Onion in Zone 6B — Mountain West

Onions are one of the most rewarding crops for Mountain West gardeners, transforming our intense high-altitude sunlight into sweet, storage-worthy bulbs that outshine anything from the store. With our cool nights and moderate summer temperatures, onions develop excellent flavor complexity while avoiding the bitter, harsh taste that comes from extreme heat. They're also remarkably well-suited to our alkaline soils and low humidity conditions.

While our short 163-day growing season and late spring start might seem challenging for a 100-day crop, onions are actually quite forgiving in our climate. Their cold tolerance means you can plant them early, and the consistent cool nights throughout summer help them size up without bolting. The key is understanding our unique timing windows and working with our late but reliable spring.

Starting onions from seed indoors makes sense if you want the largest possible bulbs or specific varieties not available as sets. Begin seeding in late January through mid-February, about 10 weeks before your planned transplant date. Our late spring means this indoor start gives onions the head start they need to reach full size.

Set up seed trays in a warm spot (65-70°F) with good light once germination occurs. Bottom watering works especially well for onion seedlings, preventing the thin stems from getting knocked over. The seedlings look like grass and grow slowly at first, which is normal.

With our consistently cool spring nights, indoor-started seedlings often perform better than direct-sown seeds, giving you more control over timing. However, onion sets remain the easiest option for most Mountain gardeners, offering reliability without the indoor setup.

Transplant your indoor-started onion seedlings from mid-April through early May, once soil is workable and nighttime temperatures stay above 25°F consistently. Harden them off gradually over a week, as our intense UV can shock tender seedlings that haven't adapted to high-altitude sun exposure.

Space transplants 4-6 inches apart in well-prepared soil. Our alkaline conditions are actually favorable for onions, though adding compost helps with drainage and nutrition. Plant them at the same depth they were growing in their containers.

Watch for our typical spring temperature swings during this period. A late cold snap won't kill established transplants, but row cover protection helps them establish more quickly. The key is getting them in the ground early enough to take advantage of our cool, moist spring conditions before summer's dry period begins.

Direct sowing onion seeds works well in the Mountain West, especially for storage varieties. Sow from late March through mid-June, though earlier plantings generally produce larger bulbs. Our cool spring soils and consistent moisture from snowmelt provide ideal germination conditions.

Prepare your bed with loose, well-draining soil and sow seeds about 1/2 inch deep. Space them 4-6 inches apart, or sow more densely and thin later. Soil temperature needs to be at least 35°F for germination, which typically occurs by early April in most Mountain valleys.

The easier approach is planting onion sets (small bulbs) in early spring as soon as soil can be worked. Sets establish faster than seeds and are less affected by our variable spring weather. However, seed varieties offer better storage potential and more variety choices than what's typically available in sets.

Onions need consistent moisture throughout their growing season, which requires attention in our dry Mountain climate. They're not drought tolerant, and inconsistent watering leads to smaller bulbs or premature bolting. Plan for about 1 inch of water per week, including any natural rainfall.

Our low humidity and intense sun mean surface soil dries quickly, but check moisture 2 inches down with your finger before watering. Water deeply at the base rather than overhead, as our dry air makes foliage watering inefficient and our low humidity means disease pressure is minimal anyway.

During bulb formation in mid to late summer, maintain steady soil moisture but avoid overwatering, which can lead to soft, poorly-storing bulbs. A 2-inch layer of organic mulch helps retain moisture and keeps soil temperatures more consistent in our variable mountain weather.

Critical timing: Stop watering completely when onion tops begin to fall over and yellow, typically in late summer. This signals the start of the natural curing process, and continued watering at this stage prevents proper storage development.

Begin harvesting onions in early July through mid-October, depending on variety and planting time. The clear signal for harvest is when tops fall over and begin yellowing from the bottom up. This typically happens about 100 days from planting, though our cool nights can extend this timeline slightly.

Pull bulbs on a dry day when soil isn't muddy. Brush off excess soil but don't wash them. If tops haven't fallen naturally by early September, you can bend them over to start the curing process. Handle carefully to avoid bruising, which affects storage life.

Cure harvested onions in a warm, dry, well-ventilated space for 2-3 weeks before storing. Our low humidity is actually perfect for this curing process. Spread them in a single layer or braid the tops together for hanging.

With our first frost typically arriving in early October, make sure to harvest all remaining onions well before then. Late-harvest onions may not cure properly if caught by frost, though light frost won't immediately damage unharvested bulbs if you need a few extra days.

Onion Thrips These tiny insects cause silvery streaks and stippling on onion leaves, eventually leading to brown, withered foliage. You'll see tiny black specks (thrips excrement) on affected leaves. Our dry conditions and intense UV can stress plants, making them more susceptible to thrip damage.

Cool, wet springs reduce thrip populations, but our typically dry summers favor their development. Use row covers early in the season, and ensure consistent watering to keep plants healthy and resilient. Remove any infected plant debris and consider beneficial insects like lacewings.

Pink Root This fungal disease causes onion roots to turn pink, then purple, and eventually black and brittle. Plants may appear stunted with yellowing leaves, and bulbs remain small. The disease thrives in warm, moist conditions but can persist in soil for years.

Our alkaline soils can harbor this pathogen, especially in areas with poor drainage. Improve soil drainage and avoid overwatering. Rotate onion family crops to different areas each year. Choose resistant varieties when available, and avoid planting in areas where you've seen this problem before.

Bolting Onions send up flower stalks prematurely, shifting energy from bulb development to seed production. You'll see a thick, hollow stem emerging from the center with a round flower head. Once bolted, the bulb typically remains small and doesn't store well.

Our variable spring temperatures and long summer days can trigger bolting, especially in varieties not suited to our latitude. Plant appropriate day-length varieties (long-day types for our region), avoid planting too early when soil is still cold, and maintain consistent soil moisture. If bolting occurs, harvest immediately and use fresh rather than storing.

Mountain West Specific Challenges: Our intense UV and low humidity can stress onion plants, making them more susceptible to pest problems. However, the same dry conditions that favor thrips also reduce fungal disease pressure compared to more humid regions. Focus on consistent watering and soil health to help plants withstand our unique high-altitude growing conditions.

Onions are excellent companions for carrots, as their strong scent helps deter carrot rust flies while carrots' taproots break up soil for onion bulb development. Plant them together in alternating rows for mutual benefit. Lettuce and other greens also pair well, as onions' vertical growth doesn't compete for space, and the onions help repel aphids that commonly attack leafy crops.

Tomatoes benefit from onion companions, as onions may help deter some soil-dwelling pests and their shallow root system doesn't compete with tomatoes' deeper roots. However, avoid planting onions near beans or peas, as onions can inhibit the nitrogen-fixing bacteria these legumes rely on. In our Mountain growing conditions, focus on companions that make efficient use of space and water, since both are often at a premium in our short, dry growing season.