Onions (Allium cepa) are a biennial vegetable grown for their swollen bulbs, with hollow, blue-green leaves and a shallow root system that stays close to the soil surface. They need ample sunlight, well-draining soil, and a steady supply of nitrogen-rich fertilizer to size up properly. The chemical compounds in onions are toxic to pets, so keep animals out of the patch and store harvested bulbs out of reach. Onions anchor more recipes than any other vegetable, which is why kitchen remodeling plans for a growing family always budget extra storage and prep space for them.
Success with onions comes down to matching the variety to your day length, planting at the right time, and keeping moisture and feeding steady through the growing season. This article covers onion types, planting from sets, seeds, and transplants, soil and water needs, feeding, harvesting, curing, storage, and the pests and diseases that cause the most trouble.
Onion Types and How They Grow
Bulb formation in onions is triggered by day length, not by soil fertility or watering. Each variety is programmed to start bulbing when daylight crosses a threshold, so choosing the wrong type for your latitude produces leaves without bulbs, or bulbs that bolt to seed.
Day-Length Categories
Short-day onions bulb when daylight reaches 10 to 12 hours, which suits the southern states. Long-day onions need 14 to 16 hours and suit the north. Intermediate types sit between the two and work best in the middle latitudes. That dependence on sunlight echoes the growing residential solar market, where seasonal sun hours drive output and site selection decides the payoff.
| Type | Day length needed | Best regions | Common varieties |
|---|---|---|---|
| Short-day | 10-12 hours | Southern U.S. (zones 7+) | Texas Grano, Red Creole |
| Intermediate | 12-14 hours | Mid-latitudes (zones 5-6) | Candy, Superstar |
| Long-day | 14-16 hours | Northern U.S. (zones 3-6) | Walla Walla, Yellow Sweet Spanish |
Growth Stages of a Bulb
In the first season the plant produces leaves, then a bulb; in the second season, if left unharvested, it sends up a flower stalk and sets seed. Gardeners interrupt the cycle by harvesting at the end of year one. The bulb stores the energy the leaves produce, so any damage to foliage, from pests, disease, or rough weeding, shrinks the harvest.
Why Timing Matters
Plant too early and cold weather pushes the plant to bolt; plant too late and the bulb lacks time to size up before day length changes. Regional planting calendars exist for a reason, and following yours is the highest-value decision you will make all season.
Planting Onions from Seeds, Sets, and Transplants
You can start onions three ways, and each has a different cost, timeline, and success rate. Choose based on how much space, time, and patience you have.
Choosing Your Starting Point
- Sets are small bulbs grown the previous year; they are the fastest and most forgiving option for beginners.
- Transplants are seedlings started indoors or bought at garden centers; they give a head start in short-season regions.
- Seeds are the cheapest route and offer the widest variety selection, but they need 8 to 10 weeks of indoor growing before planting out.
Planting Depth and Spacing
Set bulbs 1 inch deep with the pointed tip just below the surface, spaced 4 to 6 inches apart, in rows 12 to 18 inches apart. Transplants go slightly deeper, up to the base of the leaves. As the bulb matures, the neck swells and pushes upward, a slow expansion not unlike the clay masonry expansion that surveyors measure and plan for in old brick walls.
- Choose a site with at least six hours of direct sun.
- Work compost into the top 6 inches of soil.
- Set bulbs 1 inch deep, pointed end up, 4 to 6 inches apart.
- Space rows 12 to 18 inches apart for weeding access.
- Water in well and cover with a light mulch.
Soil, Sun, and Water Requirements
Onions have shallow roots, so the top few inches of soil decide the crop. Loose, well-drained soil with consistent moisture produces round bulbs; compacted or soggy soil produces misshapen ones.
Soil Preparation and pH
A sandy loam with a pH between 6.0 and 7.0 is ideal. In heavy clay, build a raised bed or work in generous compost. Test the soil before adding lime or sulfur, because onions respond to pH more than most vegetables.
Watering and Weed Control
Water to keep the top 6 inches of soil evenly moist, about 1 inch per week in cool weather and more during bulb swell. Onions compete poorly with weeds, so keep beds clean, especially in the first six weeks. Onions earn their place in small gardens the way live-work units earn their keep for builders: a compact footprint with steady output and no wasted space.
- Give onions about 1 inch of water per week before bulbing begins.
- Increase to 1.5 to 2 inches per week while bulbs swell.
- Stop watering when the tops begin to fall over.
- Water at the base in the morning so foliage dries by night.
Feeding Onions Through the Season
Onions are heavy users of nitrogen, and their shallow roots cannot reach nutrients buried deep. Regular sidedressing keeps growth steady from planting to bulbing.
Nitrogen-Rich Fertilizer
Use a fertilizer with a higher first number, such as 10-10-10 or a dedicated onion feed, and work it into the soil beside the row rather than dumping it on the foliage. Sidedress every three to four weeks until the bulbs start to swell. Fish emulsion and blood meal are good organic sources; both release nitrogen quickly enough for onions’ short root reach.
When to Stop Fertilizing
Stop feeding once bulbing begins, usually when the neck thickens and the bulb shoulders rise above the soil line. Late nitrogen makes soft bulbs that rot in storage. Interplanting onions with carrots, lettuce, or beets is mixed-use development at garden scale, with two crops sharing one bed without competing for the same root zone.
- Work a balanced fertilizer into the soil at planting.
- Sidedress with a nitrogen-rich feed three weeks after planting.
- Repeat every three to four weeks while leaves are actively growing.
- Stop all feeding when bulbs begin to swell.
Harvesting, Curing, and Storing
Onions are ready when the plant tells you: tops fall over, the neck softens, and the skin turns papery. Harvesting at the right moment, then curing properly, separates onions that store for months from those that rot in weeks.
Knowing When to Harvest
- The leaf tops fall over naturally, not from frost or disease.
- The neck feels soft and bends easily.
- Bulb shoulders push above the soil surface.
- The outer skin has dried to a papery layer.
Curing and Storage Conditions
Cure bulbs in a warm, dry, airy place for two to three weeks until the necks are completely dry, then trim roots and tops. Store in a cool, dark, dry spot with good airflow, in mesh bags or braids, and check monthly for soft bulbs. Left in the ground past maturity, bulbs split or send up seed stalks, and dropped seed turns the bed into a growing family of volunteer plants, the way a growing family fills a bungalow faster than expected. Harvest on a dry day and leave the roots attached until curing ends; bulbs pulled wet invite rot.
Common Pests, Diseases, and Troubleshooting
Most onion failures trace back to three things: the wrong variety, soggy soil, or skipped rotation. Fix those and the crop is usually healthy.
Pests to Watch
Onion maggots lay eggs at the base of young plants, and the larvae tunnel into the bulbs; thrips rasp the leaves, leaving silvery streaks. Rotate the onion bed every three to four years, remove crop debris, and cover new plantings with row cover early in the season.
Diseases and Prevention
Downy mildew shows as fuzzy gray patches on leaves in cool, wet weather; botrytis leaf blight follows similar conditions. Avoid overhead watering, space plants for airflow, and pull and discard infected plants rather than composting them.
| Problem | Likely cause | Fix |
|---|---|---|
| Soft rot at the neck | Excess nitrogen or wet harvest | Stop feeding at bulbing, cure fully |
| Early bolting | Wrong day length or cold snap | Choose the right variety, plant on time |
| Small bulbs | Overcrowding | Thin to 4-6 in spacing |
| Silvery leaf streaks | Thrips | Spray with insecticidal soap, rotate beds |
| Maggots in bulbs | Onion maggot larvae | Rotate crops, use row cover |
Onions are often the first crop a child grows, and that early success builds confidence the way growing up with concrete gives future masons hands-on familiarity with materials long before formal training starts. Hand a young gardener a row of sets and let the harvest do the teaching.
Scaling up from a few rows to a full onion bed is a gradual process of adding a row or two each year, the same incremental growth that growing construction firms plan for when they expand equipment and crews. Start with one variety matched to your day length, keep the water and nitrogen steady, and the second year’s crop will practically grow itself.