Onion in Zone 7A — Mountain West

Onions are absolutely worth growing in our Mountain West climate — they thrive in our intense sunlight and alkaline soil conditions. Our cool nights help develop that sharp, complex flavor that makes storage onions so valuable for winter cooking, while the low humidity means fewer fungal diseases to worry about. You'll get varieties and flavors you simply can't find in stores, plus the satisfaction of harvesting enough onions to last through our long winters.

Our high altitude and short season do present challenges, but onions are actually well-suited to Mountain gardening. With our 173-day growing season and late spring starts, timing becomes crucial — but onions are frost-hardy enough to go out early and mature before our first frost in early October. The key is getting them established while the soil is still cool in spring.

Starting onion seeds indoors makes sense if you want the largest possible bulbs or specific varieties not available as sets. You'll want to start seeds from mid-January through early February, about 10 weeks before your transplant date. This early start gives onions maximum time to size up before summer heat triggers bulb formation.

Set up seed trays in a warm spot (65-70°F works well) with good light once they germinate. Bottom watering works best — onion seedlings are thin and delicate, so watering from above can knock them over. Keep the soil consistently moist but not soggy.

The main advantage of starting from seed is variety selection and cost — you can grow storage onions, sweet onions, or specialty varieties for a fraction of the cost of sets. However, seedlings require more babying than sets, and our late spring means you're committed to indoor growing for nearly three months.

Transplant your onion seedlings from early to late April, timing it after the worst of our spring temperature swings but while soil is still cool. Onions prefer to establish in cool weather, so don't wait too long even if nights are still chilly — they can handle light frosts just fine.

Harden off seedlings gradually over a week, starting with just a few hours outside and building up. In our intense Mountain sun, even hardy plants need time to adjust to the UV exposure they'll face. Space transplants 4-6 inches apart in well-draining soil — closer spacing gives you smaller bulbs, wider spacing allows for larger ones.

The easiest method for beginners is actually onion sets (small bulbs), not transplants. Sets are less fussy, establish quickly, and are ready to plant as soon as you can work the soil. You'll find them at local garden centers right when you need them.

Direct sowing onion seeds works well in our climate, especially for green onions or if you don't mind smaller bulbs. Sow from late March through mid-June, though earlier plantings will give you larger storage onions. The soil just needs to be workable — onions germinate in cool conditions that would stall warm-season crops.

Prepare your bed with compost worked into our typically alkaline soil. Onions aren't heavy feeders, but they appreciate well-draining soil that won't waterlog during spring snowmelt. Plant seeds about half an inch deep and 4-6 inches apart, or sow more thickly and thin as they grow.

For sets, plant them pointed end up with just the tip showing above soil level. Sets are foolproof and give you a head start — they're already a year old and ready to put energy into bulb development rather than getting established from seed.

Onions need consistent moisture during their growing season, especially during bulb formation in summer. In our dry Mountain climate with only 10-20 inches of annual rainfall, you'll be doing most of the watering yourself. Plan on about an inch of water per week, applied deeply rather than frequently.

Water at soil level rather than overhead — our low humidity means foliar diseases aren't a huge concern, but efficient water use matters more in our dry climate. Use the finger test: stick your finger 2 inches into the soil, and if it's dry, it's time to water. In our intense sun and low humidity, soil dries out faster than you might expect.

Stop watering completely when onion tops begin to fall over and yellow — this signals the start of the curing process. Continuing to water at this stage can lead to soft bulbs that won't store well. This usually happens in mid to late summer as the plants naturally prepare for dormancy.

A thin layer of straw mulch helps retain moisture in our dry conditions, but don't mulch too heavily early in the season when soil is still warming up. Our cool spring nights mean the soil needs all the solar warming it can get.

Your first onions will be ready from early July through mid-October, depending on when you planted and which varieties you chose. Green onions can be harvested anytime, but for storage onions, wait until the tops naturally begin to fall over and turn yellow. This typically happens about 100 days from planting.

Don't pull onions too early — the bulb is still sizing up as long as the tops are green and upright. But once 50-75% of the tops have fallen over, it's time to harvest the whole crop. Choose a dry day if possible, and gently lift bulbs with a garden fork rather than pulling by the tops.

After harvest, cure onions in a warm, dry place with good air circulation for 2-3 weeks. Our low humidity is perfect for this — spread them on screens or hang them in braids in a garage or covered porch. Properly cured onions will keep through winter in cool, dry storage.

As first frost approaches in early October, harvest any remaining onions even if the tops haven't fully died back. Green-topped onions won't store as long, so use these first while your properly cured onions wait in storage.

Onion Thrips These tiny insects cause silvery streaks and stippling on onion leaves, eventually leading to yellowing and reduced bulb size. In our intense Mountain sun, damaged leaves can't photosynthesize efficiently, stunting growth. Thrips thrive in hot, dry conditions — exactly what we have in summer.

Row covers early in the season help prevent initial infestation. Blue sticky traps can monitor and catch adults. If thrips appear, insecticidal soap works well, though you'll need repeat applications. Our dry air means treatments dry quickly and may be less effective than in humid climates.

Pink Root This soil-borne fungal disease turns onion roots pink to red, eventually killing them and stunting bulb development. You'll notice plants that seem to struggle despite adequate water, with roots that look discolored when you dig them up. Pink root thrives in warm, alkaline soils — our alkaline Mountain soils can make this more common.

Rotate onion crops to different beds each year and avoid planting in the same spot for at least three years. Improve drainage and add organic matter to help soil structure. Choose resistant varieties when possible, and avoid overwatering, which creates conditions the fungus loves.

Bolting Onion plants send up a flower stalk prematurely, making the bulb bitter and woody. The plant shifts energy from bulb development to seed production, leaving you with small, poor-quality onions. Cold spells followed by warm weather can trigger bolting, and our spring temperature swings make this a real concern.

Plant at the right time for your area — too early and cold snaps trigger bolting, too late and heat does the same. Remove flower stalks immediately if they appear, though the bulb quality is already compromised. Choose bolt-resistant varieties and keep soil consistently moist during temperature swings.

Mountain West Specific Challenges Our intense UV can stress onion plants if they're not properly established, while alkaline soil can tie up nutrients and encourage pink root disease. Sudden hail storms can shred leaves just when bulbs are sizing up. The good news is our dry climate and cool nights reduce many common onion diseases that plague more humid regions.

Onions make excellent companions for carrots, beets, and lettuce in Mountain gardens. Carrots and onions planted together help each other — onions repel carrot flies while carrots help break up soil for onion roots. Both crops appreciate similar cool-season growing conditions and moderate water needs in our dry climate.

Tomatoes also benefit from onion companions, as onions help deter many soil pests that affect tomatoes. In our intense Mountain sun, the upright onion foliage doesn't shade low-growing plants but can provide a bit of wind protection. Avoid planting onions with beans or peas — onions can inhibit the nitrogen-fixing bacteria that legumes depend on, reducing their productivity in our already challenging short season.