The Short Version Before It Gets Long
Most people buy spring onions at the store and think they have to start from scratch every time. That isn't necessary. The actual fastest method uses the root end of a store-bought bunch, sticks it in dirt, and waits about three weeks for a harvest. But if you want to do it properly from seed, or if you want continuous supply rather than one lucky harvest, the process is slightly more involved. I grew spring onions through a full garden season last year using both methods, and I ended up culling a whole row because I planted them too close together. That was my mistake. I'll get to the spacing issue.
How To Grow Spring Onions From Scraps Versus From Seed
The scrap method is what most home gardeners stumble into by accident. You buy spring onions, use the tops for cooking, and toss the white root end into a jar of water on the windowsill. Within two or three days you'll see green shoots coming up and roots forming below. Transfer that root end to a pot with potting soil about an inch deep, water it lightly, and place it somewhere that gets partial sun. You can harvest the green tops in roughly 20 to 25 days. The onion itself will keep producing new leaves as long as you don't cut it down to the bulb. This method has one major flaw. The resulting plants tend to be weaker and produce smaller yields because the original bulb has already spent its energy reserves. If you are growing for food rather than curiosity, it is not the best approach. It is fine for a kitchen herb situation where you want occasional garnish. Starting from seed is the method that actually produces a reliable crop. Here is how it works in practice.
Planting Details That Actually Matter
Buy seeds labeled Welsh onion, bunching onion, or simply spring onion. These are all the same category of allium that does not form a large bulb and is harvested for its green stalks. Do not confuse them with scallion seeds, which can sometimes be a different variety depending on the supplier. Sow seeds about a quarter inch deep in well-draining soil. The soil temperature should be at least 50 degrees Fahrenheit. Anything colder and the seeds will sit in the ground doing nothing for three weeks or more, and then rot. I learned this the hard way in early April when I planted too soon and lost roughly 40 percent of my row to damping off fungus. The workaround was simple: start seeds indoors in seed trays six to eight weeks before your last frost date, then transplant outside after the danger has passed. If you are planting directly outdoors, wait until the soil warms up. In most temperate zones that means mid to late spring. You can also do a fall planting in milder climates where the ground does not freeze solid.
Get the Full Details
Spacing Is Where People Mess Up
I mentioned earlier that I culled a whole row because of poor spacing. The seeds are tiny and it is tempting to sprinkle them thickly and thin them out later. This is wrong. Start with proper spacing from day one. Space your rows 12 to 18 inches apart. Within each row, thin seedlings to about 2 inches apart. If you leave them closer than that, the plants compete for nutrients and the stalks become thin and leggy. You will get a lot of green but very little substance. Two inches gives each plant enough room to develop a sturdy stalk without choking its neighbor. Transplanting seedlings is easier than thinning in place if you want to avoid disturbing the roots. Sow seeds in clusters indoors, then once they have their first true leaves, move the strongest seedling from each cluster into its final spot outdoors. This avoids the root damage that happens when you yank seedlings out of crowded soil.
Soil and Feeding
Spring onions prefer a soil pH between 6.0 and 7.0. They are not fussy but they do not thrive in heavy clay. If your native soil is dense, amend it with compost or coarse sand before planting. Good drainage is non-negotiable. Waterlogged soil will kill spring onions faster than almost anything else. Fertilizer needs are modest. A balanced all-purpose fertilizer applied at planting time is sufficient for most garden soils. If your soil is particularly poor, add a second light feeding three weeks after transplanting. Do not over-fertilize with nitrogen. Excess nitrogen produces lush green growth but weakens the overall plant and makes it more susceptible to disease. I once gave a row extra nitrogen because the leaves looked pale, and the next week several plants showed signs of downy mildew. The pale leaves were not a nutrient deficiency. They were normal for that variety in that soil condition. I stopped fertilizing and the plants recovered on their own.
Watering Routine
Water consistently. The goal is even moisture, not saturation. About one inch of water per week is the standard target, adjusted for rainfall. In hot weather you may need to water more frequently. The soil should feel moist about two inches down when you check it with your finger. Inconsistent watering causes the stalks to split. This is not a disease. It is a physical response to the plant taking up water faster than its outer layers can expand. If you experience dry spells followed by heavy rain or deep watering, you will see splitting. Mulching helps stabilize soil moisture and reduces this problem significantly.
Pests and Diseases
Onion thrips are the most common pest. They feed on the leaf tissue and leave silver or bronze streaks. In light infestations you can spray the plants with water to dislodge them. For heavier infestations, insecticidal soap applied in the evening works. Do not spray during the day when temperatures are high because the soap can burn the leaves. White rot is a soil-borne fungal disease that causes the base of the plant to turn white and fuzzy, then the whole plant collapses. There is no cure once a plant is infected. Remove and destroy affected plants immediately. Do not compost them. The fungus can persist in soil for many years, so do not plant alliums in the same spot for at least three to four years. Rotate your beds. Allium leaf miner is another pest that has become common in recent years. The adult fly lays eggs near the base of the plant, and the larvae tunnel into the leaves creating visible white trails. Row covers installed at planting time are the most effective prevention. Without physical barriers, the damage is usually cosmetic but unsightly.
Harvesting and When to Stop
You can begin harvesting spring onions about eight weeks after planting from seed, or sooner if you started from scraps. Harvest by pulling the entire plant or by cutting individual stalks as needed. If you cut stalks, leave at least half the green portion attached so the plant can continue growing. The white part of the stalk near the soil line will remain in the ground and keep producing. If you want larger mature bulbs instead of just green stalks, let some plants grow without harvesting. Eventually the bulbs will swell and you can dig them up. This takes longer and changes the flavor profile entirely. The onions become pungent and sharp rather than mild and sweet. The biggest limitation of growing spring onions is that they do not store well once harvested. Unlike mature storage onions, spring onions have high moisture content and begin to wilt within a few days even in the refrigerator. They are best eaten fresh. Plan your planting schedule so you are harvesting within a week or two of when you need them, rather than growing a massive surplus that goes to waste.
If you live in a climate with very hot summers, spring onions will bolt and go to seed quickly once temperatures exceed 80 degrees Fahrenheit. The stalks become tough and bitter. In that case, succession planting every two to three weeks gives you a rolling harvest before the heat hits, and then you switch to a fall crop if your growing season allows it.
