How to grow onions from seeds

An exhibitor inspects the red onions during the annual East African seeds Agricultural Farmers Field Day at Wambugu Farm in Nyeri. [Kibata Kihu, Standard]

Onions are a hardy, cool season crop closely related to garlic, leeks and chives.

They are easy to grow, have many varieties, and have a long storage life. Although most onion types are biennial, they are usually treated as annuals and are harvested in their first growing season.

Choose onions that are suited to your geographic location. The day length or the number of sunlight hours they receive each day is also a consideration in the choice of the onion variety. The sunlight hours signal to the onion when to stop forming green tops and switch production to bulb growth below the surface.

Soil

Onions should be planted in loamy, loose and well-drained soil with a neutral pH. Compacted and heavy clay soils can affect how onion bulbs develop. The more water it receives, the sweeter the onion will be. Give onion plants enough water daily.

Onions are heavy feeders and require a good amount of fertiliser to produce the largest bulbs. Before planting, prepare the bed by mixing compost into the soil. Add nitrogen-rich fertiliser at planting time and every few weeks throughout the season. Be sure to practice crop rotation each year too.

Starting from seed

Start onion seeds indoors. It can take up to two weeks for onion seeds to germinate but you can speed up the sprouting considerably by keeping the soil warm. Always use fresh seeds, since onion seeds are short-lived and are less likely to successfully germinate over time.

Sow seeds about one inch deep and a half inch apart. Gently mist the soil to moisten it. Onions germinate quickest at 20C. Cover with a plastic bag to help retain heat and you will see your seeds pop in as little as four days.

Once seeds have sprouted, remove the cover and move to a sunny spot. Check moisture daily and give a light spritz when needed. When the green shoots are about five inches tall, trim them back to three inches.

When the third leaf appears, onion seedlings are ready to be hardened off and transplanted into the garden. Space transplants are four to five inches apart with 12 to 18 inches between rows.

Planting onion sets

Onion sets or immature onions can be planted in the soil right away. Although onion sets are the easiest way to start your onion crop, they do not have nearly as many choices of cultivars when compared with varieties purchased as seeds.

Work some compost or manure into the soil. Add some alfalfa pellets to boost nitrogen levels. Plant onion sets no more than one inch deep, spacing them two to six inches apart in rows.

Onions need a lot of water to swell in size so keep the soil moist throughout the growing season. Retain moisture and suppress weeds by adding a layer of straw between rows.

Harvesting

After about 100 to 120 days, your onions should be ready for harvest. You'll know they are ready when a flower stalk has emerged or their green tops fall over and start to yellow.

If you plan on using the onion right away, gently loosen the soil around the bulb and pull it up. Fresh onions should be consumed within a day or two.

To prep your onion for longer storage, leave it in the ground but pull back some of the soil around the bulb to expose it to air and encourage drying. After a few days, or when the tops are dry, pull the onions out of the ground.

Storage

Strong-flavoured, pungent onions will store longer than sweeter varieties, and when properly cured, can last for up to a year. Place the freshly pulled onions on dry soil and let them sit in the sun to dry.

After a few days, lay them out on a flat surface in a single layer to fully dry for two to four weeks.

Pick a sheltered spot with good airflow for storage. When handling your onions, be careful. Any bruise or wounding of this vegetable will encourage rot to set in, which can spread to your whole crop. Only choose the best onions for storage, free of soft spots, mold, and other signs of damage.