When to Plant Onions in Colorado for a Bountiful Harvest

Written by

William Golder


Dorian Goodwin

when to plant onions in colorado

Onions are indeed perfect for planting in Colorado as they can handle moderate cold. The problem is that I plant them too soon, and they will germinate slowly. Plant them too late, and the yield suffers. The question, then arises, is, When to plant onions in Colorado?

The University of Colorado states that the best time to plant onions is 10th to 30th March. However, a few additional factors are at play (covered below).

Best Time to Plant Onions in Colorado


The planting time of mid to late March also depends on a few other factors.

1. Last frost

Planting can only be done if there hasn’t been any frost for at least 15 days. Onions can’t handle temperatures below 20 F. If, after planting onions, the temperature drops below that, the seedlings will rot, and bulbs won’t grow from seeds.

2. Soil temperature

As per Utah State University, ideal germination temperature for onion seedlings is 40°F with the ideal soil temperature pegged at 75 °F. If the temperature goes below 40°F the yield reduces.

3. Onion types

Time to plant is also dictated by the onion type as covered in the table below.

Sr. No. Onion Type Daylight Hours Needed Recommended planting timeline
1. Long day onions 14 hours or more 15th – 30th March
2. Medium day onions 12 to 14 hours 10-15th March
3. Short day onions 11 to 12 hours Not recommended in Colorado

Note: If short day or intermediate day onions are exposed to longer daylight hours in Colorado, the onion bulbs appear soon, which yields smaller-sized onions.

4. Region


Different regions of Colorado experience different weather conditions. For example, Fraser, which is west of Boulder, experiences lower temperatures than the rest of the state. In such regions, onions should be planted in late March.

Similarly, Grand Junction, Denver, Fort Collins, and Greeley have warm climates. In such areas, onions can be planted in Mid March.

The local climate is also important in determining if you should plant onions in mid or late-March. This difference of a fortnight can be the difference between the large onion yield and the seedlings rotting away.

Growing Onions in Colorado

Follow the steps below to aid onion growth after planting them right at a depth of 1 inch and a 4-6 inches gap. The depth and gap restrictions should be followed irrespective of whether they are planted in gardens or in raised beds.

Step 1: Add compost and organic matter

Onions do need more compost and other organic matter to grow. According to a post by Iowa state university, organic matter and compost help maintain soil fertility levels.

Step 2: Use fertilizer when bulbs swell


Onions cannot grow large enough without high nitrogen content in the soil. Therefore, add nitrogen-based fertilizer to the soil at an interval of 3 weeks, with the first application being 3 weeks after planting.

Step 3: Water regularly & remove the weeds


Onions need watering every 5 days during the initial stage. The frequency reduces to 7-10 days once the bulbs are removed. Overwatering will lead to yellow onion bulbs.

As highlighted above, onions also need nutrient-dense soil. To facilitate the same, remove the weeds regularly.

Step 4: Stop fertilization when bulbs appear


Once the onions push through the soil surface, it’s best to stop watering and fertilizing. The onions are now ready to harvest.

Onion Varieties to Plant in Colorado


By now, you are aware that long-day onion varieties are best suited for Colorado’s climate. However, which are the best onions to grow in colorado? Check the section below to know the types of onions to grow in Colorado.

  1. Walla walla – Walla onions need 14-15 hours of daylight, making them perfect for Colorado summers.
  2. Copra – Copra onions need 14 to 16 hours of daylight, making them another good option for Colorado summer.
  3. Candy – Candy also needs 12 to 14 hours of daylight, making it a suitable variety.

Care and Maintenance After Planting

While we spoke about fertilization and watering above, there are a few other tips to keep in mind while planting and growing onions.

  • pH level – Planting onions in well-drained soil with a pH level between 6 to 7 is best.
  • Use mulch to cover bulbs – Any immature bulbs that push through the soil should be covered with mulch. It helps them retain moisture and grow faster.

Note: Do not cover mature onion bulbs that emerge through the soil.

  • Till the soil before planting – It is best to till the soil and remove weeds before planting onion seeds to eliminate insects like cutworms, maggots, and even diseases.

Harvesting and Storing


The sign of harvesting onions is when the tops are dry or falling over. After pulling them out of the ground, follow the tips below.

  1. Store them indoors – While one can keep them outdoors, the harvest is well protected indoors.
  2. Cure them – Curing is necessary to store onions indoors. Ensure temperature between 75 F and 90 F and proper ventilation for curing. Curing takes anywhere between 2 to 4 weeks. Once the top is completely dry, you can cut it off.
  3. Store them in a dry place – It is best to store them in a cool and dry area with temperatures between 32 F and 40 F.

Frequently Asked Questions

How big do Colorado onions get?

A report by Colorado State University states that onions can range from 1 inch to 4.5 inches in diameter.

Do onions grow well in Colorado?

Copra, candy, and walla walla grow well in Colorado. These onion species need 14 to 16 hours of daylight since they are of the long day type, making them perfect for Colorado.

Are there wild onions in Colorado?

Geyer onions are wild onions that can be found all over Colorado. However, their size is much smaller than the onion varieties discussed above.

How many onions grow per plant?

One onion is produced from one seedling. That is why onion seedlings are planted in sets, 4-6 inches apart.

Do green onions grow well in Colorado?

It is best to avoid growing short-day green onions in Colorado, as they only need 6 to 8 hours of daylight which is much more than what Colorado weather offers.

Colorado weather is best for growing long-day green onion varieties like Southport White globe and Sweet Spanish.


So, if you are wondering when to plant onions in Colorado, Simply follow the guide above, and planting, caring for, and harvesting onions in Colorado won’t be a problem. Start with the long-day onion seeds, follow the guide above and you will be surprised by the yield in Colorado.

Planting calendar for Vegetables in Colorado:

5/5 - (2 votes)