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Zone 5b Flower Planting Schedule: A Month-by-Month Guide

Planting Guide · March 2026 · 7 min read

Zone 5b covers a wide swath of the northern United States — parts of Iowa, Minnesota, Wisconsin, Michigan, upstate New York, and northern Pennsylvania — with minimum winter temperatures of -15°F to -10°F and a last spring frost around May 1 to May 15. This schedule tells you exactly when to start, transplant, and direct sow the most popular flowers for your zone.

Key Dates for Zone 5b

• Last spring frost: May 1 – May 15
• First fall frost: October 1 – October 15
• Frost-free growing season: approximately 140 to 165 days

February – March: Start Seeds Indoors

Begin slow-growing annuals and perennials indoors 8 to 12 weeks before your last frost date. Use grow lights or a bright south-facing window and keep soil temperature at 65°F to 75°F for best germination.

• Snapdragons — start 10 to 12 weeks before last frost (February)
• Petunias — start 10 to 12 weeks before last frost (February)
• Impatiens — start 10 to 12 weeks before last frost (February)
• Begonias — start 12 weeks before last frost (early February)
• Pansies and violas — start 8 to 10 weeks before last frost (March)
• Echinacea (coneflower) — start 8 to 10 weeks before last frost (March)
• Lavender — start 10 to 12 weeks before last frost (February); slow germinator

April: Start More Seeds Indoors

Start faster-growing annuals 4 to 6 weeks before last frost. These grow quickly and do not want to be root-bound in pots.

• Marigolds — start 4 to 6 weeks before last frost
• Zinnias — start 4 weeks before last frost (or direct sow after frost)
• Cosmos — start 4 weeks before last frost (or direct sow after frost)
• Sunflowers — start 3 to 4 weeks before last frost (or direct sow)
• Celosia — start 4 to 6 weeks before last frost
• Dahlias — start tubers indoors in pots 4 weeks before last frost

May: Transplant and Direct Sow

After your last frost date passes (typically May 10 to May 15 in Zone 5b), harden off indoor starts and transplant outside. Direct sow warm-season flowers into prepared beds.

• Transplant outdoors: petunias, marigolds, snapdragons, impatiens, begonias, zinnias, celosia, dahlias
• Direct sow: sunflowers, cosmos, zinnias, nasturtiums, morning glories, bachelor buttons
• Plant bulbs: gladiolus, cannas, dahlia tubers (if not started indoors)

Hardening off tip: Move seedlings outdoors for 2 hours on day one, increasing by an hour each day for 7 to 10 days. Keep them out of direct afternoon sun and wind the first few days.

June – August: Succession Planting

Keep sowing quick-blooming annuals every 2 to 3 weeks for continuous color through fall:

• Zinnias: Sow every 3 weeks through mid-July for blooms until frost
• Sunflowers: Sow every 2 weeks through early July for staggered blooming
• Cosmos: Sow through June for late-summer blooms
• Marigolds: Transplant a second round in June for fresh fall color

September – October: Fall Tasks

• Plant spring-blooming bulbs: tulips, daffodils, crocuses, alliums, hyacinths. Plant 6 to 8 weeks before the ground freezes (mid-September through October).
• Collect seeds: Save seeds from zinnias, cosmos, sunflowers, and marigolds for next year.
• Dig tender bulbs: Lift dahlia tubers, gladiolus corms, and canna rhizomes after the first frost blackens the foliage. Store in a cool, dry place over winter.
• Mulch perennials: After the ground freezes, apply 3 to 4 inches of straw or shredded leaves over perennial beds to protect roots.

Perennials That Thrive in Zone 5b

These come back year after year once established:

• Black-eyed Susan (Rudbeckia) — blooms July to September
• Coneflower (Echinacea) — blooms June to August
• Daylilies — blooms June to July
• Hostas — shade-loving foliage plants
• Peonies — blooms May to June, extremely cold-hardy
• Bee balm (Monarda) — blooms July to August, attracts pollinators
• Sedum (Stonecrop) — blooms August to October, drought-tolerant