When to plant basil in Portland, Maine
May 10 – May 26
Transplant outdoors · typical Portland yearClimate basis: NOAA station Portland INTL Jetport, ME (USW00014764), 2 mi (4 km) from downtown Portland (ZCTA 04101 centroid), elevation 45 ft, on the 1991–2020 climate normals. Transplant basil outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (April 26). One year in ten still freezes after May 7.
The Portland year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to May 7
Every method, with its sources
Start seeds indoors
Mar 22 – Mar 29Iowa State: start seeds indoors 4 or 5 weeks before the last frost date. Anchored to the median last spring freeze (Apr 26). Iowa State University Extension and Outreach
Transplant outdoors
May 10 – May 26UNH: set out 2 weeks after the last frost; Oklahoma State: plant one month after the last spring frost; Iowa State: after all danger of frost. The window spans the UNH and Oklahoma State recommendations. Anchored to the median last spring freeze (Apr 26). University of New Hampshire Extension · Oklahoma State University Extension · Iowa State University Extension and Outreach
Direct-sow outdoors
May 26 – Jun 9Oklahoma State Table 1 times basil planting one month after the last spring frost; Iowa State adds direct sowing once nights stay above 50°F. Point rule given a 14-day span. Anchored to the median last spring freeze (Apr 26). Oklahoma State University Extension · Iowa State University Extension and Outreach
Variety assumption: Leaves are harvestable about 30–40 days after transplanting.
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.
Crop rule sources (3)
- Iowa State University Extension and Outreach: Growing Basil in the Home Garden (accessed 2026-07-24)"Start seeds indoors 4 or 5 weeks before the last frost date." "Plant basil outdoors after all danger of frost is past."
- University of New Hampshire Extension: Growing Vegetables: When to Plant Your Vegetable Garden [fact sheet] (accessed 2026-07-24)Per-crop 'time from seed to transplant' and 'safe to set out in garden' relative to last frost.
- Oklahoma State University Extension: Culinary Herbs for Oklahoma Gardens: Culture, Use and Preservation (accessed 2026-07-24)Table 1 planting times relative to last spring frost for common herbs.