City	Sow outdoors	Last frost
Philadelphia	Mar 30 to Apr 20	Mar 30
Pittsburgh	Apr 18 to May 9	Apr 18
Allentown	Apr 20 to May 11	Apr 20
Reading	Apr 14 to May 5	Apr 14
Erie	Apr 26 to May 17	Apr 26
Bethlehem	Apr 20 to May 11	Apr 20
Scranton	Apr 23 to May 14	Apr 23
Lancaster	Apr 19 to May 10	Apr 19
Levittown	Apr 21 to May 12	Apr 21
Harrisburg	Apr 15 to May 6	Apr 15
York	Apr 26 to May 17	Apr 26
Wilkes-Barre	Apr 23 to May 14	Apr 23
Altoona	Apr 25 to May 16	Apr 25
State College	Apr 20 to May 11	Apr 20
Penn Hills	Apr 16 to May 7	Apr 16
Norristown	Apr 8 to Apr 29	Apr 8
Chester	Mar 30 to Apr 20	Mar 30
Mount Lebanon	Apr 18 to May 9	Apr 18
Bethel Park	Apr 18 to May 9	Apr 18
Easton	Apr 18 to May 9	Apr 18
Hazleton	Apr 30 to May 21	Apr 30
Drexel Hill	Apr 2 to Apr 23	Apr 2
Monroeville	Apr 22 to May 13	Apr 22
Williamsport	Apr 25 to May 16	Apr 25
Lebanon	Apr 20 to May 11	Apr 20
Plum	Apr 16 to May 7	Apr 16
King of Prussia	Apr 8 to Apr 29	Apr 8
Chambersburg	Apr 18 to May 9	Apr 18
Pottstown	Apr 20 to May 11	Apr 20
Carlisle	Apr 23 to May 14	Apr 23
Allison Park	Apr 16 to May 7	Apr 16
New Castle	Apr 30 to May 21	Apr 30
Murrysville	Apr 22 to May 13	Apr 22
Baldwin	Apr 18 to May 9	Apr 18
Phoenixville	Apr 18 to May 9	Apr 18

Questions

When should I plant sunflowers in Pennsylvania?

In Philadelphia, sow outdoors from March 30 to April 20. Across Pennsylvania the window opens between March 30 and April 30, depending on the local last frost.

How is this date worked out?

Sow outdoors up to 3 weeks after the last frost. Frost dates are NOAA 1991–2020 averages for the weather station nearest each city.