If you want a garden full of bright, cheerful blooms, knowing when to plant sunflowers in Colorado is your first step. The timing is crucial for success in our unique climate, and getting it right means taller stalks and bigger flowers.
Colorado’s gardening zones range from 4 to 7, with most population centers in zones 5 and 6. This means our springs can be unpredictable, with late frosts a common concern. Sunflowers are warm-season annuals that need soil to be warm and all danger of frost to have passed. Planting too early can stunt them or cause the seeds to rot.
When to Plant Sunflowers in Colorado
For most of Colorado, the optimal window for planting sunflower seeds directly in the garden is from mid-May to mid-June. You should aim for soil temperatures to be consistently above 50°F, with 60-70°F being ideal for quick germination.
Here’s a more detailed breakdown by area:
- Front Range Urban Corridor (Denver, Colorado Springs, Fort Collins): Target May 10th to June 1st. Watch for that last spring frost, which can occure as late as mid-May in some spots.
- Western Slope (Grand Junction, Palisade): This area warms up earlier. You can often start planting in late April or early May.
- High Mountain Valleys (above 7,000 feet): Wait until late May or even early June. The growing season is shorter, so choose faster-maturing varieties.
- Eastern Plains: Similar timing to the Front Range, but wind protection becomes a key factor for young seedlings.
Understanding Your Last Frost Date
The average last spring frost date is your best guide. You can find this for your specific town through local extension services. A safe rule is to plant sunflowers 1 to 2 weeks after this average date. This ensures the soil has had time to warm up properly.
If you’re eager to get a head start, you can sow seeds indoors 2-3 weeks before your outdoor planting date. Use biodegradable pots to avoid disturbing the roots during transplant. Sunflowers have sensitive taproots and don’t always transplant well, so this method requires a gentle touch.
Choosing the Right Sunflower Variety for Colorado
Not all sunflowers are created equal, especially for our climate. Your choice depends on your goal: giant blooms, cut flowers, or wildlife food.
- For Giant Sunflowers: ‘Mammoth Russian’ or ‘American Giant’ are classics. They need a long season, so plant as early as safely possible in your area.
- For Cutting Gardens: Branching varieties like ‘Autumn Beauty’ or ‘ProCut Series’ produce many stems over weeks. You can make successive plantings every 2-3 weeks for continuous blooms.
- For Containers & Small Spaces: Dwarf types like ‘Sunspot’ or ‘Teddy Bear’ are perfect for patio pots and windy balconies.
- For Pollinators & Birds: Choose open-pollinated, pollen-producing varieties. The seeds they produce will also feed birds in the fall.
Step-by-Step Planting Guide
Once your timing is set, follow these simple steps for a great start.
1. Site Selection and Soil Prep
Sunflowers need full sun—at least 6 to 8 hours of direct light daily. They prefer well-draining soil. Colorado’s native clay soil can be heavy, so amending it with compost is very beneficial. Work the soil to a depth of about 2 feet for giant types, loosening it so the taproot can grow deep.
2. How to Sow the Seeds
Plant seeds 1 inch deep in lighter soils, and about 1/2 inch deep in heavier clay. Space seeds according to the variety’s mature size:
- Giant types: 18-24 inches apart
- Medium branching types: 12-18 inches apart
- Dwarf types: 6-12 inches apart
Water the seeds gently but thoroughly after planting. Keep the soil moist, but not soggy, until seedlings emerge in 7-10 days.
3. Watering and Care for Young Plants
While established sunflowers are drought-tolerant, seedlings need consistent moisture. Water deeply once a week, encouraging deep root growth. A layer of mulch around the base helps retain soil moisture and suppresses weeds. Be carful not to pile mulch against the stem.
Managing Colorado’s Gardening Challenges
Our state presents some specific hurdles for sunflower growers.
Wind Protection
Young sunflower stalks are suceptible to breaking in our strong winds. Plant them along a fence, or use stakes and soft ties for support as they grow. Planting in groups can also provide mutual support.
Wildlife Considerations
Birds, squirrels, and chipmunks love sunflower seeds—even before they’re grown. To protect planted seeds, cover the row with a lightweight row cover or netting until seedlings are a few inches tall. You can also try planting a few extra seeds as a decoy.
Soil and Nutrient Needs
Avoid over-fertilizing. Too much nitrogen can lead to lots of leaves and weak stems. A balanced, slow-release fertilizer worked into the soil at planting is usually sufficient. Sunflowers are not heavy feeders, and rich soil can sometimes cause them to grow to fast and topple.
Succession Planting for Longer Blooms
To enjoy sunflowers from midsummer into early fall, don’t plant all your seeds at once. Make a second planting about 3 weeks after your first. Choose a variety with a shorter “days to maturity” for these later sowings. This staggered approach ensures you’ll have color for months.
When to Expect Your Blooms
Most sunflower varieties bloom 70 to 100 days after planting. A mid-May planting will typically flower in August. If you planted a late succession crop, you can expect blooms into September, especially if we have a mild fall. Deadheading spent flowers on branching varieties can encourage more blooms to form.
FAQs: Planting Sunflowers in Colorado
Can I plant sunflowers in July in Colorado?
Yes, but choose a fast-maturing variety (under 70 days to bloom). Planting in early July should yield flowers before the first fall frost in many areas. Be prepared to water more frequently, as summer heat can dry soil quickly.
What is the latest month to plant sunflowers?
In lower elevations (like the Front Range), early July is generally the cutoff for a reasonable chance of blooms. In warmer microclimates, you might push it to mid-July with a very quick variety. After that, the risk of an early frost taking out the buds becomes to high.
How do I protect sunflowers from late spring frost?
If seedlings are up and a late frost is forecast, cover them overnight with frost cloth, buckets, or even cardboard boxes. Remove the covers in the morning once temperatures rise. This is a common need for Colorado gardeners who plant on the early side.
Do sunflowers grow well in Colorado’s high altitude?
They can, but the season is shorter. Stick with dwarf or early-maturing varieties. Ensure they have a sheltered, sunny spot. The intense sunlight at altitude is great for them, but the cool nights can slow growth, so every warm day counts.
By following these timing and care tips, you can grow a stunning display of sunflowers that thrive in Colorado’s unique conditions. The key is patience—waiting for the soil to warm up properly sets the stage for a whole season of spectacular, sunny blooms.