How to Care for Onion

Onion (Allium cepa) is a hardy, cool-season bulbing plant with upright, hollow blue-green foliage and a rounded underground bulb. It is one of the easiest garden plants to grow, rewarding steady care with tidy clumps of strappy leaves.

Light

Give onions full sun, at least six to eight hours of direct light daily. Onions are highly responsive to day length, and abundant sun drives the leaf growth that ultimately determines bulb size. In shade the plants stay thin, weak, and slow, so choose the brightest open spot in the garden.

Water

Keep the soil evenly and moderately moist. Onions have shallow, sparse roots and dislike drying out, so water regularly, aiming for about an inch a week. Avoid letting the ground swing between bone-dry and soggy, as inconsistent moisture stalls growth. Ease off watering late in the season as the tops begin to soften and fall over.

Soil & Potting

Plant in loose, fertile, well-drained soil rich in organic matter. Onions resent compacted or heavy clay, which restricts bulb swelling; work in compost to loosen texture and improve drainage. A slightly acidic to neutral pH around 6.0 to 6.8 is ideal. In containers use a deep, wide pot with a free-draining potting mix.

Humidity & Temperature

Onions are cool-season growers that establish best in mild temperatures between 55 and 75 degrees Fahrenheit. They tolerate light frost and actually prefer cool early-season conditions for leaf building, with warmer, longer days later triggering bulb formation. Ambient outdoor humidity is fine; good airflow helps keep foliage dry.

Feeding

Onions are moderately heavy feeders that respond to nitrogen during the leaf-building phase. Feed every few weeks with a balanced or nitrogen-forward fertilizer through the early and mid season to encourage large tops. Stop feeding once bulbs begin to enlarge and the necks start to soften, as late nitrogen delays maturity.

Propagation

Onions are grown from seed, from small starter bulbs called sets, or from young transplants. Seed is sown early indoors or directly in the garden; sets and transplants are simply pushed into prepared soil at the proper spacing. Space plants a few inches apart so each bulb has room to expand.

Repotting / Pruning

Onions are typically grown as an annual crop and are not repotted; instead, thin crowded seedlings to give remaining plants room. No pruning is needed. If flower stalks appear prematurely, remove them so the plant keeps its energy in the bulb rather than in seed production.

Common Problems & Pests

Onion thrips are the most common pest, causing silvery streaking on the foliage; strong plants and good airflow help resist them. Onion maggots may tunnel into bulbs in wet, crowded plantings. Fungal issues such as downy mildew and neck rot appear in overly wet, poorly drained soil, so prioritize drainage and spacing. Yellowing, floppy tips late in the season are usually a normal sign of maturity.

Seasonal Care Tips

Start onions in early spring while conditions are cool so foliage builds before long summer days trigger bulbing. Keep soil consistently moist through spring and early summer, then withhold water as the tops yellow and bend over. Choose varieties matched to your latitude, since short-day, intermediate, and long-day types respond to different day lengths.