Planting Guides

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

Denver, Colorado

USDA Zone 5a

Last Frost: May 10

Last updated: October 30, 2025

Learn when to plant basil in Denver with specific dates for Zone 5a. Compare 5 varieties & discover which basil grow best in Colorado's high altitude climate.

NNathan Brooks

October 30, 2025

Basil varieties growing in Denver garden with mountain backdrop showing Zone 5a climate — — Image © PlantReference.org 2026

Quick Answer

Start basil seeds indoors March 15-30 in Denver. Transplant outdoors after May 10 when soil reaches 60°F consistently.

TL;DR

Start basil seeds indoors March 15-30 for Denver's Zone 5a climate. Transplant after May 10 when soil reaches 60°F consistently. Choose cold-hardy varieties like Ocimum basilicum 'Genovese' and avoid temperature-sensitive types in Denver's high altitude conditions.

Frequently Asked Questions

When is the last frost date in Denver for basil planting?

Denver's average last frost date is May 10, but basil requires soil temperatures above 60°F consistently. Wait until May 15-25 to transplant basil outdoors, even if air temperatures are warm earlier. Check soil temperature with a thermometer—cold soil will shock basil roots and permanently stunt growth.

Which basil varieties grow best in Denver's high altitude climate?

Ocimum basilicum 'Genovese' performs most reliably in Denver's Zone 5a conditions, handling UV and temperature swings well. Ocimum basilicum 'Thai' thrives in our dry air with concentrated essential oils. Ocimum basilicum 'Purple Ruffles' benefits from intense UV that maintains vibrant color. Avoid Ocimum basilicum 'Lemon' which bolts quickly in temperature stress.

Can I direct seed basil in Denver or should I start transplants?

Start basil indoors March 15-30 for best results in Denver. Direct seeding is risky due to unpredictable spring weather and soil temperature fluctuations. One late cold snap can kill emerged seedlings, making transplants the reliable choice for our 148-day growing season.

How do I protect basil from Denver's intense UV radiation?

Provide afternoon shade using 30% shade cloth or plant basil east of taller plants like tomatoes. Maintain consistent soil moisture—drought stress amplifies UV damage. Purple basil varieties actually benefit from intense UV, while tender varieties like Ocimum basilicum 'Lemon' need protection from direct afternoon sun.

What soil amendments does basil need in Denver's alkaline clay?

Denver's alkaline soil (pH 7.5-8.0) requires amendment to basil's preferred 6.0-7.0 range. Mix equal parts native soil, aged compost, and coarse sand for drainage. Add sulfur to lower pH gradually. Consider raised beds filled with amended soil to bypass alkaline clay entirely and improve drainage.

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Tagged with

#basil planting denver #zone 5a basil #high altitude gardening #denver vegetable garden #colorado herb growing #basil varieties denver #mountain gardening tips #denver frost dates #basil transplanting #high altitude herbs

Written By

Nathan Brooks

Nathan is a garden designer in Denver who focuses on water-wise plantings adapted to Colorado's Front Range climate. He grew up in Nebraska, studied environmental science in college, and fell into garden design after spending summers working at a native plant nursery in Fort Collins. Denver's climate is genuinely challenging—low humidity, intense UV, alkaline clay soil, and wild temperature swings that can go from 70°F to snow in 24 hours. Nathan designs gardens that handle all of that without supplemental irrigation once established. He writes about xeric gardening, native plant selection for the Front Range, and the practical realities of high-altitude growing.