Colorado planting calendar: when to plant on the Front Range
Quick answer
Trees and shrubs: early fall (September to October) is the best window on the Front Range, and spring works too. Perennials: in spring once frost danger has passed, or in fall no less than a month before the first frost, which means by early September in most Front Range cities. Fall bulbs: September to October. Warm-season vegetables: mid-to-late May, once the soil hits 60 °F.
On Colorado's Front Range (USDA zones 5 to 6), plant cool-season vegetables 2 to 4 weeks before the average last frost (early-to-mid May), set out warm-season crops once soil reaches 60 °F (usually late May), plant trees, shrubs, and fall bulbs in September and October, and deep-water established plants about monthly through winter. Here's the month-by-month guide.
What to plant when
Cool-season vegetables (peas, spinach, lettuce, kale)
Direct-sow / transplant 2 to 4 weeks before your average last frost, typically late March to April.
These hardy crops tolerate light frost and bolt in summer heat, so get them in early; a second sowing in late summer gives a fall harvest. (CSU GardenNotes #720)
Warm-season crops (tomatoes, peppers, squash, basil)
After your last frost AND once soil reaches 60 °F (55 °F minimum), usually mid-to-late May.
Soil temperature matters more than the calendar: transplants set into cold soil stall. Start seeds indoors ~6 to 8 weeks before last frost and harden off before transplanting. (CSU GardenNotes #717)
Trees & shrubs
Spring or, best for the Front Range, early fall (September to October).
Fall planting lets roots establish in still-warm soil before winter; water new woody plants through their first winters. Divide perennials in September too. (PlantTalk: Trees, Shrubs & Vines)
Perennials
Spring after the danger of frost has passed, or fall no less than one month before the first fall frost.
Hardy perennials can go in whenever you can work the soil, but nursery plants that were growing in a greenhouse need the frost-free window. Plants that are marginally hardy, or going into a tough spot, do best planted in May or June so they settle in before their first winter. With first frost in early October for most Front Range cities, early September is the practical fall cutoff. (PlantTalk #1017)
Fall bulbs (tulips, daffodils, crocus)
Plant September to October, before the ground freezes.
Spring-blooming bulbs need cold-soil weeks to root; plant early in the window for best establishment, up until late October. (PlantTalk #1086)
Winter watering
About once or twice a month, October to April, only when air is above 40 °F, soil is unfrozen, and there's no snow cover; water at midday so it soaks in before night.
Front Range winters are dry and sunny; unwatered roots desiccate. Apply to trees, shrubs, lawns, and perennials, and always follow your utility's current drought restrictions. (CSU Extension 7.211)
Average frost dates by city
Frost dates are averages. Local elevation and microclimate shift them, so treat them as windows, not guarantees. Tap a city for its full local guide.
| City | Avg. last spring frost | Avg. first fall frost | Growing season |
|---|---|---|---|
| Boulder | ~May 5 | early October | 144 days |
| Colorado Springs | mid-May | ~October 5 | 145 days |
| Castle Rock | mid-to-late May (cold-air pooling can push frost later at elevation, verify locally) | mid-to-late September | 130 days |
| Fort Collins | ~May 11 | early-to-mid October | 146 days |
| Denver | mid-May | early-to-mid October | 150 days |
| Aurora | mid-May | early-to-mid October | 150 days |
| Longmont | late April to mid May | late September to mid October | 150 days |
| Loveland | early-to-mid May | early-to-mid October | 145 days |
| Greeley | ~May 4 | ~October 4 | 150 days |
Planting FAQ
- When is the best time to plant trees in Colorado?
- Early fall, September to October, is the best window on the Front Range: the soil is still warm, so roots grow before winter. Spring planting works too. Either way, water new trees through their first winters whenever the soil is not frozen.
- When should I plant perennials in Colorado?
- In spring once the danger of frost has passed, or in fall no less than a month before the first fall frost, which is early September for most Front Range cities. Marginally hardy perennials do best planted in May or June.
- When can I plant tomatoes on the Colorado Front Range?
- After your area's last frost and once the soil reaches about 60 °F (55 °F minimum), usually mid-to-late May. Soil temperature matters more than the date; transplants set into cold soil stall.
- When do I plant tulips and other fall bulbs in Colorado?
- September to October, before the ground freezes. Plant early in that window for the best root establishment.
- When is the last frost on the Front Range?
- Roughly early-to-mid May for most Front Range cities, but it varies: Greeley averages ~May 4 while higher Castle Rock can see frost into early June. See the by-city table above.
- Can I plant anything before the last frost?
- Yes. Cool-season vegetables (peas, spinach, lettuce, kale) and many trees, shrubs, and perennials go in weeks before the last frost. Only frost-tender, warm-season plants need to wait.
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