City	Sow outdoors	Last frost
Philadelphia	Mar 16 to Apr 13	Mar 30
Pittsburgh	Apr 4 to May 2	Apr 18
Allentown	Apr 6 to May 4	Apr 20
Reading	Mar 31 to Apr 28	Apr 14
Erie	Apr 12 to May 10	Apr 26
Bethlehem	Apr 6 to May 4	Apr 20
Scranton	Apr 9 to May 7	Apr 23
Lancaster	Apr 5 to May 3	Apr 19
Levittown	Apr 7 to May 5	Apr 21
Harrisburg	Apr 1 to Apr 29	Apr 15
York	Apr 12 to May 10	Apr 26
Wilkes-Barre	Apr 9 to May 7	Apr 23
Altoona	Apr 11 to May 9	Apr 25
State College	Apr 6 to May 4	Apr 20
Penn Hills	Apr 2 to Apr 30	Apr 16
Norristown	Mar 25 to Apr 22	Apr 8
Chester	Mar 16 to Apr 13	Mar 30
Mount Lebanon	Apr 4 to May 2	Apr 18
Bethel Park	Apr 4 to May 2	Apr 18
Easton	Apr 4 to May 2	Apr 18
Hazleton	Apr 16 to May 14	Apr 30
Drexel Hill	Mar 19 to Apr 16	Apr 2
Monroeville	Apr 8 to May 6	Apr 22
Williamsport	Apr 11 to May 9	Apr 25
Lebanon	Apr 6 to May 4	Apr 20
Plum	Apr 2 to Apr 30	Apr 16
King of Prussia	Mar 25 to Apr 22	Apr 8
Chambersburg	Apr 4 to May 2	Apr 18
Pottstown	Apr 6 to May 4	Apr 20
Carlisle	Apr 9 to May 7	Apr 23
Allison Park	Apr 2 to Apr 30	Apr 16
New Castle	Apr 16 to May 14	Apr 30
Murrysville	Apr 8 to May 6	Apr 22
Baldwin	Apr 4 to May 2	Apr 18
Phoenixville	Apr 4 to May 2	Apr 18

Questions

When should I plant swiss chard in Pennsylvania?

In Philadelphia, sow outdoors from March 16 to April 13. Across Pennsylvania the window opens between March 16 and April 16, depending on the local last frost.

How is this date worked out?

Sow outdoors from 2 weeks before to 2 weeks after the last frost. Frost dates are NOAA 1991–2020 averages for the weather station nearest each city.