The Merrimack Valley spans USDA hardiness zones 5b-6a — enough range to grow cool-season vegetables, hardy fruit, and warm-season crops that mature before the first hard frost.
New Hampshire's Merrimack River corridor — Concord down through Manchester and Nashua — where the valley floor runs a shade milder (zone 6a at its southern end) than the 5b hills around it, buying growers a little more frost-free room than the uplands nearby. On paper, potato, apple, sugar maple, and blueberry all suit these conditions — on the ground, soil, sun, and drainage make the final call.
The Merrimack Valley spans New Hampshire.
Your yard isn't all of the Merrimack Valley.
The Merrimack Valley spans USDA zones 5b-6a, 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 Merrimack Valley — soil, climate, flood, and parcel records. How we handle your address.
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The Merrimack Valley — the short answer
The Merrimack Valley: USDA zones 5b-6a, 173–183 frost-free days, in New Hampshire.
- USDA zones
- 5b-6a
- USDA PHZM 2023, aggregated over the footprint
- Frost-free season
- 173–183 days
- NOAA Regional Climate Centers ACIS, 1991-2020
- Counties
- 2
- 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 the Merrimack Valley
Plants matched to the Merrimack Valley's USDA zones 5b-6a, 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 the Merrimack Valley
Plants the USDA PLANTS Database documents as native and present in this region’s state, kept only where the published hardiness range meets the Merrimack Valley’s USDA zones 5b-6a — 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 the Merrimack Valley
The USDA soil survey maps this ground county by county. These are those records weighted by land area across two member counties — this region’s own profile, not a state figure.
The ground is predominantly Spodosols, at 43% of mapped acres, followed by Inceptisols (40%), Entisols (12%) and Histosols (5%). The most extensive named soils are Canton, Monadnock, and Marlow.
55% of those acres are well drained, and another 13% moderately well drained, 11% excessively drained. Topsoil pH runs 4.3–5.0 between the lowest and the highest county on record. 2% of the footprint is classed prime farmland.
Soil order
Spodosols
Drainage
Well drained
Topsoil pH
4.3–5.0
Hydric soils
14%
Soil changes across a driveway, so which of these your own ground sits on is a parcel question rather than a regional one. Merrimack County holds the most wetland ground here, at 16% — soil types explained.
USDA NRCS SSURGO via Soil Data Access
What this means for growing in the Merrimack Valley
The soil survey reads this ground county by county, and the readings above are those records summed across the Merrimack Valley. Here is what they mean for a garden here — and the move an experienced grower makes about each one.
Drainage
From county records55% of the mapped acres across the Merrimack Valley are well drained — ground that drains quickly. That is area-weighted across all two 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 the Merrimack Valley runs pH 4.3–5.0 — acidic in all two 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
Safe to Grow Here?
What the federal record shows across the Merrimack Valley — and how to grow with it.
Across the Merrimack Valley's 2 counties the federal record holds 3,596 documented sites, mostly Underground Storage Tanks (1,595), Brownfields (970) and Nitrate (794). The heaviest single county is Hillsborough County, with 2,148.
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 the Merrimack Valley to see which records sit where you actually grow.
Sources Checked
Summed across the 2 counties in the Merrimack Valley
Know Before You Grow
- •Underground tanks leak petroleum as they age — near a former filling station, test the soil.
- •Near a brownfield, raised beds of imported soil give your food clean ground.
- •On a private well, test for nitrates: above 10 mg/L the water needs treatment before irrigating.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
Your land is one point inside the Merrimack Valley
Everything above resolves to the Merrimack Valley as a whole — a footprint spanning USDA zones 5b-6a. 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 the Merrimack Valley Average
Merrimack Valley Average
- ●USDA Zones 5b-6a, averaged across Merrimack Valley
- ●One soil character for all of Merrimack Valley
- ●State-average frost dates
YOUR Parcel
- ✓Your exact zone, not Merrimack Valley's 5b-6a
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in the Merrimack Valley
Pull a site-specific report for your exact address in the Merrimack Valley — soil, sun, drainage, frost risk, contamination, and scored plant recommendations.
Three things about your lot that Merrimack Valley averages can't show:
We read public map data around The Merrimack Valley — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Merrimack Valley average
How we know this about the Merrimack Valley
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 the Merrimack Valley
What hardiness zone is the Merrimack Valley?
The Merrimack Valley spans USDA hardiness zones 5b-6a, 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 the Merrimack Valley?
Measured between 28°F hard freezes, the Merrimack Valley's counties mostly run 176 to 181 frost-free days, median 178 (NOAA Regional Climate Centers ACIS, 1991–2020, over 2 member counties). That 5-day spread is the honest read — no single season describes 2 counties, and each county guide below carries its own.
When does frost risk end in the Merrimack Valley?
Across the Merrimack Valley, the middle half of member counties see their last 28°F hard freeze between Apr 22 and Apr 25 (median Apr 24), and their first fall freeze between Oct 17 and Oct 20. Those are hard freezes, not the last light frost: tender transplants wait for about May 15, and elevation decides which end of a 3-day spread you garden in.
What grows well in the Merrimack Valley?
The Merrimack Valley's conditions suit plants such as Potato, Apple, Sugar Maple, Blueberry, Lilac. 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 the Merrimack Valley defined?
The Merrimack Valley is defined here as a curated cluster of 2 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 the Merrimack Valley cover?
The Merrimack Valley spans New Hampshire. Each state's full growing guide is linked on this page.
What other growing regions cover the same states?
The Upper Valley, White Mountains, The Lakes Region, New Hampshire Seacoast, and The Monadnock Region share at least one state with the Merrimack Valley. 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 the Merrimack Valley’s own scale
6 of the readings below were taken at the Merrimack Valley's own scale — the zone 5b-6a band across its footprint, a median 178-day season across its 2 counties and 3,596 documented federal contamination records across its 2 counties, 2,148 of them in Hillsborough County alone; one is borrowed from a wider read, named and linked below.
Measured here
- The USDA hardiness band 5b-6a aggregated across all 2 counties in the footprintUSDA PHZM 2023
- The boundary itself — the 2 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 2 counties run 176 to 181 frost-free days, median 178NOAA Regional Climate Centers ACIS, 1991-2020
- The federal contamination record over the whole footprint — 3,596 documented federal contamination records across its 2 counties, 2,148 of them in Hillsborough County aloneEPA, USGS and USDA federal site records
- The soil order mix across the footprint — 43% Spodosols, topsoil pH 4.3–5.0, area-weighted across its 2 mapped countiesUSDA NRCS SSURGO
Inherited from a wider read
- The candidate plant list — its state's curated sets, merged and filtered to zones 5b-6afrom New Hampshire
What this page can’t tell you
- Which end of the 5-day range your ground sits at — 176 days at one extreme, 181 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 the Merrimack Valley
State growing guides
Counties in this region
Other growing regions across New Hampshire
The Connecticut River's bi-state trench from Windsor to Hanover — 5a-5b valley-floor ground on both banks, warmer than the hills that rise from it, and a single growing community that has always ignored the state line.
Spanning Coos, Grafton, and Carroll counties, the White Mountains carry one of New Hampshire's shortest growing seasons — from 145 frost-free days in Coos County up to 162 in Grafton — across USDA hardiness zones 4b to 5b.
Winnipesaukee's tempering water gives the Lakes Region towns a solid 5b band and 174-191 measured frost-free days — noticeably gentler than the White Mountain country that begins one county north.
New Hampshire's Seacoast — Rockingham and Strafford counties — sits in USDA zone 6a and posts the state's two longest frost-free seasons, about 187 to 188 days, roughly two weeks more than the Concord-area interior (Merrimack County, 173 days).
Southwestern New Hampshire around Mount Monadnock and the hub city of Keene — cold zone-5b winters and a roughly 170-day growing season across the hills of Cheshire County.