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How to grow onions

How to grow onions

Onions are a staple crop in gardens worldwide, offering versatility, flavor, and nutritional benefits. Whether you're aiming to harvest sweet onions, storage onions, or flavorful spring onions, these resilient plants adapt well to various climates and gardening methods. Growing onions is straightforward and rewarding, making them a great choice for gardeners of all skill levels.

How to grow onions from seed

Growing onions from seed is cost-effective and gives you access to a wider variety of onion types. Start onion seeds indoors about 8 to 10 weeks before the last frost. Sow seeds ¼-inch deep in seed trays filled with seed-starting mix, and keep soil evenly moist. Seedlings will emerge within 7-14 days. Harden seedlings off outdoors gradually before transplanting them into your garden, spacing them according to the mature size of the variety. Alternatively, direct-seed onion outdoors in warmer climates or use onion sets (small bulbs) for a quicker harvest.

When to plant onions

Onions are typically planted in early spring, as soon as the ground can be worked, or in the fall for an early summer harvest in milder climates. For northern gardeners, planting onion sets or seedlings outdoors about two to four weeks before your last frost is ideal. In southern regions with milder winters, onions can be planted in the fall and grown through winter for harvest in early spring. Planning your planting schedule according to your local climate ensures healthy, productive onions.

How far apart to plant onions

Spacing onions properly is crucial for good growth and bulb development. Plant onion seedlings or sets about 4 to 6 inches apart in rows spaced 12 to 18 inches apart. If you're planning to harvest green onions or scallions, spacing can be as close as 2 inches. Adequate spacing helps bulbs expand fully and ensures good airflow, reducing diseases and improving yields.

How much space onions need

Onions don't require significant horizontal space, but they do need room for bulb growth. For standard onion varieties grown for bulbs, spacing should be 4 to 6 inches apart, while green onions and scallions can be planted much closer—around 2 inches apart. Good spacing allows bulbs room to expand, leading to larger, healthier onions. Rows spaced about 12 to 18 inches apart facilitate weeding, watering, and harvesting.

Do onions need a trellis

Onions do not need a trellis. They have upright, sturdy foliage and develop bulbs underground, so no additional support structure is required. Onions' compact growth habit makes them excellent for planting in garden beds, raised beds, or containers without extra support.

Where to plant onions

Plant onions in a sunny location with fertile, well-drained soil enriched with compost or aged manure. Avoid areas prone to waterlogging, as excess moisture can cause bulbs to rot. Onions thrive in raised beds or garden plots with good airflow and sunlight. Soil should have a neutral pH (around 6.0 to 7.0), and beds should be cleared of weeds regularly, as onions don't compete well against invasive plants.

How much sunlight onions need

Onions require full sun, meaning at least 6 to 8 hours of direct sunlight per day, for optimal bulb development. Good sunlight is essential for photosynthesis, growth, and producing large, healthy bulbs. Shaded locations or insufficient sunlight will cause onions to grow poorly and produce smaller, less flavorful bulbs.

What temperature onions grow in

Onions prefer cooler weather during early growth but mature best under moderately warm conditions. Ideal temperatures for growing onions range from 55°F to 75°F (13°C to 24°C). Cooler temperatures encourage foliage growth, while warmer temperatures promote bulb formation. Temperatures above 85°F (29°C) may slow bulb development and cause onions to bolt prematurely, reducing harvest quality.

Can onions handle frost

Onions are cold-tolerant and can handle light frosts easily, often surviving temperatures as low as 20°F (-6°C). However, prolonged exposure to extreme cold or heavy freezes may damage or slow down growth. If severe frost is predicted, consider using row covers or mulch to protect young onion plants. Generally, mild frost won't negatively impact onion growth or flavor.

How much water onions need

Onions require consistent watering, especially during bulb formation, and typically need about 1 inch of water per week. Soil should remain consistently moist but never waterlogged, as onions are susceptible to rot. Mulching with straw, grass clippings, or compost can help maintain moisture and keep soil temperatures consistent.

How often to water onions

Water onions deeply once or twice a week, ensuring the soil remains moist to about 2 inches deep. Adjust the watering frequency based on rainfall and temperature. During hotter, dry spells, onions may need watering every two to three days. As bulbs reach maturity and tops start falling over, reduce watering to allow bulbs to cure properly in preparation for harvesting.

How long onions take to grow

Onions typically take between 90 to 120 days from planting to harvest, depending on the variety and growing conditions. Onion sets can mature faster, often in about 75 to 90 days, while onions grown from seed may take up to four months. Green onions or scallions mature quickly, usually within 60 days, making them ideal for fast, successive plantings.

When to harvest/pick onions

Harvest onions when the tops begin to yellow and naturally fall over, typically late summer or early autumn. This signals that the bulbs have stopped growing and are ready to be harvested. Harvest green onions or scallions much earlier, when stems reach about pencil thickness, usually around 60 days after planting.

How to harvest/pick onions

To harvest bulb onions, gently lift them from the soil using a garden fork or your hands, being careful not to bruise the bulbs. Shake off excess soil and leave them in a dry, warm, well-ventilated location to cure for about two weeks. Curing toughens the outer skin, enhancing storage life. For scallions or green onions, simply pull them gently from the soil as needed.

Can you grow onions indoors

Onions can be grown indoors for green onions or scallions, but full-sized bulb onions require more space and conditions typically challenging to replicate indoors. To grow scallions indoors, plant onion seeds or sets in containers placed near a sunny window or under grow lights. Ensure consistent watering, and you'll have fresh green onions throughout the year.

How to grow onions in pots/containers

Choose a container at least 8 inches deep and wide, with proper drainage holes. Use a high-quality potting mix enriched with compost. Plant onion sets or seedlings spaced appropriately for their intended use—2 inches apart for scallions or about 4 inches for full-sized onions. Place containers in full sunlight and keep the soil consistently moist. Container-grown onions are ideal for patios, balconies, or limited garden space, providing convenient access to fresh, homegrown produce.