When to Plant Radishes in Boulder, CO
Published: April 24, 2026
Radish Planting Dates for Boulder, CO
| Start seeds indoors | Year-round |
| Last frost (average) | March 30 |
| Direct sow outdoors | February 23–March 5 |
| Minimum soil temperature | 40°F |
| Expect first harvest | March 17 – March 25 |
| Fall crop planting | September 27–October 7 |
| Fall crop harvest | October 19 |
| First fall frost (average) | November 1 |
⚠ Direct sow radishes 4–6 weeks before last frost — one of the fastest crops in the garden. Ready in as little as 22 days. Succession plant every 2 weeks for continuous harvest.
Best Radish Varieties for Boulder, CO
Consult a Boulder-area nursery or your state extension office for radish varieties proven in Zone 6a.
Growing Radishes in Boulder
Boulder sits in Zone 6a, with an average last frost of March 30 and first fall frost around November 1 — giving a 216-day frost-free growing season. Cool-season crops like radish benefit from Boulder's cooler spring and fall windows, when temperatures stay in the 55–75°F sweet spot that produces the best flavor and least bolting.
Boulder's moderate climate supports radish on the standard transplant calendar with minimal special accommodation. Succession planting — sowing fresh seeds or setting new transplants every 2–3 weeks through spring — stretches the harvest window and gives you a backup crop if pests hit the first planting.
Boulder's well-draining loam soils are among the best for radish growing — focus on annual organic matter additions (2 to 3 inches of compost) and consistent moisture during drier months. Water radish at 1 inches per week; loam holds moisture evenly without the drought-crack cycle of heavy clay or the rapid-drain losses of pure sand.
Radish Calendar for Boulder
| Month | Task |
|---|---|
| Year | Start seeds indoors under grow lights |
| March | Last frost — soil warming, prepare bed |
| February | Direct sow seeds into warm soil |
| March | Expect first harvest window to open |
| September | Start fall crop — transplants or direct sow |
| November | First fall frost — harvest remaining, end of outdoor season |
Radish Tips for Boulder Gardeners
- •Thin to 2-inch spacing when seedlings are an inch tall — crowded radishes produce small woody roots.
- •Succession plant every 10–14 days from early spring through late spring for continuous harvest.
- •Succession plant radish in Boulder every 2–3 weeks through spring to extend the harvest window into summer.
Common Radish Pests in Boulder
- •Flea Beetle — peaks July in Zone 6a (active May–September); tiny black beetles that riddle leaves with holes; row covers protect seedlings.
- •Cabbage Root Maggot — peaks July in Zone 6a (active May–September); monitor closely during peak season and treat early.
- •Aphid — peaks July in Zone 6a (active May–September); monitor closely during peak season and treat early.
Check plants every 2–3 days during peak season — early intervention prevents most infestations from becoming serious.
What to Plant with Radishes in Boulder
In Boulder's moderate climate, Lettuce and Carrot are the most beneficial plants to grow alongside radish. Lettuce improves nearby plant health and pest resistance. Keep radish away from Hyssop — it competes for nutrients or shares pest pressure.
Frequently Asked Questions
When should I plant radish in Boulder, CO?
In Boulder (Zone 6a), direct sow radish around February 23–March 5. Soil must be at 40°F or warmer at 2-inch depth before sowing — cold soil rots the seeds.
What zone is Boulder, CO for radish growing?
Boulder is USDA Zone 6a. For radish, this means a frost-free growing season of roughly 31 weeks running from March 30 to November 1. Cool-season crops like radish thrive in this zone with both spring and fall planting windows available.
When is radish harvest season in Boulder?
Expect the first radish harvest in Boulder around March 17, with harvest continuing through March 25. This is based on 22–30 days from transplant or direct sow. A fall crop planted September 27–October 7 adds a second harvest around October 19.
How long does it take to grow radish in Boulder, CO?
From transplant or direct sow to first harvest, radish takes 22–30 days in Boulder's climate. Based on a typical planting date of February 23–March 5, expect your first harvest around March 17. Boulder's Zone 6a transition-zone climate produces maturity times right in the middle of the stated range.
What soil does radish need in Boulder?
Boulder's loam soil is near-ideal for radish. Work in 2–3 inches of compost before planting to boost organic matter and nutrient content. Minimal amendment is needed beyond that — loam holds moisture evenly without the drought-crack cycle of clay or the nutrient-loss issues of sandy soil.