Planting Guides
When to Plant Kale in Boston: Complete Guide + Best Varieties for Zone 6b
Boston, Massachusetts
USDA Zone 6b
Last Frost: Apr 20
Last updated: October 30, 2025
Learn when to plant kale in Boston with specific dates for Zone 6b. Compare 5 varieties & discover which kale grow best in Massachusetts climate.
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Quick Answer
Plant kale in Boston March 15-22 for spring harvest, July 15-August 8 for fall. Fall crops taste superior after frost sweetening.
TL;DR
Plant kale in Boston March 15-22 for spring harvest and July 15-August 8 for superior fall crops. Fall planting produces sweeter kale due to frost exposure. Boston's Zone 6b climate allows Brassica oleracea 'Winterbor' to overwinter with protection, providing harvest through January.
Frequently Asked Questions
When should I start kale seeds indoors in Boston?
Start kale seeds indoors February 15-March 1 for spring planting and June 15-July 1 for fall planting. Indoor starting provides 6-8 week old transplants ready for outdoor planting when conditions are optimal. Spring transplants go outside March 22-April 5, while fall transplants move outdoors August 1-15. This timing avoids the stress of direct sowing during Boston's variable spring weather or hot summer conditions.
Can kale survive Boston winters outdoors?
Brassica oleracea 'Winterbor' survives Boston winters with protection. Use row covers or cold frames when temperatures drop below 20°F (-7°C). Plants go dormant during January and February but resume growth in March. Mulch heavily around plant bases to prevent frost heaving. Coastal areas of Boston have better survival rates due to ocean moderation of temperature extremes.
Why does my Boston kale bolt in spring but not fall?
Spring bolting occurs because kale experiences vernalization—cold exposure followed by warming temperatures triggers flowering. Boston's cool April followed by warm May mimics the plant's natural two-year cycle. Fall plantings develop as temperatures drop, avoiding the cold-then-warm sequence. Additionally, fall kale benefits from decreasing daylight hours which suppress flowering hormones. Plant Brassica oleracea 'Vates' for best spring bolt resistance.
What's the best kale variety for small Boston gardens?
Brassica oleracea 'Vates' works perfectly for Boston container gardening and small spaces. This dwarf variety reaches only 12-16 inches tall while producing abundantly. It tolerates Boston's urban heat island effect better than tall varieties and fits in 12-inch deep containers. Space plants 8-10 inches apart in small beds compared to 12 inches for full-size varieties. Brassica oleracea 'Red Russian' also works well due to its upright, narrow growth habit.
How do I know when Boston kale is ready to harvest?
Harvest kale leaves when they reach 6-8 inches long, typically 6-8 weeks after planting. Brassica oleracea 'Red Russian' matures fastest at 50-60 days, while Brassica oleracea 'Winterbor' takes 60-75 days. For sweetest flavor, harvest after Boston's first frost (around October 19) when cold exposure converts starches to sugars. Leaves should feel crisp and show deep color—avoid yellowing or tough, fibrous leaves which indicate overmaturity.
Should I remove kale flowers if the plant bolts?
Remove flower stalks immediately when Boston kale bolts to redirect energy into leaf production. Cut flowering stems at the base using sharp garden shears. However, once kale bolts, leaf quality declines significantly—they become bitter and tough. Consider harvesting flower buds and young flowers as "kale raab" for stir-fries. Plant Brassica oleracea 'Winterbor' for better bolt resistance in Boston's variable spring weather.
What spacing works best for kale in Boston raised beds?
Space kale plants 12 inches apart in all directions in Boston raised beds. This spacing provides adequate air circulation to prevent fungal diseases common in Boston's humid conditions while maximizing production. For Brassica oleracea 'Vates' dwarf variety, use 8-10 inch spacing. Closer spacing increases disease pressure and reduces individual plant size, while wider spacing wastes valuable Boston garden space. Row spacing of 18 inches allows easy access for harvesting and maintenance.
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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.
