If you’re a gardener in Colorado, you know timing is everything. Getting your planting schedule right is key to a successful harvest, especially for crops like onions. This guide will help you understand exactly when to plant onion sets in Colorado for the best results.
Onions are a fantastic crop for our state. They’re relatively low-maintenance and store well. But planting them at the wrong time can lead to small bulbs or premature bolting. Let’s break down the ideal timing and methods to ensure you get big, beautiful onions from your sets.
When to Plant Onion Sets in Colorado
The best time for planting onion sets in Colorado is typically from mid-March to late April. This window targets early spring, as soon as your garden soil is workable. You need to get them in the ground early because onions prefer cooler weather for initial growth.
Here’s a more detailed look at the timing across the state:
- Front Range & Urban Corridor (Denver, Colorado Springs, Fort Collins): Aim for late March through mid-April. The lower elevations warm up a bit sooner.
- Western Slope (Grand Junction, Palisade): You can often start a little earlier, in mid-March, thanks to the milder winters and warmer springs.
- High Mountain Valleys & San Luis Valley: Wait until late April or even early May. Frost lingers much longer at higher altitudes.
The ultimate signal is the soil itself. When the ground thaws, dries out enough that it’s not a muddy clump, and can be worked, it’s time. A good test is to squeeze a handful; if it crumbles easily, you’re good to go.
Why Timing Matters So Much
Planting too early in cold, wet soil can cause the sets to rot. Planting too late, however, means they won’t have enough time to grow large bulbs before the long, hot days of summer trigger bulbing. Onions are photoperiodic, meaning bulb formation is triggered by day length. We need to give the plants plenty of time to establish green tops before that signal kicks in.
Understanding Colorado’s Onion Growing Seasons
Onions are categorized by the ammount of daylight they need to form bulbs: short-day, intermediate-day, and long-day. For most of Colorado, long-day varieties are the only reliable choice. These varieties begin forming bulbs when daylight lasts 14-16 hours, which matches our summer conditions perfectly.
- Popular Long-Day Varieties for Colorado: Walla Walla, Yellow Sweet Spanish, Red Zeppelin, Sterling, and Patterson.
- Avoid Short-Day Varieties: These are suited for southern latitudes and will bulb too early here, resulting in very small onions.
Choosing Between Seeds, Sets, and Transplants
While this article focuses on sets, it’s good to know your options. Sets are small, dormant onion bulbs grown the previous year. They are the easiest for beginners and give you a head start. Seeds offer more variety but require starting indoors very early. Transplants are young started plants, similar to buying vegetable starts for your garden.
How to Plant Your Onion Sets Step-by-Step
Follow these steps for planting success:
- Prepare the Soil: Choose a spot with full sun. Onions need well-draining soil. Work in several inches of compost or well-rotted manure. They prefer a slightly acidic to neutral pH (6.0-7.0).
- Planting Depth & Spacing: Plant the sets about 1 inch deep. Space them 4-6 inches apart in rows that are 12-18 inches apart. The pointed tip should be just barely visible at the soil surface.
- Watering: Water thoroughly after planting. After that, provide about 1 inch of water per week, including rainfall. Consistent moisture is crucial for good bulb development.
- Fertilizing: Use a nitrogen-rich fertilizer at planting and again when the plants are about 6 inches tall. Stop fertilizing once the bulbs begin to swell, as too much nitrogen late can hurt storage quality.
Common Problems and Solutions
Even with perfect timing, you might face a few challenges:
- Bolting: This is when an onion sends up a flower stalk. It’s often caused by temperature fluctuations or planting sets that are too large. Once an onion bolts, the bulb stops growing. Use sets that are about 3/4 inch in diameter—no larger.
- Pests: Onion thrips and onion maggots can be issues. Use floating row covers as a barrier and practice crop rotation each year.
- Diseases: Good soil drainage and air circulation between plants helps prevent fungal issues like botrytis or downy mildew.
Caring for Your Growing Onions
Weeding is critical. Onions have shallow roots and don’t compete well. Mulching with straw can help suppress weeds and retain soil moisture. As the bulbs mature and the tops begin to yellow and fall over, stop watering. This allows the outer skins to toughen up for harvesting.
Harvesting and Curing for Storage
Harvest time in Colorado is usually from late July through August. When about half the tops have fallen over naturally, gently loosen the soil and lift the bulbs. Don’t just yank them by the stems.
Proper curing is essential for storage. Lay the bulbs in a single layer in a warm, dry, well-ventilated place out of direct sun for 2-3 weeks. Once the necks are tight and dry and the outer skins are papery, you can trim the roots and tops and store them in a cool, dry location.
FAQ: Planting Onions in Colorado
Can I plant onion sets in the fall in Colorado?
Generally, no. Our winters are too severe for most onion sets to survive in the ground. Spring planting is the reliable method.
What if I miss the spring planting window?
You can still try planting sets up until late May, but the bulbs will likely be smaller. It’s better to aim for the prime spring window.
How do I protect young plants from a late frost?
Onion sets are quite cold-tolerant. A light frost usually won’t harm them. If a hard freeze is predicted after planting, you can cover them with a frost cloth or row cover for protection.
Can I grow onions in containers on my Colorado patio?
Absolutely. Choose a deep container (at least 10-12 inches) with good drainage holes. Use a quality potting mix and follow the same spacing and care guidelines. They might need water more frequently.
By following this guide and paying attention to your local conditions, you’ll be well on your way to a bountiful onion harvest. Remember, the simple act of getting your timing right—when to plant onion sets in Colorado—is the most important step you can take. With a little planning and care, you’ll enjoy homegrown onions for months to come.