Onion in Zone 6A — Mountain West

Onions are perfect for Mountain West gardens, thriving in our intense sunlight and cool nights that concentrate their flavor. These hardy bulbs store beautifully through our long winters, giving you homegrown onions from your July harvest well into the following spring. Our alkaline soil actually suits them well, and they're one of the few crops that can handle our intense UV without missing a beat.

While our 153-day growing season might seem challenging for a 100-day crop, onions are remarkably cold-hardy and can be planted early in spring. Our late spring start and early October frost mean timing matters, but these tough plants handle our temperature swings and cool nights better than most vegetables.

Starting onion seeds indoors makes sense if you want the largest possible bulbs or specific varieties not available as sets. Start seeds late January through mid-February, about 10-12 weeks before transplanting. This timing works well in our climate since spring arrives late anyway.

Use seed trays with good drainage and keep them warm (65-70°F) for germination. Once seedlings emerge, they need bright light—a south-facing window works, but grow lights prevent the leggy growth that can happen during our short winter days. Bottom watering prevents damping-off disease and keeps seedlings sturdy.

The main advantage of starting from seed is variety selection and larger final bulbs. However, onion sets are much easier for beginners and still produce excellent results in our Mountain climate.

Transplant your indoor-started seedlings mid-April through early May, once soil can be worked and nighttime temperatures stay consistently above 20°F. Harden them off gradually over a week—our intense UV and temperature swings can shock tender seedlings if you rush this step.

Space transplants 4-6 inches apart in rows, planting them at the same depth they were growing in their containers. Our cool spring nights actually help transplants establish strong root systems before summer growth kicks in.

Watch for late spring weather surprises—while onions handle frost well, a sudden warm spell followed by snow can stress newly transplanted seedlings. Row covers provide insurance during those unpredictable April and early May weeks when Mountain weather keeps us guessing.

Direct sowing works well in the Mountain West, either with seeds or onion sets. Plant early April through early June once soil temperature reaches 50°F—even if night temperatures still dip into the 20s, onions are tough enough to handle it.

For seeds, plant ¼ inch deep and thin to 4-6 inches apart once they germinate. Onion sets (small bulbs) are the easier option: plant them pointed end up with just the tip showing above soil level. Our alkaline soil suits onions well, but work in some compost if your soil is heavy clay.

Sets give you a head start and are nearly foolproof, making them ideal for beginners or gardeners dealing with our short season. Seeds offer more variety choices but take longer to size up—both methods work well in our climate.

Onions need consistent moisture during bulb formation but hate waterlogged soil. In our dry Mountain climate with only 10-20 inches of annual rainfall, you'll be the primary water source. Check soil moisture by sticking your finger 2 inches deep—if it's dry, it's time to water.

Water deeply once or twice per week, providing about 1-1.5 inches total. Our low humidity means water evaporates quickly from the soil surface, so water at the base of plants rather than overhead. This also prevents fungal issues that can develop when water sits on leaves, even in our dry climate.

During bulb formation (mid-summer), consistent moisture is critical. Signs of underwatering include wilted, bluish-green tops and smaller bulbs. Overwatering shows up as yellowing leaves and soft bulbs. A thin layer of straw mulch helps retain moisture without keeping soil too wet.

Stop watering completely once tops begin to fall over and yellow in late summer—this signals that bulbs are mature and beginning their natural curing process. Continued watering at this stage can cause bulbs to rot or sprout new growth.

Your first onions will be ready mid-July through early October, depending on variety and planting timing. Harvest when tops naturally fall over and begin yellowing—don't wait for them to completely die back, especially as our early October frost approaches.

Pull bulbs carefully, brushing off excess soil but leaving roots and tops intact. The papery outer skin should be forming, and necks should feel firm when gently squeezed. If some bulbs haven't sized up by late September, harvest them anyway as green onions rather than risk frost damage.

Cure harvested onions in a warm, dry, well-ventilated area for 2-3 weeks before storing. Our low humidity is perfect for this process—spread them on screens or hang in bunches in a garage or covered porch. Properly cured onions from Mountain gardens often store better than those from more humid climates.

Any onions with thick necks or soft spots should be used first, as they won't store well. Well-cured bulbs with tight necks will keep in cool, dry storage well into spring, giving you homegrown onions through the winter months.

Onion Thrips Tiny, slender insects that cause silvery streaks and stippling on leaves, eventually leading to yellowing and stunted growth. You might see the small, yellowish bugs themselves on closer inspection. Our intense UV and low humidity actually help control thrips compared to more humid regions, but they can still be problematic during warm spells.

Thrips thrive in hot, dry conditions—ironically, the same conditions our Mountain climate often provides. Encourage beneficial insects with diverse plantings nearby, and use reflective mulch to confuse thrips. Insecticidal soap works for severe infestations, but good garden hygiene usually keeps them manageable here.

Pink Root A fungal disease that turns roots pink to purple and eventually kills them, stunting bulb development. Affected plants may appear to grow normally at first but then yellow and decline. This disease persists in soil and is worse in heavy, poorly drained soils.

Our alkaline soil can contribute to pink root problems, especially in areas where drainage isn't perfect. Improve soil drainage and avoid planting onions in the same location year after year. Choose resistant varieties when possible, and ensure good air circulation between plants.

Bolting Plants send up a flower stalk prematurely, making bulbs smaller and leaves bitter. You'll see a thick, round stem shooting up from the center with a round flower head forming. Once bolting starts, the bulb stops growing and quality declines.

Bolting happens when plants experience stress from temperature swings, drought, or day length changes. Our variable spring weather—warm days followed by cold nights—can trigger this. Plant at the right time for your area, maintain consistent soil moisture, and choose bolt-resistant varieties for our challenging climate.

Mountain West Specific Challenges Our combination of intense UV, low humidity, and alkaline soil creates unique growing conditions for onions. While the dry air reduces many fungal problems common in humid regions, it increases water needs and can stress plants during hot afternoons, even at our moderate 85°F summer highs. Mulching and consistent watering become even more critical in our climate.

Onions make excellent companions for carrots, beets, and lettuce in Mountain gardens. The strong scent deters many pests that trouble these crops, while their upright growth doesn't compete for space. Carrots and onions particularly benefit each other—onions repel carrot flies while carrots help break up soil around onion bulbs as they grow.

Tomatoes also pair well with onions, as the onions help deter aphids and other soft-bodied insects that can damage tomato plants in our intense sun. Avoid planting onions near beans or peas, as onions can inhibit the nitrogen fixation that these legumes provide to your soil—something especially important in our often nutrient-poor Mountain soils.