When to plant bok choy in Portland, Maine
April 26 – May 10
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 bok choy 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 – Apr 5Maryland's pac-choi-scoped instruction to "start seeds indoors and plant 3 to 5-week old transplants into garden soil" supplies the 21–35 day seed-to-transplant lead, counted back from the start of the UNH-derived transplant window (day 0) as the registry's indoor-start policy requires, giving last spring frost minus 35 to minus 21 days; UNH's generic "four to six weeks" indoor period is not crop-specific and is not used. Anchored to the median last spring freeze (Apr 26). University of Maryland Extension · University of New Hampshire Cooperative Extension
Transplant outdoors
Apr 26 – May 10UNH Cooperative Extension states that Asian greens "like bok choi" bolt prematurely after cool exposure "so they should be set out after last frost" — a point rule, encoded per the registry window policy as the 14 days beginning at the last spring frost (0 to +14 days). Anchored to the median last spring freeze (Apr 26). University of New Hampshire Cooperative Extension
Direct-sow outdoors
Mar 29 – Apr 5University of Maryland Extension, on a page whose planting facts are explicitly scoped to "collards, kale, mustard, turnips, and pac choi", directs spring sowing "3 to 4 weeks prior to frost date", giving last spring frost minus 28 to minus 21 days; UNH's "set out after last frost" guidance for Asian greens governs setting out transplants and is encoded as the transplant window rather than treated as a competing sowing interval. Anchored to the median last spring freeze (Apr 26). University of Maryland Extension
Fall planting
Not calculatedNo fall window is encoded because Maryland's fall rule is a days-to-maturity calculation (time to maturity plus a 10–14 day "short day factor", counted back from the first fall frost) that yields a different offset for every cultivar across the 40–80 day maturity range Maryland states for these greens, while Minnesota Extension's fall guidance is fixed-calendar (direct seeding in July) rather than frost-anchored.
Variety assumption: Offsets assume leaf-harvested pac choi within the 40–80 day from-seed maturity range Maryland gives for this group of greens; both UNH and Minnesota Extension warn that this crop bolts after cold exposure or late spring frosts, so the pre-frost sowing window carries real bolting risk and the after-frost transplant window is the conservative option.
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 (2)
- University of Maryland Extension: Growing Leafy Greens in a Home Garden (accessed 2026-07-25)"Planting leafy green facts - collards, kale, mustard, turnips, and pac choi"
- University of New Hampshire Cooperative Extension: Cool Season Vegetables (accessed 2026-07-25)"Asian greens (like bok choi), and kohlrabi, are known to bolt prematurely if they are exposed to cooler temperatures, so they should be set out after last frost."