Planting Guides
When to Plant Onions in Boston: Complete Guide + Best Varieties
Boston, Massachusetts
USDA Zone 6b
Last Frost: Apr 20
Last updated: December 8, 2025
Plant onions in Boston April 5-26 for Zone 6b success. Learn long-day varieties, exact timing, and complete growing guide for Massachusetts gardeners.
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Quick Answer
Plant onions in Boston April 5-26 when soil reaches 50°F consistently. Use long-day varieties for Zone 6b success.
TL;DR
Plant onions in Boston from April 5-26 using long-day varieties like Allium cepa 'Walla Walla' for Zone 6b success. Plant garlic cloves October 15-November 5 for next summer's harvest. Boston's 183-day growing season and 42°N latitude require long-day onion varieties that bulb when daylight reaches 14+ hours in late June.
Frequently Asked Questions
When exactly should I plant onions in Boston?
Plant onions in Boston from April 5-26 when soil temperature reaches 50°F consistently. This timing allows 2-3 weeks before the average last frost date of April 19th while giving onions the full growing season needed for proper bulb development. Planting earlier in cold, wet soil leads to rot; planting later reduces final bulb size as day length triggers bulbing in late June regardless of planting date.
What onion varieties grow best in Boston's Zone 6b climate?
Long-day varieties are essential for Boston's 42°N latitude and Zone 6b climate. Allium cepa 'Walla Walla' produces sweet, large bulbs perfect for fresh eating through late summer. Allium cepa 'Red Wing' develops into excellent storage onions lasting 8+ months when properly cured. Allium cepa 'Yellow Sweet Spanish' creates the largest bulbs, often exceeding one pound with proper care. Never plant short-day varieties in Boston—they won't form bulbs in our latitude.
Can I plant onions in fall in Boston?
Fall onion planting is not recommended for Boston gardeners growing bulb onions. Boston's Zone 6b climate lacks the mild winters necessary for overwintering onion varieties. However, fall garlic planting is essential—plant Allium sativum 'Music' or Allium sativum 'California Early' cloves from October 15-November 5 for next summer's harvest. Garlic requires winter cold exposure to form proper cloves and performs best with fall planting.
Why are my Boston onions not forming bulbs?
Onions fail to bulb in Boston due to wrong variety selection or late planting. Short-day and intermediate-day onions won't form bulbs at Boston's 42°N latitude because our summer days (15+ hours) exceed their trigger points. Plant only long-day varieties that require 14-16 hours of daylight to bulb. Late planting after April 26th also reduces bulbing as plants lack sufficient leaf growth before day length triggers the bulbing process in late June.
How do I prepare Boston's clay soil for onions?
Boston's heavy clay soil requires significant amendment for successful onion growing. Work 2-3 inches of compost into the top 8 inches of soil each spring, adding coarse sand (never fine sand) to improve drainage. Build raised rows 4-6 inches high even within raised beds to ensure proper drainage. Test soil pH and add sulfur if above 7.0. Never work clay soil when wet—wait until it crumbles easily in your hand to avoid compaction.
When should I harvest onions in Boston?
Harvest onions in Boston when 50-75% of tops fall over naturally, typically mid-August through early September depending on planting date and variety. Allium cepa 'Red Wing' planted April 10th usually matures by August 25th. Never force tops to fall by stepping on them—this creates wounds that cause storage rot. Bulbs should feel firm when squeezed gently at soil level, with outer skins beginning to develop papery texture.
How do I store onions through Boston winters?
Cure onions first in a warm (75-85°F), dry location with good air circulation for 2-3 weeks until skins are papery and necks tight. Store cured onions in Boston basements or unheated garages that maintain 32-40°F through winter. Use ventilated containers or mesh bags—never plastic. Check monthly and remove any soft bulbs immediately to prevent spoilage spread. Properly cured Allium cepa 'Red Wing' stores 8-10 months in Boston conditions.
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Written By
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Kevin Doyle
Kevin is a landscape contractor in the Boston area who specializes in cold-climate perennial gardens and hardscaping. He studied landscape design and worked at a prominent nursery on the North Shore before starting his own design-build firm. Kevin's gardens are built to handle New England weather—freeze-thaw cycles, nor'easters, late springs, and heavy snow loads. He's particularly interested in native perennials and shrubs that provide four-season interest without constant maintenance. Kevin writes about cold-hardy plant selection, winter garden structure, and building landscapes that look good even in February.
