Planting Guides
When to Plant Kale in Boise: Complete Guide + Best Varieties for Zone 6a
Boise, Idaho
USDA Zone 6a
Last Frost: May 5
Last updated: October 30, 2025
Plant kale in Boise April 1-15 for spring harvest or August 1-15 for superior fall crops in Zone 6a's 158-day season.
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Quick Answer
Plant kale in Boise April 1-15 for spring harvest or August 1-15 for fall crops. Fall plantings develop superior flavor after frost.
TL;DR
Plant kale in Boise April 1-15 for spring crops or August 1-15 for superior fall harvests in Zone 6a. Fall plantings survive Idaho's winters and develop the sweetest flavor after frost. Choose Brassica oleracea 'Winterbor' for maximum cold hardiness or Brassica oleracea 'Lacinato' for tender leaves and excellent flavor.
Frequently Asked Questions
When should I plant kale in Boise for the best flavor?
Plant kale in Boise between August 1-15 for fall harvest. Fall-planted kale develops superior flavor because Idaho's first frost around October 9th converts starches to sugars, creating sweet, tender leaves. Spring plantings (April 1-15) work well but lack the frost-enhanced sweetness that makes fall kale exceptional. The gradual cooling temperatures and increased rainfall in fall create ideal growing conditions that spring plantings often struggle to match.
Which kale varieties handle Boise's Zone 6a winters best?
Brassica oleracea 'Winterbor' and Brassica oleracea 'Redbor' are your most reliable winter survivors in Boise. Both handle temperatures down to 5°F (-15°C) with light row cover protection. Brassica oleracea 'Vates' also overwinters well and stays compact in windy conditions. Brassica oleracea 'Red Russian' may suffer damage below 15°F (-9°C) but recovers quickly. Brassica oleracea 'Lacinato' is moderately hardy but benefits from cold frame protection during January and February cold snaps.
Can I grow kale during Boise's hot summers?
Summer kale production in Boise requires careful variety selection and management. Brassica oleracea 'Lacinato' handles heat better than curly types and may survive with afternoon shade and consistent irrigation. Most varieties bolt (flower and turn bitter) when temperatures consistently exceed 80°F (27°C), typically mid-July through August. Focus on spring harvest finishing by July 1st, then plant fall crops in August for better results. Summer success requires daily watering, shade cloth, and accepting lower leaf quality.
How do I prevent cabbage worms on my Boise kale plants?
Cabbage worms are the white butterfly larvae that create large holes in kale leaves. In Boise, use floating row covers immediately after planting spring crops to exclude egg-laying adults. Remove covers once plants are established (4-6 weeks). For uncovered plants, spray with Bt (Bacillus thuringiensis) weekly during active butterfly periods (May-June and August-September). Hand-picking works for small gardens—check plants every 2-3 days and remove green caterpillars. Fall plantings typically have fewer problems as butterfly activity decreases with cooling temperatures.
What soil amendments work best for kale in Boise's alkaline soil?
Boise's alkaline soil (pH 7.5-8.5) needs organic matter and slight acidification for optimal kale production. Add 2-3 inches of compost annually to improve soil structure and buffer pH. Apply sulfur at 1 pound per 100 square feet in spring to gradually lower pH to 6.5-7.0. Bone meal provides phosphorus and calcium, while kelp meal supplies trace minerals often lacking in Idaho soils. Mycorrhizal inoculant helps establish strong root systems in our mineral-rich but biologically poor volcanic soils. Avoid lime—our soils are already too alkaline.
How long can I harvest kale through Boise's winter?
With proper protection, kale harvest can continue through February in most Boise winters. Brassica oleracea 'Winterbor' and Brassica oleracea 'Redbor' survive under row covers or in cold frames when temperatures drop below 15°F (-9°C). Snow actually insulates plants, allowing harvest by brushing snow away. January and February provide the sweetest leaves as cold temperatures maintain high sugar content. Harvest during warmer periods (above 25°F/-4°C) to prevent leaf damage. Plan for 2-4 month winter harvest period depending on variety and protection methods used.
Should I start kale from seeds or transplants in Boise?
Both methods work well in Boise, but timing determines the best approach. For spring planting, start seeds indoors March 15-22 and transplant April 10-20, or direct sow April 1-15 when soil reaches 45°F (7°C). Fall plantings work best as direct seeding August 1-15, allowing full season growth in ideal conditions. Direct seeding costs less and avoids transplant shock, while transplants give you a head start and better control over spacing. In Boise's clay soils, cover seeds with vermiculite rather than native soil to prevent crusting that blocks seedling emergence.
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Written By
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Lily Nakamura
Lily is a Master Gardener and high-desert homesteader outside Boise. She and her husband moved to Idaho from the Bay Area and had to completely relearn gardening for a climate with alkaline soil, intense sun, hard freezes, and limited water. Lily now grows a large vegetable garden, maintains a small orchard of cold-hardy fruit trees, and raises cut flowers for the local farmers market. She writes about the specific challenges of intermountain gardening—short seasons, temperature swings, and the importance of soil amendment in high-pH soils. Lily is honest about the learning curve: high-desert gardening is humbling, and she thinks more gardening writers should admit when conditions are genuinely difficult.
