When To Plant Onions In Colorado

If you’re wondering when to plant onions in Colorado, timing is everything for a succesful harvest. Our state’s unique climate, with its short growing season and unpredictable spring weather, makes getting the date right crucial.

Onions are a rewarding crop for any Colorado gardener. They store well and are a kitchen staple. But planting them at the wrong time can lead to small bulbs or premature bolting. This guide will give you the clear, expert advice you need to grow great onions this year.

The best time to plant onions in Colorado depends on your method: seeds, sets, or transplants. Here’s the simple breakdown.

Planting Onion Seeds

For seeds, you need to start early indoors. Begin sowing onion seeds inside about 10-12 weeks before your last expected spring frost. For most Front Range areas, this means starting seeds in late January to early February.

This long lead time is essential because onions need a long growing period to form large bulbs.
Starting indoors gives them a vital head start before transplanting them outside.

Planting Onion Sets & Transplants

This is the most common method for Colorado gardeners. Onion sets (small dormant bulbs) and homegrown or store-bought transplants go into the ground in early spring.

Plant them as soon as the soil is workable—typically from late March through mid-April.
The soil temperature should be at least 50°F. They can tolerate a light frost after planting.

For a fall harvest of green onions or scallions, you can plant a second crop of sets in late summer, about 8 weeks before your first fall frost.

Understanding Colorado’s Frost Dates

Your local average last frost date is your key calendar marker. It varies greatly across our state:

Front Range Urban Corridor (Denver, Boulder, Fort Collins): May 5-15
Western Slope (Grand Junction, Palisade): April 15-30
High Mountain Valleys (San Luis Valley, high elevations): June 1-15 or later
Eastern Plains: May 1-10

Always use your specific location’s date. Your county extension office is a great resource for this information.

Day Length: The Onion’s Secret Clock

This is a critical concept often missed. Onions bulb in response to day length. You must choose the right type for Colorado’s latitude.

Long-Day Onions: Need 14-16 hours of daylight to trigger bulbing. These are the only type you should grow in Colorado.
Short-Day Onions: Bulb with 10-12 hours of daylight. They will not form proper bulbs here and should be avoided.
Day-Neutral Onions: A good, flexible option. They bulb under a wider range of daylight hours (12-14) and perform well in Colorado.

Stick to long-day or day-neutral varieties. Popular long-day choices include ‘Walla Walla’, ‘Yellow Sweet Spanish’, and ‘Copra’. ‘Candy’ is a reliable day-neutral hybrid.

Step-by-Step Planting Guide

1. Choosing Your Onion Type

First, decide what you want: storage onions or sweet onions. Storage onions have thicker skins and last for months. Sweet onions have higher water content and are best eaten fresh but don’t store as long. Select varieties known for our climate.

2. Preparing Your Garden Soil

Onions need loose, well-draining soil with plenty of nutrients. Colorado’s heavy clay soil needs ammendment.

Work in 2-3 inches of compost or well-rotted manure into the top 6-8 inches of soil.
Ensure the planting bed is free of rocks and clumps so bulbs can expand easily.
Onions prefer a slightly acidic to neutral pH (6.0-7.0). A soil test can help you adjust if needed.

3. The Planting Process

Follow these steps for planting sets or transplants:

Space Correctly: Plant sets/transplants 4-6 inches apart in rows that are 12-18 inches apart. For larger bulbs, use the wider spacing.
Plant at the Right Depth: Place them so the tip is just barely visible above the soil surface. If planting from seed indoors, harden off seedlings for a week before transplanting.
Water In: Give them a gentle but thorough watering after planting to settle the soil around the roots.

4. Ongoing Care and Maintenance

Consistent care is simple but non-negotiable.

Watering: Onions have shallow roots. Provide 1 inch of water per week, more during hot, dry spells. Drip irrigation or soaker hoses are ideal to keep foliage dry and prevent disease.
Weeding: Keep the area weed-free. Weeds compete fiercely for nutrients and water. Mulch lightly with straw to supress weeds and retain moisture.
Fertilizing: Use a nitrogen-rich fertilizer early in the season to support green growth. Switch to a balanced or lower-nitrogen fertilizer once bulbing begins (when the base of the stem starts to swell).

Common Colorado Onion Growing Challenges

Bolting (Premature Flowering)

Bolting happens when an onion sends up a flower stalk. The bulb stops growing and won’t store well. It’s often caused by temperature fluctuations—a cold snap after warm weather. To minimize risk:

Choose bolt-resistant varieties.
Avoid planting too early if a long cold spell is forcasted.
If a stalk appears, harvest that onion immediately for fresh use.

Pests and Diseases

Onions have few pest problems but stay vigilant.

Onion Thrips: Tiny insects that cause silvery streaks on leaves. Blast them off with water or use insecticidal soap.
Onion Maggots: More rare, but can damage roots. Crop rotation is the best prevention.
Rot: Caused by overly wet soil. Ensure excelent drainage and avoid overwatering.

Poor Bulb Formation

Small bulbs can result from several factors: incorrect day-length type, overcrowding, insufficient water during bulbing, or a lack of nutrients. Double-check you’re growing a long-day variety and review your care practices.

Harvesting and Curing Your Onions

Harvest time is usually late July through August. Signs of readiness include the tops falling over and yellowing. Here’s what to do:

Gently lift bulbs from the soil on a dry, sunny morning.
Lay them or hang them in a warm, dry, airy place out of direct sun for 2-3 weeks. This curing process is essential for storage onions.
Once the necks are tight and papery and the outer skins are crisp, trim the roots and cut the tops back to about 1 inch.
Store in a cool, dry, dark place in mesh bags or a single layer.

FAQ: Your Colorado Onion Questions Answered

Can I plant onions in the fall in Colorado?

Planting full-sized bulb onions in the fall is generally not recommended for Colorado due to our harsh winters. However, you can plant a crop of onion sets in late summer for a fall harvest of green onions before the ground freezes.

What are the best onion varieties for Colorado?

Stick to long-day or day-neutral types. Excellent choices include ‘Yellow Sweet Spanish’ (storage), ‘Walla Walla’ (sweet, best used fresh), ‘Copra’ (excellent long-storer), ‘Redwing’ (red storage), and the versatile day-neutral ‘Candy’.

How often should I water my onions?

Aim for about 1 inch of water per week, including rainfall. The key is consistency—don’t let the soil dry out completely, especially when the bulbs are forming. Reduce watering as harvest approaches to let them mature.

Why are my onion tops turning yellow?

A little yellowing at the base as they mature is normal. Widespread yellowing earlier could signal overwatering, poor drainage, a nutrient deficiency, or pest damage like thrips. Check soil moisture and inspect the leaves for insects.

Can I grow onions in containers on my Colorado patio?

Absolutely. Choose a container at least 10-12 inches deep with good drainage holes. Use a quality potting mix and maintain consistent watering, as pots dry out faster. Select varieties suited for closer spacing, like scallions or smaller bulb onions.

Growing onions in Colorado is very achievable with the right timing and knowledge. By planting the correct day-length variety at the right time in prepared soil, you’ll be well on your way to a bountiful harvest. Remember, the extra effort you put into soil preparation and consistent watering pays off with crisp, flavorful onions from your own garden.