When to Plant Flower Seeds in Colorado for Best Blooms

Planting flower seeds in Colorado requires timing that matches the state’s unique climate and elevation. With cold winters, late frosts, and varying hardiness zones, knowing when to sow is key to success. This guide covers everything from last frost dates to seed-starting strategies for stunning blooms all season.

Key Takeaways

• Know your hardiness zone: Colorado ranges from Zone 3 to Zone 7, so planting times vary by region.
• Wait for the last frost date: Most flower seeds should be planted after the last spring frost, typically between late April and early June.
• Start seeds indoors for a head start: Begin 6–8 weeks before the last frost for tender annuals like petunias and zinnias.
• Use cold frames or row covers: These protect early plantings from unexpected cold snaps.
• Choose native and cold-hardy flowers: Plants like columbine, gaillardia, and snapdragons thrive in Colorado’s climate.
• Monitor soil temperature: Many seeds need warm soil (above 60°F) to germinate properly.
• Extend the season with succession planting: Sow seeds every 2–3 weeks for continuous blooms into fall.

Understanding Colorado’s Unique Growing Conditions

Colorado is a state of extremes—high elevations, intense sunlight, dry air, and dramatic temperature swings. These factors make gardening here both rewarding and challenging. Unlike milder climates, Colorado gardeners must carefully time their planting to avoid late frosts and ensure seeds have the right conditions to sprout and thrive.

The state spans USDA hardiness zones 3 through 7, with most of the Front Range (including Denver, Boulder, and Colorado Springs) falling in zones 5 and 6. Mountain towns like Aspen or Vail may be in zone 4 or even 3, while lower-elevation areas like Grand Junction enjoy zone 7 warmth. This wide range means there’s no one-size-fits-all planting calendar. What works in Pueblo might not work in Steamboat Springs.

Another key factor is elevation. At 5,000 feet and above, the growing season is shorter, and nighttime temperatures can drop suddenly—even in summer. Frost can occur as late as mid-May in many areas, and as early as September in the mountains. That’s why understanding your local microclimate and frost dates is essential before planting flower seeds.

Why Timing Matters for Flower Seed Success

Planting too early can expose delicate seedlings to frost damage or cold soil that prevents germination. Planting too late may shorten the blooming season, especially for slow-growing flowers. For example, marigolds and cosmos may take 60 to 90 days to bloom from seed. If you wait until June to plant, you might not see flowers until August.

On the flip side, some cool-season flowers like pansies, snapdragons, and sweet peas actually prefer cooler soil and can be planted in early spring—sometimes as early as March or April, depending on your zone. These flowers can tolerate light frosts and will bloom beautifully in spring and fall.

Last Frost Dates: Your Planting Compass

One of the most important tools for Colorado gardeners is the average last frost date. This is the date after which frost is unlikely to occur in spring. Planting flower seeds after this date greatly reduces the risk of losing your seedlings to a sudden cold snap.

Here’s a general guide to last frost dates across Colorado:

• Denver and Front Range (Zone 5b–6a): April 20–May 10
• Colorado Springs (Zone 6a): April 25–May 10
• Boulder (Zone 5b): April 15–May 5
• Fort Collins (Zone 5b): April 20–May 5
• Grand Junction (Zone 7a): April 1–April 15
• Mountain towns (Zone 3–4): May 15–June 1

Keep in mind these are averages. Always check a 10-day weather forecast before planting, and be ready to cover plants if a late frost is predicted.

How to Use Frost Dates for Seed Planting

Once you know your last frost date, you can plan your planting schedule. For direct-sown seeds (those planted outdoors in the ground), wait until after the last frost—unless the seed packet says otherwise. For example, lettuce and radishes can go in earlier, but tomatoes and basil must wait.

For tender annual flowers like impatiens, begonias, and geraniums, it’s safer to wait until two weeks after the last frost. These plants are highly sensitive to cold and won’t survive even a light frost.

Cool-season flowers are the exception. Seeds for pansies, calendula, and larkspur can be sown in early spring—sometimes as early as March—as long as the soil isn’t frozen. These flowers actually benefit from a period of cold and will bloom earlier than warm-season varieties.

Starting Seeds Indoors: A Smart Strategy

One of the best ways to get a jumpstart on the growing season in Colorado is to start flower seeds indoors. This is especially helpful for long-season annuals that need 8–10 weeks to mature.

Begin sowing seeds indoors 6 to 8 weeks before your last frost date. For most of the Front Range, that means starting in late February to early March. Use seed trays or small pots filled with a sterile seed-starting mix. Keep the soil moist and place the trays in a warm spot (around 70°F) with plenty of light—either a sunny south-facing window or grow lights.

Some great flowers to start indoors include:

• Petunias: Need 10–12 weeks to bloom; start in February.
• Zinnias: Fast growers, but benefit from an early start.
• Cosmos: Can be direct-sown, but indoor starts lead to earlier blooms.
• Snapdragons: Prefer cooler starts and can be sown indoors in late winter.
• Marigolds: Easy to grow from seed and great for pest control.

Hardening Off: The Crucial Step Before Transplanting

Before moving your seedlings outdoors, you must “harden them off.” This means gradually exposing them to outdoor conditions over 7–10 days. Start by placing them outside in a sheltered spot for a few hours a day, increasing the time and sun exposure each day.

This process helps seedlings adjust to wind, sun, and temperature fluctuations. Skipping hardening off can shock the plants, causing wilting or even death. After hardening off, transplant them into the garden after the last frost date—preferably on a cloudy day or in the evening to reduce stress.

Direct Sowing: When and How to Plant Outdoors

Not all flower seeds need to be started indoors. Many, like sunflowers, nasturtiums, and poppies, prefer to be direct-sown into the garden. These seeds are either large, fast-germinating, or sensitive to transplanting.

For direct sowing, wait until the soil has warmed to at least 60°F—usually late April to early June, depending on your location. Use a soil thermometer to check the temperature at a 2-inch depth. Cold soil can cause seeds to rot or fail to sprout.

Best Flowers for Direct Sowing in Colorado

Here are some top choices for direct sowing:

• Sunflowers: Plant after last frost; they grow quickly and love full sun.
• Nasturtiums: Edible flowers that thrive in poor soil and cool weather.
• California poppies: Drought-tolerant and perfect for xeriscaping.
• Sweet alyssum: Low-growing, fragrant, and great for borders.
• Morning glories: Fast climbers that bloom all summer.

When planting, follow the depth and spacing instructions on the seed packet. Lightly cover seeds with soil and water gently. Keep the soil moist until germination, which can take anywhere from 5 days to 3 weeks, depending on the flower.

Choosing the Right Flowers for Colorado’s Climate

Not all flowers are created equal when it comes to Colorado’s climate. Some struggle with the dry air, intense UV rays, and temperature swings. Others—especially native species—are perfectly adapted.

Top Native and Drought-Tolerant Flowers

Native plants are a smart choice because they’re already suited to local conditions. They require less water, fewer pesticides, and support pollinators like bees and butterflies.

Consider planting:

• Colorado blue columbine: The state flower, blooms in late spring.
• Gaillardia (blanket flower): Bright red and yellow blooms all summer.
• Penstemon: Tubular flowers that attract hummingbirds.
• Yarrow: Drought-tolerant with clusters of tiny flowers.
• Blanket flower: Heat-loving and long-blooming.

These plants can often be direct-sown in spring or started from transplants. They’re also great for rock gardens, meadows, and low-water landscapes.

Annuals vs. Perennials: What to Plant When

Annuals complete their life cycle in one season and must be replanted each year. They’re great for adding instant color. Perennials come back year after year and often take a season or two to establish.

For best results, combine both. Plant annuals like petunias and marigolds for summer color, and perennials like coneflowers and black-eyed Susans for long-term structure.

Most perennial flower seeds can be sown in fall or early spring. Fall planting allows seeds to go through natural cold stratification, which improves germination. Spring planting works too, but may require extra care to keep soil moist.

Season Extension Techniques for Longer Blooms

Colorado’s short growing season doesn’t have to limit your flower garden. With a few smart techniques, you can extend the blooming period from late spring to early fall.

Using Cold Frames and Row Covers

Cold frames and floating row covers protect plants from frost and wind. You can plant cool-season flowers earlier under cover and keep warm-season plants safe during unexpected cold snaps.

Row covers are lightweight fabric sheets that allow light and water through but trap heat. Drape them over hoops or frames for easy protection.

Succession Planting for Continuous Color

Instead of planting all your seeds at once, try succession planting. Sow small batches of seeds every 2–3 weeks. This ensures that as one batch finishes blooming, another is just starting.

For example, plant zinnias in early May, then again in late May and early June. You’ll enjoy blooms from July through September.

Fall Planting for Spring Blooms

Some flower seeds benefit from fall planting. Species like larkspur, poppies, and sweet peas can be sown in late September or October. The seeds will lie dormant over winter and sprout in early spring—giving you a head start on the season.

Make sure to mark your planting spots, as it’s easy to forget where you sowed seeds under snow.

Final Tips for a Stunning Colorado Flower Garden

Gardening in Colorado is all about working with nature, not against it. Pay attention to your microclimate, protect plants from extremes, and choose flowers that love your conditions.

Water deeply but infrequently to encourage strong roots. Mulch around plants to retain moisture and regulate soil temperature. And don’t forget to deadhead spent blooms—this encourages more flowers and keeps your garden looking fresh.

With the right timing and a little patience, your Colorado flower garden can be a vibrant, colorful oasis—even in the shadow of the Rockies.

Frequently Asked Questions

When is the best time to plant flower seeds in Colorado?

The best time to plant flower seeds in Colorado depends on your location and the type of flower. For most areas, wait until after the last frost date—typically late April to early June. Cool-season flowers can go in earlier, while tender annuals should wait until soils warm.

Can I plant flower seeds in March in Colorado?

Yes, but only certain types. Cool-season flowers like pansies, snapdragons, and sweet peas can be planted in March if the soil isn’t frozen. Avoid planting warm-season flowers like petunias or marigolds until after the last frost.

Should I start flower seeds indoors in Colorado?

Yes, starting seeds indoors 6–8 weeks before the last frost gives you a head start, especially for long-season annuals. This is especially helpful in higher elevations with shorter growing seasons.

What flowers grow best in Colorado?

Native flowers like Colorado blue columbine, gaillardia, and penstemon thrive in Colorado’s climate. Drought-tolerant annuals like zinnias, marigolds, and sunflowers also do well with proper care.

Can I plant flower seeds in fall in Colorado?

Yes, some seeds—like larkspur, poppies, and sweet peas—can be planted in fall. They’ll go dormant over winter and sprout in spring. This is called fall sowing and works well for cold-stratified seeds.

How do I protect flower seedlings from frost in Colorado?

Use row covers, cold frames, or cloches to protect young seedlings. You can also water the soil before a frost—wet soil holds heat better than dry soil. Always monitor the weather and be ready to cover plants on cold nights.