, allowing soil to partially dry between irrigations.

During bulb formation (usually 60-90 days after planting), onions become particularly thirsty. This critical period determines final bulb size. I've found that one inch of water weekly, delivered in a single deep soaking rather than daily sprinkles, produces the best results.

Here's an insider trick: stop watering when tops begin falling over naturally. This signals the onion's transition from growing to curing mode. Continued watering at this stage leads to soft bulbs that won't store well.

Feeding for Bulb Development

Onions respond dramatically to proper nutrition, but timing matters as much as type. Early in the season, nitrogen drives leaf growth. Those leaves later feed bulb development through photosynthesis, so don't skimp on early feeding.

I side-dress with blood meal or fish emulsion every three weeks until mid-season, then switch to a balanced organic fertilizer. The transition coincides with day length—when days exceed 14 hours, most onion varieties shift focus from tops to bulbs.

Avoid high-nitrogen fertilizers after this shift. Late nitrogen produces thick necks that refuse to cure properly, ruining storage potential. If you notice exceptionally dark green, lush growth in midsummer, you're probably overfeeding.

Managing Pests Without Panic

Onion sets face fewer pest problems than many vegetables, but they're not immune. Onion maggots pose the biggest threat in many regions. These fly larvae tunnel into developing bulbs, causing seemingly healthy plants to suddenly yellow and collapse.

Prevention beats treatment. Covering beds with floating row cover immediately after planting blocks egg-laying flies. Remove covers once hot weather arrives—maggot activity drops off above 80°F.

Thrips, those tiny insects that leave silver streaks on leaves, rarely cause serious damage to home garden onions. Commercial growers spray because cosmetic damage affects marketability. Your onions will taste identical whether pristine or thrip-scarred.

Companion Planting Truths and Myths

Garden folklore assigns magical properties to onion companion planting. Supposedly, onions repel aphids, rabbits, and various other pests. In reality, the effects are subtle at best. I've watched aphids colonize plants directly adjacent to onions without hesitation.

What does work: interplanting onions with crops that mature at different rates. Lettuce grows quickly between onion rows, harvested long before bulbs need the space. Carrots and onions make excellent neighbors, their root systems occupying different soil depths without competition.

Avoid planting onions near beans or peas. This isn't superstition—alliums can inhibit nitrogen-fixing bacteria that legumes depend on. Keep them separated by at least a few feet.

Harvesting at Peak Perfection

Knowing when to harvest separates good onion growers from great ones. Don't wait for all tops to fall—begin harvesting when about half have toppled naturally. Waiting longer risks bulbs splitting or sprouting, especially after summer rain.

Gently lift bulbs with a fork, avoiding bruising. Brush off loose soil but resist the urge to remove papery outer layers. Those layers protect bulbs during curing and storage.

Cure onions in a warm, dry, ventilated area for 2-3 weeks. I spread them on old window screens propped on sawhorses in my garage. Properly cured onions develop tight necks and papery skins that rustle when handled.

Storage Secrets from Old-Timers

Traditional storage methods still work best. Braid cured onions together if you're feeling crafty, or simply store in mesh bags or old pantyhose. The key is air circulation—never store onions in plastic.

Temperature matters more than most gardeners realize. Onions store longest at 32-40°F with 65-70% humidity. Few homes maintain these conditions, but an unheated garage or basement often comes close. At room temperature, expect 1-2 months storage for most varieties.

Sweet onions store poorly regardless of conditions. Eat these within weeks of harvest. Storage onions, typically with higher sulfur content and stronger flavor, last months when properly cured.

Varieties Worth Growing

Not all onion sets are created equal. 'Stuttgart' remains my favorite for reliable production and storage. 'Red Baron' adds color to the garden and plate. 'Candy' produces impressively large sweet onions, though they won't keep long.

Day length sensitivity affects variety choice more than climate. Short-day onions bulb when days reach 10-12 hours, suitable for southern gardens. Long-day varieties need 14-16 hour days, performing best in northern regions. Intermediate-day onions split the difference.

Many garden centers sell generic "yellow onion sets" without variety names. These usually perform adequately but rarely excel. Whenever possible, seek out named varieties suited to your day length.

Beyond Basic Growing

Once you've mastered standard onion growing, experiment with techniques that push boundaries. Succession planting every 2-3 weeks extends harvest through summer. In mild climates, a fall crop planted from sets in late summer provides fresh onions when stored bulbs run low.

Try growing onions in containers—a 5-gallon bucket holds 4-5 sets comfortably. This works especially well for gardeners with poor native soil or limited space. Just ensure adequate drainage and consistent watering.

Some gardeners swear by hilling onions like potatoes, gradually mounding soil around stems as they grow. I've tried this technique with mixed results. While it doesn't seem to increase bulb size significantly, hilled onions do stand more upright and suffer less from soil splash during rain.

Growing onions from sets might lack the bragging rights of seed-starting, but it delivers something more valuable: consistent success. In a world where gardening often feels like gambling against weather and pests, onion sets offer reassuring predictability. They transform novices into successful growers and give experienced gardeners one less thing to worry about during busy spring planting season.

Authoritative Sources:

Brewster, J.L. Onions and Other Vegetable Alliums. 2nd ed., CABI Publishing, 2008.

Schwartz, Howard F., and S. Krishna Mohan, editors. Compendium of Onion and Garlic Diseases and Pests. 2nd ed., American Phytopathological Society, 2008.

United States Department of Agriculture. "Onions: Planting, Growing, and Harvesting Onions." USDA National Agricultural Library, nal.usda.gov/legacy/afsic/onions-planting-growing-and-harvesting-onions.

Voss, R.E., et al. "Onion Production in California." University of California Agriculture and Natural Resources Publication 7242, 2013, anrcatalog.ucanr.edu/Details.aspx?itemNo=7242.