Planting Guides
When to Plant Onions in Milwaukee: Complete Guide + Best Varieties for Zone 5b
Milwaukee, Wisconsin
USDA Zone 5b
Last Frost: May 15
Last updated: October 30, 2025
Learn when to plant onions in Milwaukee with specific dates for Zone 5b. Compare 6 varieties & discover which onions grow best in Wisconsin's climate.
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Quick Answer
Plant onion sets April 15-May 1 in Milwaukee, 2-3 weeks before May 14 last frost. Long-day varieties like Walla Walla and Red Wing require Wisconsin's 14+ hour summer days to bulb properly.
TL;DR
Milwaukee gardeners in Zone 5b should plant long-day onions between April 15-May 1, about 2-3 weeks before the May 14 last frost date. Best varieties include Allium cepa 'Walla Walla', Allium cepa 'Red Wing', and Allium cepa 'Yellow Sweet Spanish' which require 14-16 hours of daylight to form bulbs. Plant garlic cloves in October for summer harvest, using hardneck varieties like Allium sativum 'Music' that thrive in Wisconsin's cold winters.
Frequently Asked Questions
When should I plant onions in Milwaukee to avoid frost damage?
Plant onion sets and transplants between April 15-May 1 in Milwaukee, which is 2-3 weeks before our May 14 last frost date. Onions handle light frost down to 25°F (-4°C), but cover them with row cover during severe cold snaps. The key is getting them established early enough to maximize vegetative growth before day length triggers bulbing in mid-June. Later planting results in smaller bulbs due to the shortened growing period before the photoperiod switch occurs.
What onion varieties grow best in Milwaukee's Zone 5b climate?
Milwaukee requires long-day onion varieties that need 14-16 hours of daylight to form bulbs. The best varieties for our climate are Allium cepa 'Red Wing' for storage (keeps 8-10 months), Allium cepa 'Walla Walla' for fresh eating (sweet, large bulbs), and Allium cepa 'Yellow Sweet Spanish' for both cooking and moderate storage. Avoid short-day and intermediate-day varieties—they won't bulb properly at our 43°N latitude and will remain as large scallions instead of forming bulbs.
Can I plant onions in fall for overwintering in Milwaukee?
Fall-planted onions for overwintering generally don't succeed in Milwaukee's Zone 5b climate due to our harsh winters with temperatures reaching -10 to -20°F (-23 to -29°C). Unlike milder climates (Zones 7-8), our onions would freeze-kill without expensive protection. Stick to spring planting of long-day varieties. However, garlic should be planted in fall—plant Allium sativum 'Music' cloves in October for July harvest. Garlic requires winter cold exposure (vernalization) to form proper bulb divisions.
How do I prevent my onions from bolting in Milwaukee?
Bolting prevention in Milwaukee focuses on temperature management and proper variety selection. Use small onion sets (smaller than a dime) rather than large sets, which are more prone to bolting when stressed. Plant long-day varieties appropriate for our latitude—never short-day types. Cover plants with row cover during late spring cold snaps below 25°F (-4°C), as temperature stress after warm periods triggers bolting. Plant within the proper timing window (April 15-May 1) and ensure consistent soil moisture to minimize stress factors that encourage premature flowering.
When do I harvest onions in Milwaukee and how do I store them?
Harvest storage onions in Milwaukee when 50-75% of the tops fall over naturally, typically late August to early September. Never manually bend tops over—this prevents proper neck sealing and causes storage rot. Cure onions in a warm (80-85°F), dry, well-ventilated area for 2-4 weeks until outer skins become papery and necks feel tight and wiry. Store properly cured onions at 32-40°F (0-4°C) with 65-70% humidity and good air circulation. Allium cepa 'Red Wing' stores 8-10 months under these conditions, while sweet varieties like Allium cepa 'Walla Walla' should be used within 1-3 months.
Should I grow onions from sets, transplants, or seeds in Milwaukee?
Onion sets work best for Milwaukee gardeners due to our relatively short 139-day growing season and variable spring weather. Sets handle temperature fluctuations better than transplants and give you a 4-6 week head start over seeds. Choose sets smaller than a dime to minimize bolting risk. Transplants are a good second choice and offer better variety selection, but they're more expensive and need careful handling. Avoid starting from seeds unless you have a heated greenhouse—our growing season is too short to achieve full bulb size when starting seeds outdoors, and indoor starting requires 10-12 weeks before transplanting.
What's the difference between hardneck and softneck garlic for Milwaukee?
Milwaukee requires hardneck garlic varieties like Allium sativum 'Music' because they're bred for cold climates and need vernalization (winter cold exposure) to form proper clove divisions. Hardneck types produce edible flower stalks (scapes) in late spring that should be removed to maximize bulb size. They store 6-8 months and have complex, rich flavors preferred by chefs. Softneck varieties struggle in Zone 5b because they're adapted to milder climates (Zones 7-10) and may not receive adequate cold hours for proper bulb development. Plant hardneck cloves in October for July harvest.
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Patricia "Pat" O'Brien
Pat has been gardening in Milwaukee for over thirty years, through every kind of Wisconsin weather. She worked as a school librarian and started gardening seriously when her kids were small—she wanted them to know where food came from. Now retired, she's a certified Master Gardener and runs the plant sale at her local garden club every spring. She specializes in cold-hardy perennials, native wildflower meadows, and the art of getting a vegetable garden producing in Wisconsin's short but intense growing season. Pat is practical and patient—she's seen enough Wisconsin winters to know that gardening here is a long game, and she writes with the steady confidence of someone who's been doing this a long time.
