Onion in Zone 5B — Midwest

Growing onions in our Zone 5B Midwest gardens is one of the most rewarding crops you can tackle. These hardy bulbs love our fertile soil and consistent growing season, developing that sharp, sweet flavor that store-bought onions just can't match. With our 168-day growing season, you have plenty of time to grow large, storage-quality onions that'll last you through winter.

Our Midwest weather can throw some curveballs with late spring frosts and summer heat spells, but onions are tough customers. They actually prefer our cool spring starts and handle our variable weather better than most crops. The key is getting your timing right and working with our natural growing patterns.

Starting onion seeds indoors makes sense if you want the largest possible bulbs or specific varieties that aren't available as sets. You'll want to start seeds from late January through mid-February, giving yourself about 10 weeks before transplanting outdoors. Set up seed trays in a warm spot (65-70°F) with good grow lights or a sunny south window.

Onion seedlings look like thin grass at first, so don't worry if they seem wimpy. Keep them warm and use bottom watering to prevent damping-off disease. A heat mat helps with germination in our cold Midwest winters.

The main downside is that seedlings need consistent care for months before they can go outside. Most Midwest gardeners find onion sets easier and more reliable, but starting from seed gives you access to storage varieties and heirloom types you won't find otherwise.

You can transplant onion seedlings outdoors from mid-April through early May, once our soil has warmed and the worst frost danger has passed. Watch the weather forecast carefully during this period - late frosts can still sneak in and set back young transplants.

Harden off seedlings gradually over a week, bringing them outdoors for longer periods each day. Our spring weather can be unpredictable, so be ready to cover plants if temperatures drop below 32°F. Transplant seedlings 4-6 inches apart in rows, setting them at the same depth they were growing indoors.

Space them properly from the start - crowded onions won't develop large bulbs. The cool, moist conditions of Midwest spring are perfect for getting transplants established before summer heat arrives.

Direct sowing onion seeds works well in our region, especially if you plant from late March through mid-June. Wait until soil temperature reaches about 50°F and you can work the ground without it clumping. Our fertile Midwest soil gives seeds a good start once conditions are right.

Plant seeds about half an inch deep and 4-6 inches apart. You can also direct plant onion sets during this same window - sets are actually easier for beginners since they're already established small bulbs. Sets give you a head start and are less fussy about soil conditions.

The main advantage of direct sowing is simplicity - no indoor setup required. However, you'll get smaller bulbs compared to transplanted seedlings, and germination can be spotty if we get a cold, wet spring.

Onions need consistent moisture throughout the growing season, but our Midwest climate makes this easier than in drier regions. With our typical 30-40 inches of annual rainfall, you may not need much supplemental watering except during dry spells. Check soil moisture by sticking your finger 2 inches deep - it should feel like a wrung-out sponge.

During bulb formation in mid-summer, aim for about 1 inch of water per week including rainfall. Water at the base of plants rather than overhead - our moderate humidity can encourage fungal problems if leaves stay wet too long. A soaker hose or drip irrigation works perfectly for onion beds.

Stop watering completely once the tops start to fall over and yellow in late summer. This signals that bulbs are maturing and need to begin the curing process. Too much water at this stage can cause bulbs to rot in storage.

Mulch around plants with grass clippings or straw to maintain consistent soil moisture and suppress weeds. Our clay soil tends to either stay too wet or dry out hard, so mulch helps moderate those extremes.

Your onions will be ready for harvest from early July through mid-October, depending on when you planted and which varieties you chose. The key signal is when the tops naturally fall over and start turning yellow-brown - this means the bulbs have stopped growing and are ready to cure.

Pull bulbs on a dry day and brush off excess soil, but don't wash them. Lay them out in a warm, well-ventilated area like a garage or covered porch for 2-3 weeks to cure. The papery outer skin should feel dry and rustle when touched.

Green onions can be harvested anytime during the growing season by pulling young plants before bulbs form. For storage onions, patience pays off - fully cured bulbs will keep through winter in a cool, dry place.

As our first frost approaches in early October, harvest any remaining onions even if the tops haven't completely died back. A light frost won't hurt the bulbs, but you'll want them out of the ground before hard freezes arrive.

Onion Thrips These tiny, slender insects cause silvery streaks and stippling on onion leaves, making plants look bleached or damaged. You might see the small, yellowish thrips moving around if you shake the leaves over white paper. Our warm, dry summer spells create perfect conditions for thrip populations to explode.

Heavy infestations weaken plants and reduce bulb size. Spray with insecticidal soap early in the morning or evening, and encourage beneficial insects with nearby flowering plants. Row covers during early growth can prevent initial infestations.

Pink Root This soil-borne fungal disease turns onion roots pink to red, eventually killing them and stunting plant growth. Affected plants have yellowing leaves and small bulbs. Pink root thrives in our heavy clay soils, especially when they stay waterlogged from spring rains or poor drainage.

Improve soil drainage by adding compost or planting in raised beds. Rotate onions to different garden areas each year and choose resistant varieties when possible. Avoid overwatering, particularly during our humid summer periods.

Bolting Onions send up flower stalks prematurely, shifting energy from bulb development to seed production. You'll see a tall, thick stem emerging from the center with a round flower head. This often happens after our variable spring weather stresses plants - warm spells followed by cold snaps trigger bolting.

Plant onions at the right time for your area and choose varieties suited to our day-length. Remove flower stalks immediately if they appear, though the bulbs won't store as well. Consistent moisture and avoiding heat stress help prevent premature bolting.

Midwest Specific Challenges Our moderate-to-humid summers and clay soil create conditions where fungal diseases can develop quickly. Good air circulation and proper spacing become especially important here. Summer heat spells stress plants just when bulbs are sizing up, so consistent watering and mulching are critical for success in our region.

Onions make excellent companion plants in Midwest gardens, and they pair especially well with carrots and beets. The onions help repel carrot flies and other root pests, while their upright growth doesn't compete with these root vegetables for space. Lettuce grows beautifully between onion rows, taking advantage of the light shade and benefiting from the pest-deterrent effects.

Tomatoes and onions are classic partners - onions may help reduce some tomato pests while tomatoes provide light shade during our hottest summer days. Avoid planting onions near beans and peas, as onions can inhibit the nitrogen-fixing bacteria that help legumes thrive. In our fertile Midwest soil, this companion effect seems more pronounced than in other regions.