San Juan County, in Washington, sits in USDA hardiness zone 9a — a range where zone-matched perennials and frost-aware annual timing set what succeeds.
Well-matched crops include grape, garlic, kale, and western red cedar, and the gap between "grows in the area" and "grows in your yard" is closed by soil, sun, and drainage.
San Juan County lies within the Pacific Northwest — a regional growing area with its own character.
Grounded in USDA PHZM 2023 · Growable Ground suitability scoring
San Juan County holds more than one microclimate.
Soils and elevations shift across San Juan County, so your frost dates and drainage aren't the county average. Enter your address and we'll score 1,112 plants against your land's actual soil, sun, and frost.
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
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Quick Facts
USDA Zones
9a
Last Frost (state avg.)
Mar 1 - Jun 1
First Frost (state avg.)
Sep 15 - Nov 15
County Area
111K acres
Hardiness Zone Range
Growing Season
Zone maps are averages across San Juan County. Your yard's slope, trees, and frost pockets shift what actually grows — see your land's exact reading.
Soil in San Juan County
Across San Juan County, the ground is predominantly Inceptisols, where Cady, Mitchellbay, and Doebay are the most extensive named soil series. The soil is generally well drained with a loam surface. Topsoil pH runs about 5.1–5.7, strongly acidic. Rainfall drains through hydrologic group C/D soils.
Soil order
Inceptisols
Drainage
Well drained
Prime farmland
16%
Hydric soils
14%
Soil still varies lot by lot — soil types explained.
What Grows in San Juan County
Plants matched to San Juan County's USDA zones 9a — each links to its full growing profile.
Growing Challenges in Washington
What an experienced grower plans around here — each one has a move.
Extreme rain divide: 90+ inches west, 6 inches east of Cascades
Plant to your side of the Cascades, not to the state — your exact spot's rainfall decides the whole plan.
East side requires irrigation — no rain from June through September
With no summer rain, drip lines and deep mulch are the growing season — set them up before June.
Slug and root rot pressure on the wet west side
Raise the beds, bait the slugs, and water mornings only — the wet-side trio that keeps roots and leaves healthy; extension has the details.
Short seasons at elevation in the Cascades and northeast corners
In the short-season corners, fast varieties plus a cold frame or tunnel reliably close the gap.
For cultivar selection, pest pressure, and planting-time guidance specific to Washington, the WSU Extension is the authoritative local source.
Safe to Grow Here?
What the federal record shows across San Juan County — and how to grow with it.
We checked the federal record across San Juan County — 79 documented sites across 4 of the 9 source types we track.
The most significant on record: 7 brownfield sites. Former commercial or industrial land where legacy contamination may persist.
The federal record across San Juan County is a modest one — a typical footprint for a growing area. Nothing here calls for alarm; it's worth knowing which recorded sites sit closest to where you grow, and each one on the map carries its type and location. If one turns out to be a near neighbor, a one-time soil test settles the question.
Sources: EPA, USGS — 1.8M documented sites tracked nationwide across 9 federal source types.
Environmental Intelligence
Understanding what's nearby helps you make informed decisions about where and how to grow.
Sources Checked
across San Juan County
Severity Distribution
across San Juan County
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around San Juan County, Underground Storage Tanks runs higher than the national average — 54 sites nearby. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
Underground Storage Tanks: Underground storage tanks are the single most common source of soil contamination near homes and gardens.
Use raised beds with imported soil — this eliminates the primary soil-contact pathway.
Check your specific parcel in San Juan County
Get exact proximity distances to contamination sources for your specific parcel — plus soil, sun, drainage, and 1,112 plant recommendations.
Three things about your exact spot that zone averages miss:
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources analyzed in seconds
Your Specific Parcel Matters
San Juan County Average
- ●USDA Zones 9a
- ●Generic soil type for the area
- ●State-average frost dates
YOUR Parcel
- ✓Your exact hardiness zone
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your parcel in San Juan County
Pull a site-specific report for your exact address in San Juan County, Washington — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your exact spot that zone averages miss:
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources analyzed in seconds
Key Growing Facts for San Juan County, Washington
- USDA Hardiness Zones: 9a (USDA PHZM 2023)
- Last Spring Frost (state avg.): Mar 1 - Jun 1 (NOAA 30-Year Climate Normals)
- First Fall Frost (state avg.): Sep 15 - Nov 15 (NOAA 30-Year Climate Normals)
- County Land Area: 111K acres (US Census TIGER 2025)
Zone data: USDA ARS Plant Hardiness Zone Map 2023. Climate data: NOAA NCEI. County boundaries: US Census TIGER/Line 2025.
Frost dates here are the San Juan County average. Low spots and tree cover move them by days on any one yard — see your exact frost windows.
Frequently Asked Questions
What hardiness zone is San Juan County, Washington?
San Juan County sits in USDA hardiness zone 9a, per the USDA Plant Hardiness Zone Map 2023. Zones reflect average annual extreme minimum temperatures from 1991–2020 weather data.
When does frost risk typically end in San Juan County?
San Juan County follows Washington's statewide frost window: last spring frost around Mar 1 - Jun 1 and first fall frost around Sep 15 - Nov 15, per NOAA 30-year climate normals (1991–2020). Frost dates shift with elevation and local microclimate, so watch your own site's cold pockets.
What vegetables grow in San Juan County?
San Juan County's zone 9a supports a wide range — strong performers include Grape, Garlic, Kale, Western Red Cedar, and Potato. What actually takes on any one site comes down to its soil, sun, and drainage, and we score each plant against the real conditions at your address.
Which hardiness zone is San Juan County, really?
Officially, San Juan County sits in USDA zone 9a (USDA PHZM 2023) — but a zone is a 30-year average of winter's coldest night across an area, and it can't see any one yard. A south-facing slope, a tree line, or a low frost pocket can shift a single site by half a zone either way, which is why neighboring gardeners often quote different numbers. We read the conditions at your exact address — soil, sun, slope, and frost — and score 1,112 plants against what's actually there.
Is the soil safe to grow vegetables in San Juan County?
The federal record around San Juan County shows 79 documented sites — a typical footprint for a growing area, and proximity to a documented site is information, not a diagnosis of any one yard. It's worth seeing which recorded sites sit closest to where you grow, and testing the soil before new food beds near any of them.
Just moved to San Juan County — what should I know before planting?
Start with three facts. San Juan County sits in USDA zone 9a, which sets what survives winter; the statewide frost window runs about Mar 1 - Jun 1 to Sep 15 - Nov 15 (NOAA 30-year climate normals); and 79 documented sites sit on the federal record — a typical footprint for a growing area, worth a look on the contamination map before food beds. From there, matching plants to your actual soil and sun is the fun part.
Everything on this page is a San Juan County average. Your yard writes its own version — we read soil, sun, drainage, and frost at your exact address. Try it for 14 days — no card required.
Will It Grow Here?
Zone fit is the first question — each answer below reads Washington's frost window, season length, and soil profile against the plant's real requirements.