Colorado’s high altitude means onions need extra time to develop full bulb size before harvest. If you are wondering about the best time to harvest onions in Colorado, the short answer is mid-July to early September. But the exact timing depends on your variety, planting date, and local weather patterns. This guide will walk you through every sign and step to get it right.
Onions are a cool-season crop that thrive in Colorado’s sunny days and cool nights. However, our short growing season and unpredictable frost dates can make harvest timing tricky. Harvest too early and you get small bulbs. Harvest too late and they may rot or bolt.
Best Time To Harvest Onions In Colorado
The ideal window for harvesting onions in Colorado is when the tops have fallen over naturally and the necks have begun to dry. This usually happens 100 to 120 days after planting sets or 130 to 150 days after seeding. For most of the state, that falls between late July and early September.
But don’t rely on the calendar alone. Colorado’s microclimates vary wildly. A garden in Denver might be ready two weeks earlier than one in Colorado Springs or Grand Junction. The key is to watch the plant, not the date.
Signs Your Onions Are Ready
Look for these clear indicators before you pull any bulbs:
- About 50% to 80% of the onion tops have flopped over naturally.
- The neck of the onion feels soft and starts to shrivel.
- The bulb is firm and has reached a decent size for its variety.
- The outer skin begins to papery and dry.
If you see these signs, it is time to start the harvest process. Do not wait for all tops to fall. Once most have tipped, the bulbs stop growing.
What If Tops Don’t Fall Over?
Sometimes Colorado’s cool nights or dry conditions keep tops standing tall. If the necks are still thick and green, the onion is still bulking up. Give it another week or two. But if frost is predicted, pull them anyway. Frost damage ruins storage quality.
Colorado Climate Factors That Affect Harvest
Colorado’s high altitude, low humidity, and intense sun all impact onion growth. Here is how each factor shifts your harvest timing:
- Altitude: Higher elevations mean cooler temps and slower growth. Onions in mountain towns like Leadville may need until September.
- Sunlight: Long summer days speed up bulbing. But intense UV can also cause sunscald on exposed bulbs.
- Soil: Colorado’s clay or sandy soils drain fast. Onions in sandy soil may mature quicker due to warmer root zones.
- Water: Inconsistent watering can stress onions and delay maturity. Keep soil evenly moist until tops fall.
Because of these factors, the best time to harvest onions in Colorado is not a single date but a range. You need to adjust for your specific garden.
Short-Day Vs. Long-Day Onions
Colorado is a long-day onion region. That means you should plant varieties that bulb when day length reaches 14 to 16 hours. Short-day onions, common in the South, will not bulb properly here.
Popular long-day varieties for Colorado include:
- Yellow Sweet Spanish
- Walla Walla
- Red Burgundy
- White Sweet Spanish
These typically mature in 100 to 120 days. Check your seed packet for days to maturity and count backward from your average first fall frost date. That gives you a rough target.
Step-By-Step Harvest Process
Once you have confirmed your onions are ready, follow these steps for the best results:
- Stop watering about two weeks before harvest. This helps the skins dry and reduces rot risk.
- Gently loosen the soil around each bulb with a garden fork. Do not pull by the tops alone or they may break.
- Lift the bulbs carefully, shaking off loose soil. Do not wash them yet.
- Leave them in the garden for a few hours to dry if the weather is sunny. Bring them inside if rain is expected.
- Move to a curing area once the outer skin is dry to the touch.
Curing Onions Properly
Curing is essential for long-term storage. Without it, onions rot quickly. Here is how to cure onions in Colorado’s dry climate:
- Place onions in a single layer in a well-ventilated, shady spot. A garage or covered porch works well.
- Keep them out of direct sunlight. Sun can cook the bulbs and cause green patches.
- Let them cure for 2 to 3 weeks. The necks should be completely dry and the outer skin papery.
- Once cured, cut off the tops about an inch above the bulb. Trim the roots close to the base.
Colorado’s low humidity actually helps curing. But if you have a wet spell, move them indoors to avoid mold.
What About Green Onions?
If you are growing scallions or green onions, harvest them anytime after the tops are 6 to 8 inches tall. They do not need curing. Just pull, wash, and use fresh.
Common Harvest Mistakes In Colorado
Even experienced gardeners make errors. Avoid these common pitfalls:
- Harvesting too early: Small bulbs mean less yield. Wait for tops to fall.
- Harvesting too late: Overripe onions may split or rot in the ground.
- Pulling wet onions: Moisture leads to mold during curing.
- Storing uncured onions: They will spoil fast in the pantry.
- Ignoring frost warnings: A hard freeze can ruin your crop overnight.
If you are unsure, err on the side of waiting. Onions can stay in the ground for a week or two after tops fall without major issues, as long as the soil is not waterlogged.
Storing Onions After Harvest
Proper storage extends your harvest for months. Here is what works best in Colorado:
- Store in a cool, dry, dark place. Ideal temperature is 32°F to 40°F.
- Use mesh bags, pantyhose, or wooden crates for airflow.
- Do not store onions with potatoes. Potatoes release moisture and ethylene gas that spoils onions.
- Check regularly for soft or sprouting onions and remove them.
With good curing and storage, Colorado-grown onions can last until spring. Some varieties, like Yellow Sweet Spanish, store better than others.
Best Varieties For Colorado Storage
Not all onions store equally. For long-term storage, choose:
- Yellow storage onions (e.g., Copra, Redwing)
- White onions (e.g., White Bermuda, but shorter storage)
- Red onions (e.g., Red Creole, but moderate storage)
Sweet onions like Walla Walla are best eaten fresh because they have higher water content and spoil faster.
Regional Timing Differences In Colorado
Colorado spans several climate zones. Here is a rough guide by region:
- Denver Metro: Harvest typically late July to mid-August.
- Front Range (Colorado Springs, Fort Collins): Early to mid-August.
- Western Slope (Grand Junction, Montrose): Mid-July to early August due to warmer temps.
- Mountain Towns (Leadville, Breckenridge): Late August to early September. Use short-season varieties.
- San Luis Valley: Early to mid-August. This region is known for commercial onion production.
Always check your local frost dates and adjust. A late spring or cool summer can push harvest back by two weeks.
Frequently Asked Questions
When is the best time to harvest onions in Colorado?
The best time is when 50% to 80% of the tops have fallen over naturally, usually from late July to early September depending on your location and variety.
Can I harvest onions after a frost in Colorado?
It is not recommended. Frost damages the bulbs and reduces storage life. Harvest before the first hard freeze.
How do I know if my onions are ready in Colorado’s high altitude?
Look for soft necks, fallen tops, and firm bulbs. High altitude slows growth, so count days from planting and watch for these signs.
What happens if I harvest onions too early in Colorado?
You get small bulbs that do not store well. They may also have thick necks that rot during curing.
Should I water onions before harvest in Colorado?
Stop watering about two weeks before harvest. This helps the skins dry and prevents rot.
Final Tips For Colorado Onion Growers
Growing onions in Colorado is rewarding but requires attention. The best time to harvest onions in colorado is not a fixed date but a condition. Watch your plants, check the weather, and be patient.
If you are new to onion growing, start with a small patch and note the harvest date. Next year you will have a better idea. Also, consider planting multiple varieties with different maturity dates to spread out your harvest.
Remember that Colorado’s dry climate is actually an advantage for curing. Use it to your benefit. With proper timing and care, you can enjoy homegrown onions all winter long.
One last thing: do not forget to label your varieties. It is easy to mix up sweet onions and storage onions after curing. A simple marker saves confusion later.
Happy gardening, and may your onions be big and flavorful.