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

I've been planting onions for nearly two decades now, and I still remember the first time I held those tiny onion sets in my palm, wondering if these dried-up looking bulbs could really transform into the plump, papery-skinned beauties I saw at farmers' markets. Spoiler alert: they absolutely can, and the process is surprisingly forgiving once you understand what these little powerhouses actually need.

Onion sets are essentially baby onions that were grown from seed the previous year, then harvested when they're about the size of a marble. Someone else has already done the hard part for you – coaxing those microscopic seeds into actual bulbs. What you're getting is basically a head start, a shortcut that shaves months off your growing time. It's like adopting a teenager instead of raising a baby from scratch.

The beauty of working with sets lies in their eagerness to grow. Unlike seeds, which can be finicky about temperature and moisture, sets are already primed and ready. They've got stored energy packed inside, just waiting for the right conditions to explode into growth. I've seen sets sprout in my garage when I've forgotten about them – that's how determined they are to become full-sized onions.

The Art of Timing Your Planting

Here's something most gardening books won't tell you straight out: onions are photoperiodic, which is a fancy way of saying they're obsessed with daylight hours. They'll grow leaves when days are short and start bulbing when days get longer. This isn't just botanical trivia – it's the key to understanding when to plant.

In my zone 6 garden in Ohio, I plant my sets as soon as I can work the soil in spring, usually sometime in early April. But I've got a neighbor who swears by fall planting for certain varieties. The truth is, onions are tougher than most people think. I've had sets survive surprise snowstorms that would make tomatoes weep.

The old-timers in my area say to plant onions when the spring peepers start singing – those tiny frogs that herald the end of winter. It's not scientific, but after years of following this advice, I've found it remarkably accurate. The soil temperature should be around 50°F, but honestly, I rarely check anymore. When the earth smells ready and crumbles in your hand instead of clumping, it's time.

Preparing Your Plot: More Than Just Digging

Onions are like that friend who seems easygoing but actually has very specific preferences. They want loose, well-draining soil with a pH between 6.0 and 7.0. I learned this the hard way my third year gardening when I planted sets in heavy clay that hadn't been amended. The resulting onions were stunted, misshapen things that looked more like shallots having an identity crisis.

Now I prepare my onion beds in fall, working in a generous amount of compost. Not fresh manure – that's asking for trouble. Fresh manure promotes lush leaf growth at the expense of bulb development. You'll end up with onions that are all top and no bottom, which sounds like a bad fashion choice and makes for disappointing harvests.

I've developed a ritual of adding a handful of bone meal to each planting area. Some gardeners think this is overkill, but I've noticed my onions develop stronger root systems with this phosphorus boost. The difference might be psychological, but gardening is as much about faith as it is about science.

The Physical Act of Planting

When you're ready to plant, resist the urge to bury those sets deep. This isn't a tulip bulb that needs protection from freezing. Onion sets should be planted just deep enough that the tip is barely visible above the soil surface – think of it as giving them a snorkel.

I space mine about 4 inches apart, though I've been experimenting with closer spacing lately. The conventional wisdom says wider spacing equals bigger bulbs, but I've found that slightly crowded onions still size up nicely if you keep them well-fed and watered. Plus, I'd rather have more medium onions than fewer large ones – they store better and are more practical for everyday cooking.

Make your rows about a foot apart. Any closer and you'll curse yourself come weeding time. I learned this during my ambitious phase when I tried to maximize every square inch of garden space. Crawling between too-tight rows while trying to distinguish young grass from young onion shoots is a special kind of torture.

Push each set into the prepared soil with a firm but gentle pressure. You want good soil contact but not compaction. I use my index finger, creating a small depression, placing the set, then firming the soil around it. Some gardeners use dibbers or special tools, but I find the tactile feedback of fingers helps me gauge the soil condition better.

The Growing Season: Patience and Observation

Once planted, onions begin their deceptively simple journey. For the first few weeks, you might wonder if anything's happening. Then suddenly, green shoots appear, growing with surprising vigor. This is when I get excited – it's like watching a slow-motion fireworks show.

Water is crucial during this early growth phase. Onions have shallow root systems, which means they can't reach deep for moisture like tomatoes can. I aim for about an inch of water per week, including rainfall. During dry spells, I've been known to stand in the garden with a hose, moving slowly down the rows, thinking about nothing in particular. It's meditative, really.

But here's where many gardeners go wrong: they keep watering heavily right up until harvest. You need to start backing off on water as bulbs begin forming. Too much moisture late in the season leads to soft bulbs that won't store well. I start reducing water when I see the bulbs pushing up out of the soil, usually by mid-July in my area.

Feeding onions is an art form. They're heavy feeders but sensitive to too much nitrogen late in the season. I side-dress with compost tea every three weeks until mid-June, then switch to a lower-nitrogen feed. My grandmother used to save her wood ash specifically for the onion patch, claiming it made them sweeter. I've continued the tradition, though I can't swear it makes a difference.

Common Challenges and Real Solutions

Let's talk about onion flies. These pests lay eggs at the base of young plants, and their maggots tunnel into the bulbs. I've lost entire rows to these demons. Now I cover my newly planted sets with row cover until they're established. It's not pretty, but it works.

Weeding onions requires patience and a gentle touch. Their shallow roots mean aggressive weeding can damage them. I've become a devotee of mulching between rows with grass clippings. Some people worry about nitrogen tie-up, but I've never had issues. The clippings suppress weeds and retain moisture – a double win.

Disease can strike, particularly in wet years. Downy mildew shows up as grayish patches on leaves. Botrytis causes neck rot. The best defense is good air circulation and avoiding overhead watering late in the day. I've also found that onions grown in the same spot year after year are more prone to problems. Rotation isn't just good practice; it's essential.

Harvesting: Knowing When to Pull the Trigger

The biggest mistake I see new gardeners make is harvesting too early. Onions tell you when they're ready – their tops begin to yellow and fall over naturally. This isn't disease or neglect; it's the plant's signal that it's done growing.

Once about half the tops have fallen, I knock over the rest by hand. This stops growth and begins the curing process. Then comes the waiting game. I leave them in the ground for another week or two, weather permitting. You want a dry spell for this. Pulling onions during wet weather is asking for storage problems.

When you do harvest, choose a dry day and pull them gently. Shake off excess soil but resist the urge to clean them thoroughly. That papery outer skin is protection. Lay them out in a single layer somewhere warm and airy. I use old window screens propped up on sawhorses in my garage.

The curing process takes two to three weeks. You'll know they're ready when the necks are completely dry and the outer skins rustle like paper. Only then should you clean them up, trimming roots and cutting tops to about an inch.

Storage and Final Thoughts

Properly cured onions can last months in the right conditions. I braid soft-neck varieties and hang them in my basement. Hard-neck types go into mesh bags or old pantyhose (clean ones, obviously). The ideal storage temperature is just above freezing with low humidity, but few of us have perfect conditions. I make do with my 50-degree basement and still get onions lasting until spring.

After all these years, planting onion sets still feels like an act of faith. You're trusting these humble bulbs to transform themselves, trusting the soil to nurture them, trusting yourself to provide what they need. It's a partnership, really. And when you slice into a homegrown onion in December, releasing that sharp, sweet aroma, you remember why you bother.

The truth is, growing onions from sets isn't complicated. It's about understanding their basic needs and rhythms, then getting out of their way. They want to grow – your job is simply to create conditions where they can thrive. Every garden is different, every season brings new challenges, but onions are forgiving teachers. They'll reward your efforts even as you're still learning.

So go ahead, order those sets. Clear a patch of ground. Join the ancient tradition of coaxing food from soil. Your kitchen will thank you, and you might just find, as I have, that there's something deeply satisfying about reaching for your own homegrown onion when a recipe calls for one. It's a small independence, but in our increasingly disconnected world, these small independences matter more than ever.

Authoritative Sources:

Brewster, J. L. Onions and Other Vegetable Alliums. 2nd ed., CABI, 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 Onion Plants." USDA Plant Hardiness Zone Map, Agricultural Research Service, www.ars.usda.gov.

Voss, R. E., et al. "Onion Production in California." University of California Agriculture and Natural Resources, Publication 7242, 2013.