Planting Guides

When to Plant Kale in Denver: Complete Guide + Best Varieties for Zone 5a

Denver, Colorado

USDA Zone 5a

Last Frost: May 10

Last updated: October 30, 2025

Plant kale in Denver for spring harvest April 15-25, fall harvest July 15-August 1. Zone 5a's 148-day season and May 9 frost date create perfect conditions.

DDorothy "Dot" Williams

October 30, 2025

Kale plants growing in Denver garden with mountain backdrop showing ideal Zone 5a growing conditions — — Image © PlantReference.org 2025

Quick Answer

Plant kale in Denver April 15-25 for spring harvest or July 15-August 1 for fall crops. Zone 5a's May 9 frost date allows excellent spring and fall growing windows.

TL;DR

Plant kale in Denver April 15-25 for spring harvest or July 15-August 1 for superior fall crops. Zone 5a's 148-day season and May 9 last frost create ideal conditions for Brassica oleracea 'Winterbor' and Brassica oleracea 'Lacinato'. Fall plantings survive Denver's October 4 first frost and produce sweeter leaves through winter with protection.

Product Recommendations

Winterbor Kale Seeds

($3-4)— Ultimate cold-hardy variety for Denver winters

Winterbor Kale F1 Hybrid Seeds - Premium Cold Hardy Variety

https://www.johnnyseeds.com/vegetables/greens/kale/winterbor-f1-kale-seed-2550.html

Winterbor F1 hybrid delivers exceptional cold hardiness for Denver's Zone 5a winters, surviving temperatures to -15°F with minimal protection. Deeply curled blue-green leaves develop superior sweetness after frost exposure. Fast 60-75 day maturity fits perfectly within Denver's 148-day growing season. Vigorous growth habit produces consistent harvests from fall through spring. Resistant to common kale pests and diseases while maintaining tender texture even in large mature leaves. Ideal for succession planting and overwintering in Colorado's high-altitude climate.

Price: $3.25-$4.50

Lacinato Kale Seeds

($3-4)— Heat-tolerant variety for Denver spring planting

Lacinato Dinosaur Kale Seeds - Heirloom Heat-Tolerant Variety

https://www.burpee.com/vegetables/kale/kale-lacinato-prod000463.html

Lacinato kale excels in Denver's challenging spring conditions with superior heat tolerance and UV resistance. Distinctive dark blue-green strap leaves handle intense high-altitude sunlight better than curly varieties. Nutty, sweet flavor develops without requiring frost treatment, making it perfect for spring harvest before summer heat. Plants mature in 60-75 days with continuous harvest capability. Flat leaf structure resists wind damage common in Denver's exposed locations. Italian heirloom variety maintains tenderness longer than hybrid types during temperature fluctuations.

Price: $3.50-$4.25

Heavy Weight Row Cover

($18-25)— Essential protection for Denver's temperature swings

Agribon AG-50 Heavy Weight Floating Row Cover 10ft x 20ft

https://www.gardeners.com/buy/agribon-ag-50-floating-row-cover/8590438.html

Heavyweight 1.5 oz spun fabric provides 8-10°F frost protection crucial for Denver's unpredictable weather swings. Allows 50% light transmission while protecting from hail, wind, and temperature extremes common at Denver's 5,280-foot elevation. Durable construction withstands Colorado's intense UV exposure and frequent chinook winds. Large 10x20 foot size covers multiple kale rows efficiently. Reusable for multiple seasons with proper storage. Essential for extending Denver's kale season into winter and protecting spring transplants from late frost.

Price: $18.95-$24.50

Premium Organic Compost

($8-12)— Clay soil amendment for Denver gardens

Black Gold Garden Compost Blend 2 cu ft - Organic Soil Amendment

https://www.blackgold.bz/product/garden-compost-blend/

Premium aged compost blend specifically formulated to improve Denver's challenging clay soils. Rich organic matter breaks up clay structure while adding essential nutrients for kale production. Balanced carbon-to-nitrogen ratio prevents burning while feeding beneficial soil microbes. Improves drainage in heavy clay while increasing water retention in Denver's dry climate. pH-neutral formulation works well with Denver's naturally alkaline soils. Two cubic foot bags provide convenient sizing for raised bed preparation and annual soil improvement. OMRI listed for organic gardening certification.

Price: $7.95-$11.50

Soil Thermometer

($12-18)— Monitor soil temperature for optimal planting timing

REOTEMP Heavy Duty Compost and Soil Thermometer 20-inch Stem

https://www.reotemp.com/product/heavy-duty-compost-soil-thermometer/

Professional-grade soil thermometer with 20-inch stem reaches proper depth for accurate soil temperature readings in Denver's clay soil. Stainless steel construction withstands Colorado's freeze-thaw cycles and alkaline soil conditions. Large dial face readable in Denver's bright sunlight with temperature ranges perfect for timing kale planting. Helps determine optimal planting dates by monitoring when soil reaches consistent 45°F needed for kale germination. Dual Fahrenheit and Celsius scales accommodate all gardening preferences. Built-in hang hole for convenient greenhouse or tool shed storage between seasons.

Price: $12.25-$17.95

Frequently Asked Questions

When should I plant kale seeds in Denver for the best harvest?

Plant kale in Denver July 15-August 1 for the superior fall harvest or April 15-25 for spring crops. Fall plantings consistently outperform spring ones because Denver's October frost triggers the sweetening process that makes kale tender and flavorful. Spring planting works but must be timed carefully to harvest before summer heat causes bolting and bitterness.

What kale varieties grow best in Denver's Zone 5a climate?

Brassica oleracea 'Winterbor' and Brassica oleracea 'Lacinato' perform best in Denver's high-altitude conditions. Winterbor handles Denver's dramatic temperature swings and survives winter temperatures to -15°F. Lacinato tolerates the intense UV radiation and low humidity better than curly varieties. Both varieties mature in 60-75 days, perfect for Denver's 148-day growing season.

How cold can kale survive in Denver winters?

Kale survives Denver winter temperatures down to 20°F without protection and to -10°F with row covers. Brassica oleracea 'Winterbor' and Brassica oleracea 'Redbor' show the best cold tolerance. Frost actually improves flavor by converting starches to sugars, making post-frost kale dramatically sweeter. Mulch plants heavily and use row covers during the coldest periods for continuous winter harvest.

Why does my spring kale bolt so quickly in Denver?

Denver's rapid temperature swings from cool nights to hot days trigger stress bolting in spring kale. When temperatures jump from 40°F to 85°F in 24 hours—common during chinook winds—kale interprets this as seasonal change and bolts to flower. Plant bolt-resistant varieties like Brassica oleracea 'Lacinato' for spring crops and provide afternoon shade during heat waves above 85°F.

How often should I water kale in Denver's dry climate?

Water kale 1-2 inches weekly in one deep application rather than daily light watering. Denver's clay soil and low humidity require modified watering techniques. Water early morning to minimize evaporation losses and apply 2-3 inches of mulch to conserve soil moisture. Check soil moisture 3 inches deep—clay holds water longer than it appears on the surface.

Can I grow kale year-round in Denver?

Yes, with proper varieties and season extension techniques. Plant Brassica oleracea 'Winterbor' in July for harvest through February, then start spring crops in March for June harvest. Use cold frames and row covers during winter to maintain harvesting access. Denver's intense winter sun heats protected plants effectively, making year-round growing possible with 2-3 months of season extension infrastructure.

What's the biggest mistake Denver gardeners make with kale?

Planting too late for fall crops. Many gardeners wait until September to plant fall kale, but Denver's October 4 first frost requires planting by August 1 at the latest for full-size plants. Late plantings produce only baby greens before hard freeze. Fall kale planted July 15-August 1 develops cold tolerance and produces sweet, tender leaves all winter with minimal protection.

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Tagged with

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Written By

Dorothy "Dot" Williams

Dot grew up on a small farm in rural Virginia and has maintained a vegetable garden for decades. After retiring from teaching elementary school, she became a Master Gardener volunteer and spends her time mentoring new gardeners at community garden plots in Richmond. She's especially knowledgeable about heirloom varieties, seed saving, and traditional growing methods passed down from her grandmother. Dot's no-nonsense advice comes from extensive trial and error—she's seen every tomato disease, pest problem, and weather disaster imaginable. Her biggest pet peeve is gardeners who overcomplicate simple tasks. "Plants want to grow," she often says. "Your job is to not get in their way."