Annual Flower

When to Plant Cosmos in Pennsylvania

Delicate, daisy-like blooms that dance in the breeze. Cosmos thrive in poor soil and practically grow themselves.

The Short Answer

Cosmos are the no-effort flower of the Pennsylvania garden — plant once and enjoy self-seeded volunteers for years.

Pennsylvania Frost Dates

Your planting dates depend on which part of Pennsylvania you're in. Here are the frost date ranges by region:

Region Zones Last Frost (Spring) First Frost (Fall)
Northern Pennsylvania 5a, 5b, 6a May 5 - May 20 Sep 20 - Oct 5
Central Pennsylvania 5b, 6a, 6b Apr 25 - May 10 Oct 1 - Oct 15
Philadelphia Region 7a, 7b Apr 5 - Apr 20 Oct 15 - Nov 5

Cosmos Planting Schedule for Pennsylvania

Northern Pennsylvania (Zones 5a, 5b, 6a)

Average last frost: May 5 - May 20 · Average first frost: Sep 20 - Oct 5

Start Seeds Indoors
4 wks before frost
Transplant Outside
1 wks after frost
Direct Sow
1 wks after frost

Central Pennsylvania (Zones 5b, 6a, 6b)

Average last frost: Apr 25 - May 10 · Average first frost: Oct 1 - Oct 15

Start Seeds Indoors
4 wks before frost
Transplant Outside
1 wks after frost
Direct Sow
1 wks after frost

Philadelphia Region (Zones 7a, 7b)

Average last frost: Apr 5 - Apr 20 · Average first frost: Oct 15 - Nov 5

Start Seeds Indoors
4 wks before frost
Transplant Outside
1 wks after frost
Direct Sow
1 wks after frost

Growing Cosmos in Pennsylvania

State-Specific Growing Tips

Direct sow from mid-May. Poor soil, full sun. Let some flowers go to seed for next year's display.

Recommended Varieties for Pennsylvania

Sensation Mix, Double Click, Bright Lights.

Common Challenges in Pennsylvania

Tall plants may lean after thunderstorms. Otherwise pest and disease free.

Growing Tips

Don't over-fertilize — rich soil produces more leaves and fewer flowers. Let some flowers go to seed for next year's crop.

Companion Planting

Plant cosmos alongside these companions for better growth:

Tomatoes Squash

The Bottom Line

Cosmos can be grown successfully in Pennsylvania with proper attention to regional frost dates and local growing conditions. Timing varies across the state — Northern Pennsylvania gardeners work with a last frost around May 5 - May 20, while Philadelphia Region sees frost end around Apr 5 - Apr 20. Choose varieties suited to your region, amend your soil based on its specific needs, and monitor for the pests and diseases most common in your area. For exact dates based on your zip code, use our free planting date finder.

Related Guides

Cosmos Growing Guide National planting guide with all states Pennsylvania Planting Calendar All plants for Pennsylvania Cosmos in Alabama Alabama planting dates Cosmos in Alaska Alaska planting dates Cosmos in Arizona Arizona planting dates Cosmos in Arkansas Arkansas planting dates
Note: All dates are based on NOAA 30-year Climate Normals and represent historical averages, not predictions for any specific year. Always check your local weather forecast before planting frost-sensitive crops. Learn about our data sources.

Last reviewed: March 29, 2026

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