How to Plant Onion Bulbs and Grow Another Onion

onion bulbs in the soil

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Project Overview
  • Working Time: 2 - 3 hrs
  • Estimated Cost: $8 to $14/pound

Onions are a kitchen staple and one of the easiest, most rewarding crops to grow. You get a lot of edibles from a small space, they keep well, and homegrown varieties add delicious flavor to any savory dish.

Can You Grow Another Onion From an Onion Bulb?

Planting bulbs is a popular way to start onions growing in your garden. Often referred to as sets, these are small, dried onions grown and harvested as immature bulbs, They're sold the following spring in retail outlets including garden centers, feed stores, and even groceries. They are usually available before the final frost. Bulbs take 12 to 14 weeks to grow into mature onions, but even gardeners with short seasons can grow them for green onions.

When to Plant Onion Bulbs

Plant onion bulbs in early spring as soon as the ground can be worked. Or you can plant in autumn about a month before the first frost. Plant the bulbs when you get them. If this isn't possible, store them in a cool, dry location for up to two weeks.

Once established, onions tolerate temperatures of 20 degrees Fahrenheit along with frost and light freezing. Hard freezes damage young plants, so protect bulbs with natural mulch, such as straw. grass clippings, or pine needles. Another way to prevent frost damage is using row covers.

Before You Begin

Start by preparing your planting bed. Onions need lots of sun and perform best in fertile soil on the sandy side with a pH of 6.0 to 7.0. If your soil is heavy with clay, consider planting in a raised bed. Otherwise, amend the soil with a well-aged compost. If you want to grow a few onions in pots, add compost to the potting mix and choose plastic pots with plenty of drainage holes.

What You'll Need

Equipment / Tools

  • Garden hoe
  • Gloves


  • Onion bulbs/sets
  • Aged compost
  • 10-10-10 Fertilizer
  • Bone meal and blood meal
  • Large pot/s
  • Potting soil
  • Straw, grass clippings, and pine needles
  • Row cover (optional)


  1. Select Onion Bulbs to Plant

    Onion bulbs come in three types: white, yellow, and red. Size can vary so look for ones about 3/4 of an inch in diameter. Locate a retailer that lets you select instead of buying prepackaged bulbs.

    You can discard soft, damaged, or big ones. Large bulbs tend to flower or produce smaller onions. Bulbs that flower quickly rot and should be harvested and used immediately.

    Opinions differ about removing green sprouts before planting. Some gardeners say snipping them off results in better root growth. Others say sprouted bulbs give a head start on leafy growth. Try both ways and see what works best in your patch.

  2. Prepare the Planting Site

    Choose a spot that receives sun all day. Work the aged compost into the soil or potting mix. In the garden, use the hoe to dig a narrow trench about 1 inch deep. Potted onions need 3 inches of space so choose a 12-inch pot to accommodate four bulbs.

  3. Soak the Bulbs (Optional)

    Some experts say that soaking bulbs before planting causes them to sprout more quickly. Dry bulbs planted in fertile, well-draining soil also sprout in four to seven days, so this is a personal choice. If you decide to soak, place the bulbs in compost tea or room temperature water for up to 24 hours and drain them thoroughly before planting out.

  4. Place the Bulbs

    Place the bulbs, flat/root side down and 4 inches apart, in the bottom of the trench. Space the rows at least 8 inches apart. To harvest green onions, space bulbs 3 inches apart. Make holes 1 inch deep and 3 inches apart if growing in containers.

  5. Set the Bulbs and Cover

    Use one gloved hand to hold the bulb in place and the other to cover it with soil up to the stem. Pack the soil firmly around the bulb keeping the stem and top above soil level. This sets the bulb and keeps it in the correct position during planting. Upside-down bulbs rarely sprout.

  6. Water Gently and Mulch

    Drip irrigation is ideal for watering onions. Otherwise, use a hose with a spray nozzle or a watering can with a rose spout.

    Cover the planting bed or pot with a light natural mulch.

  7. Maintenance and Care

    Provide 1 to 2 inches of water per week until the onions reach your desired size. Green onions are ready in three to four weeks. Mature onions grow above or partially above soil level so you can monitor the size. Reduce or stop watering to allow their tops to dry and die back before harvest.

    Fertilize every two weeks during its leafy growth with an NPK 10-10-10 or use organic bone meal and blood meal once a month.

    Fun Fact

    Every leaf on an onion plant represents one ring of growth for the bulb.

  8. Harvest and Cure

    Harvest green onions when leaves are about 8 inches tall. Full-size onions are ready to harvest when the green tops and leaves fall over. Stop watering and leave the plants for a week to allow the stems and necks to dry or dig the onions and lay them out on a flat surface to cure.

    After leaves and stems dry, check the top around the stem. If the entire onion is solid without soft spots, cut the stem leaving about 1/2 to 1 inch attached. Continue to cure the onions for a few days before storing them in a cool, dark location.

  • What month do you plant onion bulbs?

    Plant as early as the soil can be worked in your growing zone. Onions take 12 to 14 weeks to reach full size. Bulbs planted later can be harvested as a green onion.

  • Do you soak onion bulbs before planting?

    This is your choice. Soaking may result in quicker sprouting but dry sets can sprout as early as four days after planting.

  • How deep should onion bulbs be planted?

    Plant according to the size of the bulb, but generally the depth is about 1 inch. Keeping the root side down and the top above the soil level is the most important.

Article Sources
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  1. How Do I Get My Onions to Grow Bigger?, University of New Hampshire Extension