Casco Bay / Greater Portland spans USDA hardiness zone 5b — a zone band wide enough that plant choice, not possibility, is the interesting question.
Coastal southern Maine around Portland and Casco Bay — the three counties of the Census Portland-South Portland metro (Cumberland, Sagadahoc, York) all share USDA hardiness zone 5b and a growing season of roughly 166 to 188 frost-free days, the ocean holding the first fall freeze off until late October. Reliable performers under these conditions include potato, kale, blueberry, and apple; what your own ground favors still comes down to its soil, sun, and drainage.
Casco Bay / Greater Portland spans Maine.
Your yard isn't all of Casco Bay / Greater Portland.
Casco Bay / Greater Portland spans USDA zones 5b, but your parcel sits in exactly one — and slope, tree cover, and low spots nudge it further. Enter your address and we'll score 1,086 plants against your land's actual soil, sun, and frost.
We read public map data around Casco Bay / Greater Portland — soil, climate, flood, and parcel records. How we handle your address.
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Casco Bay / Greater Portland — the short answer
Casco Bay / Greater Portland: USDA zones 5b, 166–188 frost-free days, in Maine.
- USDA zones
- 5b
- USDA PHZM 2023, aggregated over the footprint
- Frost-free season
- 166–188 days
- NOAA Regional Climate Centers ACIS, 1991-2020
- Counties
- 3
- US Census TIGER/Line 2025
- Region type
- County cluster
- Growable Ground growing-region registry
Region boundary: Growable Ground growing-region registry — curated county clusters, EPA Level III ecoregions, and town clusters drawn finer than any county. Where a region publishes no measured season, no season figure is shown.
Hardiness Zone Range
What Grows in Casco Bay / Greater Portland
Plants matched to Casco Bay / Greater Portland's USDA zones 5b, ordered by how much hardiness margin each has across the region — the ones with room to spare at both ends of this band lead. Each links to its full growing profile.
Native Plants of Casco Bay / Greater Portland
Plants the USDA PLANTS Database documents as native and present in this region’s state, kept only where the published hardiness range meets Casco Bay / Greater Portland’s USDA zones 5b — closest range match first. Presence is recorded by state, so a plant can be documented across the state and still be scarce in your corner of the region.
The ground across Casco Bay / Greater Portland
The USDA soil survey maps this ground county by county. These are those records weighted by land area across three member counties — this region’s own profile, not a state figure.
The ground is predominantly Spodosols, at 67% of mapped acres, followed by Inceptisols (21%), Entisols (7%) and Histosols (5%). The most extensive named soils are Lyman, Hermon, and Adams.
29% of those acres are somewhat excessively drained, and another 19% moderately well drained, 16% poorly drained. Topsoil pH runs 4.2–5.5 between the lowest and the highest county on record. 5% of the footprint is classed prime farmland.
Soil order
Spodosols
Drainage
Somewhat excessively drained
Topsoil pH
4.2–5.5
Hydric soils
24%
Soil changes across a driveway, so which of these your own ground sits on is a parcel question rather than a regional one. York County holds the most wetland ground here, at 31% — soil types explained.
USDA NRCS SSURGO via Soil Data Access
What this means for growing in Casco Bay / Greater Portland
The soil survey reads this ground county by county, and the readings above are those records summed across Casco Bay / Greater Portland. Here is what they mean for a garden here — and the move an experienced grower makes about each one.
Drainage
From county records29% of the mapped acres across Casco Bay / Greater Portland are somewhat excessively drained — ground that drains quickly. That is area-weighted across all three of its mapped counties.
The upside is soil you can work early in spring and roots that never drown; the cost is that fast-draining ground dries out fast and leaches nutrients away before plants can use them.
Work in compost and mulch to hold moisture and feed, and water more often in short doses than deep and rarely.
USDA NRCS SSURGO
Surface pH
From county recordsTopsoil across Casco Bay / Greater Portland runs pH 4.2–5.5 — acidic in all three of them.
Blueberries, potatoes, and rhododendrons thrive in it, but most vegetables start locking out nutrients below about pH 6.0 and grow pale and stunted no matter how much you feed them.
A measured lime application raises pH for the crops that need it — but test first, because the right rate depends on your own soil, not the county average.
USDA NRCS SSURGO
Wet ground
From county recordsAbout 24% of the ground across Casco Bay / Greater Portland is mapped as hydric — wetland — soil. York County carries the most, at 31%.
If your lot falls in one of those pockets, a standard in-ground bed will fight waterlogging all season, and roots that need air will struggle.
Check how your own ground drains before you dig; raised beds or moisture-loving plantings turn a wet corner from a problem into a niche.
USDA NRCS SSURGO
Safe to Grow Here?
What the federal record shows across Casco Bay / Greater Portland — and how to grow with it.
Across Casco Bay / Greater Portland's 3 counties the federal record holds 2,960 documented sites, mostly Underground Storage Tanks (1,579), Brownfields (808) and Nitrate (366). The heaviest single county is Cumberland County, with 1,771.
That is a regional count, not a reading of any one lot — proximity to a documented site is information, not a diagnosis. Open the map outlined to Casco Bay / Greater Portland to see which records sit where you actually grow.
Sources Checked
Summed across the 3 counties in Casco Bay / Greater Portland
A note from Gnorman
What an experienced grower watches for around here
In and around Casco Bay / Greater Portland, Superfund runs higher than the national average — 37 sites across the 3 counties in Casco Bay / Greater Portland. That's not a problem with your land — it's information about it.
Superfund: Superfund sites represent the most severe contamination in the federal system.
Commission professional soil testing before any food production (test for heavy metals, VOCs, and SVOCs).
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
Your land is one point inside Casco Bay / Greater Portland
Everything above resolves to Casco Bay / Greater Portland as a whole — a footprint spanning USDA zones 5b. Your parcel sits in exactly one of them. Slope, tree cover, a low spot that pools cold air, and what is in the soil are all resolved at the address, and none of them are visible at region scale. That is the honest boundary of this page, and the reason the next step is a reading of your own ground rather than a better regional average.
Your Lot vs Casco Bay / Greater Portland Average
Casco Bay / Greater Portland Average
- ●USDA Zones 5b, averaged across Casco Bay / Greater Portland
- ●One soil character for all of Casco Bay / Greater Portland
- ●State-average frost dates
YOUR Parcel
- ✓Your exact zone, not Casco Bay / Greater Portland's 5b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in Casco Bay / Greater Portland
Pull a site-specific report for your exact address in Casco Bay / Greater Portland — soil, sun, drainage, frost risk, contamination, and scored plant recommendations.
Three things about your lot that Casco Bay / Greater Portland averages can't show:
We read public map data around Casco Bay / Greater Portland — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not Casco Bay / Greater Portland average
How we know this about Casco Bay / Greater Portland
The boundary: a cluster of neighboring counties. Growing regions have no official boundary, so this footprint is our published one, and every figure on this page was measured over the counties inside it rather than borrowed from the state around it.
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, aggregated across the footprint; the growing season from NOAA Regional Climate Centers ACIS 1991–2020 28°F normals, read at each member’s own record; boundaries from US Census TIGER/Line 2025; the contamination record from the federal sources named in that beat above; the soil from the USDA NRCS SSURGO survey, read at each member county’s own record and weighted to this footprint by land area. Where a member carries no record of its own, it is absent from the figure rather than filled in.
Common questions about Casco Bay / Greater Portland
What hardiness zone is Casco Bay / Greater Portland?
Casco Bay / Greater Portland spans USDA hardiness zones 5b, aggregated from the USDA Agricultural Research Service Plant Hardiness Zone Map 2023 across the region's counties. Zones reflect average annual extreme minimum temperatures from 1991–2020 data.
How long is the growing season in Casco Bay / Greater Portland?
Measured between 28°F hard freezes, Casco Bay / Greater Portland's counties mostly run 176 to 187 frost-free days, median 186 (NOAA Regional Climate Centers ACIS, 1991–2020, over 3 member counties). That 11-day spread is the honest read — no single season describes 3 counties, and each county guide below carries its own.
When does frost risk end in Casco Bay / Greater Portland?
Across Casco Bay / Greater Portland, the middle half of member counties see their last 28°F hard freeze between Apr 20 and Apr 25 (median Apr 20), and their first fall freeze between Oct 18 and Oct 24. Those are hard freezes, not the last light frost: tender transplants wait for about May 11, and elevation decides which end of a 5-day spread you garden in.
What grows well in Casco Bay / Greater Portland?
Casco Bay / Greater Portland's conditions suit plants such as Potato, Kale, Blueberry, Apple, White Pine. What actually thrives on any one site still turns on its specific soil, sun, and drainage — we score each plant against the real conditions at your address.
How is Casco Bay / Greater Portland defined?
Casco Bay / Greater Portland is defined here as a curated cluster of 3 neighboring counties that share a growing character. Growing regions have no official boundary, so this footprint is our published one — every county in it is named on this page.
Which states does Casco Bay / Greater Portland cover?
Casco Bay / Greater Portland spans Maine. Each state's full growing guide is linked on this page.
What other growing regions cover the same states?
Midcoast Maine, Down East Maine, Western Maine Mountains (High Peaks / Rangeley Lakes), Aroostook County, Greater Bangor / Penobscot Valley, and Kennebec Valley (Central Maine) share at least one state with Casco Bay / Greater Portland. Growing regions overlap and interlock — a single address can sit inside more than one, which is why the precise read is always the parcel, not the region.
What this page reads at Casco Bay / Greater Portland’s own scale
6 of the readings below were taken at Casco Bay / Greater Portland's own scale — the zone 5b band across its footprint, a median 186-day season across its 3 counties and 2,960 documented federal contamination records across its 3 counties, 1,771 of them in Cumberland County alone; one is borrowed from a wider read, named and linked below.
Measured here
- The USDA hardiness band 5b aggregated across all 3 counties in the footprintUSDA PHZM 2023
- The boundary itself — the 3 counties this region is curated fromGrowable Ground subregion registry
- The growing character that makes this region distinct from the states it crossesGrowable Ground subregion registry
- The 28°F season across the footprint — the middle half of its 3 counties run 176 to 187 frost-free days, median 186NOAA Regional Climate Centers ACIS, 1991-2020
- The federal contamination record over the whole footprint — 2,960 documented federal contamination records across its 3 counties, 1,771 of them in Cumberland County aloneEPA, USGS and USDA federal site records
- The soil order mix across the footprint — 67% Spodosols, topsoil pH 4.2–5.5, area-weighted across its 3 mapped countiesUSDA NRCS SSURGO
Inherited from a wider read
- The candidate plant list — its state's curated sets, merged and filtered to zones 5bfrom Maine
What this page can’t tell you
- Which end of the 11-day range your ground sits at — 176 days at one extreme, 187 at the other, decided mostly by elevationfrost dates explained →
- Which of those soils your own ground sits on — the survey maps them county-wide, and they change across a drivewaysoil types explained →
- The soil pH, drainage, sun hours, and frost pockets on one specific lotwhy one lot differs from the next →
Explore Casco Bay / Greater Portland
State growing guides
Counties in this region
Other growing regions across Maine
Maine's rocky-peninsula coast, where the Atlantic tempers winters into USDA zones 5b–6a and stretches the frost-free season to roughly 186–222 days — Knox County's 222-day window is the longest of any county in the state.
Down East Maine spans Hancock and Washington counties along the state's easternmost coast, where a maritime-tempered USDA zone 5b holds a short frost-free season of roughly 178 to 188 days.
The only true mountain regime in Maine. Franklin County runs just a 143-day growing season in zone 4b — the coldest, shortest-season part of the state outside Aroostook — because high elevation plus valley cold-air pooling in the High Peaks and Rangeley Lakes country pushes last frosts into late May and first frosts into early September. That is a short, sharp mountain season that neither Maine's milder coast nor Aroostook's flat lowland potato country shares, and it changes what you can start from seed, when you transplant, and which long-season crops need a head start indoors.
Northern Maine's vast potato country — hard winters, a short intense summer, and deep glacial loams behind one of the country's great seed-potato regions.
The Penobscot Valley's settled corridor — Bangor to Old Town river ground whose measured 177-187-day seasons run a solid week past the 2-million-acre northern-forest county around it.
Central Maine's inland exception to an all-coastal map. Where the Kennebec River threads Augusta, Waterville and Gardiner, the corridor holds Zone 5b with roughly 190 frost-free days — one of the warmer pockets of Maine's continental interior, and warmer than the 4b uplands that ring it. But "continental" is the operative word: with no Atlantic moderation like a Casco Bay parcel gets, growers here trade steadier summer heat for harder radiational-frost nights that can fall well below the nominal-zone number on a clear, still spring or fall evening. Read your own cold pockets off the river — not just the corridor's headline zone.