How to Plant Onion Seeds? A Complete Step-by-Step Guide for Beginners
Introduction: Why plant onion seeds?
Want bigger, cheaper, tastier onions? Learning how to plant onion seeds gives you access to dozens of varieties, lower costs, and control over timing and disease resistance. A single $2 seed packet can produce hundreds of plants, compared with pricey onion sets in stores. For example, try Walla Walla for sweet summer onions, or Red Baron for storage. In this guide you will get exact timing for long day and short day types, seed selection tips, soil prep, sowing depth and spacing, watering and feeding routines, thinning and transplanting steps, and common pest solutions.
Onion seeds versus sets, bulbs and seedlings
If you’re asking how to plant onion seeds, know they differ from sets and transplants. Sets are small immature bulbs, they grow fast and suit short seasons, but often bolt and limit variety. Transplants cut garden time, but cost more and risk shock. Seeds are cheapest, give the widest cultivar choice, and produce better storage onions, but need indoor sowing 8 to 12 weeks before last frost and care. Use seeds when you want specific varieties or larger yields.
When to plant onion seeds
Start by picking the right day-length type. North of about 37° N choose long-day onions, south choose short-day, intermediate-day fits the transition zone. This matters more than variety names when you learn how to plant onion seeds?
Indoor sowing, long-day and intermediate-day: start 8 to 12 weeks before your last frost. Example, if last frost is April 15, sow seeds late February to early March. Direct sowing, long-day: as soon as soil can be worked in spring. Intermediate-day, aim 6 to 8 weeks before last frost. Short-day onions, in mild climates, plant in fall or late winter for spring harvest, for example September to November in Zone 8.
Choose the right onion varieties for your region
Onions form bulbs when day length reaches a critical threshold. Long day varieties need 14 to 16 hours of daylight, ideal for northern climates above 36° N. Short day types set bulbs at 10 to 12 hours, best for the deep south. Day neutral or intermediate varieties suit mid-latitudes. Before you sow, check the seed packet’s day length classification so your how to plant onion seeds efforts produce large bulbs.
Prepare soil and containers for healthy starts
If you typed "how to plant onion seeds?" here’s the first move, soil. Use a light, well-drained seed-starting mix, for example two parts coconut coir or peat-free compost, one part perlite, one part vermiculite. Aim for pH 6.0 to 7.0, test with a $10 kit and add lime for acidity correction if needed. Mix in a tablespoon of bone meal or a slow-release 5-10-10 fertilizer per gallon of mix for early root development. Do not use fresh manure, it burns seedlings.
For containers, pick trays or cell packs with drainage holes and at least 2.5 to 3 inches of depth. Fill to 3/4, firm lightly, water until evenly moist. For direct sowing, loosen garden soil, amend with compost, and keep the seed bed consistently damp.
Step-by-step: how to plant onion seeds
Choose a sunny bed or a seed tray filled with fine, well-drained potting mix. Moisten the mix so it feels like a wrung-out sponge. If you are asking how to plant onion seeds? follow these exact measurements.
Sow seeds about 1/4 inch deep. For rows in the garden, space rows 12 inches apart. Place seeds about 1 inch apart in the row if you plan to thin later. For trays, sprinkle seeds evenly at a density of roughly 100 seeds per 12-inch row length, then thin later to final spacing.
Cover seeds with a very light dusting of soil or vermiculite, about 1/8 to 1/4 inch. Press gently with your hand or a board to ensure seed-to-soil contact. Label each row or tray with variety and sowing date using a weatherproof tag.
First watering matters, use a fine mist or a watering can with a rose; avoid heavy streams that wash seeds away. Keep the soil consistently moist, misting once or twice daily if the surface dries. Germination usually starts in 7 to 14 days, after which thin seedlings to 3 to 4 inches for bunching onions, or 6 to 8 inches for bulb onions.
Seedling care, thinning and feeding
If you searched "how to plant onion seeds?" start with light. Seedlings need 12 to 14 hours of bright light, either full sun or a grow light about 2 inches above tops, raised as they grow. Keep temps near 60 to 75 F and rotate trays for even growth.
Water lightly but regularly, keeping the top quarter inch of soil barely moist. Bottom watering works best to prevent damping off, check daily and avoid waterlogging.
Thin when seedlings reach 3 to 4 inches or show true leaves. First thin to about 1 inch apart, then later space to 3 to 4 inches for bulb onions, snipping extras at soil level with scissors to avoid root shock. Feed starting three weeks after emergence, use a balanced liquid fertilizer at half strength weekly, then reduce nitrogen and add potassium when bulbs begin to swell.
Hardening off and transplanting outdoors
Start hardening off about 7 to 10 days before transplanting. Put seedlings outside in morning sun for two hours on day one, add an hour each day, and bring them indoors at night if temperatures drop. Cut back watering slightly to toughen roots, but never let the soil dry out completely.
Transplant when seedlings have 3 to 4 true leaves and stems are pencil thick, or when soil temperature is consistently above 50°F. Plant so the top of the root ball is level with the soil, firm the soil around the stem, and water in well. Space bunching onions 4 to 6 inches apart, bulb varieties 8 to 12 inches apart, with rows 12 to 18 inches apart. For best bulb development give full sun, steady moisture, and a light nitrogen feed three weeks after transplanting.
Troubleshooting common problems and quick fixes
If seeds fail to sprout after 10 to 14 days, check seed age and soil temperature. Old onion seed loses viability fast, so use fresh seed and keep soil steady at 65 to 75°F. Improve germination by pressing seeds into moist, fine soil, covering very lightly, and using a clear plastic dome or tray for consistent moisture.
If seedlings bolt or send up flower stalks, they experienced temperature stress or crowding. Pull bolting plants and start again with the correct day length variety for your region, and thin to recommended spacing.
For pests and diseases, act fast. Use floating row covers to block onion maggots and thrips, remove infected bulbs, improve drainage to prevent fungal rots, and apply neem oil for minor pest outbreaks. These small fixes get you back on track when learning how to plant onion seeds?
Harvesting, curing and storing onions from seed
When to pull onions, test this rule of thumb, when about 90 percent of the tops have flopped and browned, the bulbs have stopped growing. Check necks, if the necks feel soft you are ready to harvest. If you planted onion seeds, some varieties finish later, so note days to maturity on the seed packet.
Curing is simple and critical. Pull onions on a dry day, brush off excess soil, then lay them in a single layer in sun or a well ventilated shed for 7 to 14 days. Rotate once, avoid stacking, and stop curing if rain is coming.
For long-term storage, trim tops to 1 inch after curing, cut roots short, and sort out any bruised bulbs. Store in mesh bags or slatted crates in a cool, dry place, ideally 32 to 40 degrees Fahrenheit with 65 to 70 percent humidity. Check monthly and remove soft ones to prevent rot.
Quick checklist and final tips
Before you start, follow this quick checklist so learning how to plant onion seeds goes smoothly.
- Soil prep: loosen and amend with compost, aim for well-drained, fertile soil.
- Seed depth: sow 1/4 inch deep, cover lightly.
- Spacing: thin seedlings to 4 inches apart, rows about 12 inches apart.
- Water: keep soil evenly moist, about 1 inch per week.
- Feed: side-dress with balanced fertilizer at 4 weeks.
- Mulch: straw to retain moisture and suppress weeds.
Tip: start a small patch, track dates, and expect better bulbs next season.
