When to plant lemon balm in Philadelphia, Pennsylvania

March 30 – April 13

Transplant outdoors · typical Philadelphia year

Climate basis: NOAA station Phila Franklin Inst, PA (USC00366886), 1 mi (1 km) from downtown Philadelphia (ZCTA 19107 centroid), elevation 60 ft, on the 1991–2020 climate normals. Transplant lemon balm outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (March 30). One year in ten still freezes after April 12.

Last spring freezeMarch 301-in-10 after Apr 12

First fall freezeNovember 171-in-10 before Nov 4

Frost-free season≈ 232 daysmedian at 32°F

Hard freeze, 28°Mar 17 – Nov 29kills even hardy greens

USDA zone7b5 to 10°F winter low — not frost timing

The Philadelphia year

Start seeds indoorsFeb 16 – Mar 2

Transplant outdoorsMar 30 – Apr 13

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to Apr 12

Lemon Balm in Philadelphia · freeze context from Phila Franklin Inst, PA, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Feb 16 – Mar 2

Wisconsin Horticulture sows lemon balm indoors about 4 to 6 weeks before the last spring frost, so the window runs 6 to 4 weeks before that date. Anchored to the median last spring freeze (Mar 30). University of Wisconsin-Madison Division of Extension, Wisconsin Horticulture · Virginia Cooperative Extension, Virginia Tech

Transplant outdoors

Mar 30 – Apr 13

Wisconsin Horticulture ties lemon balm transplanting to seedling height rather than a date, so the frost anchor comes from Virginia Cooperative Extension's herb publication — whose herb table lists lemon balm — which sets all herb plants out after the danger of severe frost has passed. Anchored to the median last spring freeze (Mar 30). Virginia Cooperative Extension, Virginia Tech

Variety assumption: A hardy perennial that self-seeds freely; these windows apply to first-year plantings from seed or nursery plants.

These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.

This week, not a typical year:

Crop rule sources (2)

University of Wisconsin-Madison Division of Extension, Wisconsin Horticulture: Lemon Balm, Melissa officinalis (accessed 2026-07-25)
"Lemon balm can be started from seed sown indoors about 4 to 6 weeks before the last spring frost, although seeds may be somewhat slow to germinate."
Virginia Cooperative Extension, Virginia Tech: Herb Culture and Use (Publication 426-420) (accessed 2026-07-25)
"In addition, many culinary herbs, such as tarragon, can only be propagated asexually; seed-grown plants lack the oils that give them flavor."

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