Bulbing Onions

By September 28, 2018

People have grown onions for over 5,000 years. One of the reasons for their popularity is the fact they can keep for long periods of time without spoiling. Another is that onions are a key ingredient in just about any savory meal you can imagine.

How to Grow Onions


There are two important points to keep in mind when planting and growing onions. First, you need to select the type of onions that match the daylight hours in your garden. Second, you need to give onions plenty of food so they can grow healthy leaves.


  • There are three ways you can plant onions in your garden: seeds, onion sets or onion transplants. The Garden Center has all options.  
  • Most gardeners in Colorado climates choose to plant either sets or transplants. But if you want to start from seed, plant them indoors 8-10 weeks before transplanting.
  • Onion sets are small bulbs that will grow into a larger bulb. Onion sets have already been through one bulbing process so sets grow onions faster than starting from seed.
  • Onion transplants are small onion plants that haven’t formed a bulb yet.
  • Prepare a spot that gets full sun and amend the soil with compost or other organic matter and broadcast preplant fertilizer over the planting bed. Keep in mind that onions may need twice the amount of fertilizer as other garden plants. Make sure to dig in deeply so roots can get established quickly.
  • There are many kinds of onions to plant and grow. No matter which onions you choose, be sure to select only long-day varieties for growing in Colorado. These are the onion varieties that need 14 to 16 hours of daylight to start the bulbing process. Long-day varieties are meant to mature in late summer and take 90 to 100 days or more to mature.


  • Plant onions when soil temperatures are around 60 degrees and as soon as the soil is dry enough to work in spring. The size of the finished onion bulb depends on the number of healthy green leaves the plant can grow. More leaves mean bigger bulbs, so work to grow as many leaves as possible before the bulbs start forming.
  • Don’t wait too long to plant onions, because they don’t grow well once temperatures warm above 85 degrees.
  • Plant onion sets and transplants 2-3 inches deep; and 1-2 inches apart in rows. Rows should be at least 15 inches apart.
  • Add an organic mulch to help the soil retain moisture and to avoid weeding around the delicate, shallow roots.
  • Drip irrigation or a soaker hose system will help keep soil consistently moist (not soggy) and leaves dry. It’s important to keep the leaves dry to prevent fungal problems.
  • To avoid insect pest problems, use floating row cover cloth to protect plants.


  • Keep up good watering and fertilizing through the season. Sidedress plants about a month after planting by digging in fertilizer between the rows and away from the plants. Be sure to water after fertilizing.
  • When the onion tops start to fall over, stop watering. Without water, the onions bulbs will start to harden, which is important for long-term onion storage.


  • Carefully lift onions after the tops have fallen, but before the leaves have a chance to dry completely. Cure the onions in the garden by letting the leaves of one row cover the bulbs in another for between 7-14 days.
  • If the weather is rainy and wet, cure onions under cover. Make sure air is allowed to circulate around bulbs so they can dry completely.
  • After the tops are completely dry, cut them from the bulb, leaving about an inch above the bulb. Gently brush off any soil and store on a flat surface, spread apart in a cool dry place.

Companion Plants

Plant onions with these companions:

  • Beets
  • Cabbage family members
  • Carrots
  • Lettuce
  • Parsnips

Avoid planting near beans and peas.

Materials for Success

  • Soil thermometer
  • High-quality compost and manure
  • Dry, balanced fertilizer (5-10-10)
  • Organic mulch
  • Soaker hose or drip irrigation system
  • Row cover cloth
  • Garden fork for harvesting

To learn more about growing bulbing onions or about growing your own edible vegetable garden, contact the pros at Nick’s Garden Center. 

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