Unlikely — Zone Mismatch
sunn hemp (zones 7-11) does not overlap with Wisconsin's zone range (3b-6a). Growing outdoors is unlikely without significant protection.
One zone short — what would close it
Sunn Hemp is hardy to zone 7, and the warmest ground in Wisconsin is zone 6 — 1 zone short. Planted in open ground it will not survive a Wisconsin winter. To grow it here, grow sunn hemp in a container and move it under cover for the coldest weeks, or give it a greenhouse.
Season extension and overwintering under cover both work at this margin. How to buy the weeks →
Zone math from 7-11 (plant) against 3-6 (Wisconsin). Lift modeled by crop_engine Sprint 20B effective_zone_modifier (+2 zones) via api/fast_sections.py Unlock Engine v2.
Your yard isn't the whole zone.
Sunn Hemp is grown as an annual, so your winter zone isn't the deciding factor — your frost-free window is. A typical Wisconsin site runs ~173 frost-free days, but that is a state median, not your yard: slope, trees, and low spots move the last-frost date across a single parcel. Enter your address and we'll score sunn hemp against your land's actual frost dates, sun, and soil.
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Zone Comparison
Sunn Hemp Needs
- USDA Zones: 7-11
- Frost-Free Days: 60+
- Sun: Full Sun
- Mature size: 6 ft tall
- Picking window: ~30 days of harvest
- Drought tolerance: Medium
Wisconsin Has
- USDA Zones: 3b-6a
- Last Hard Freeze (28°F): Apr 17 - May 2
- First Hard Freeze (28°F): Oct 9 - Oct 21
- Annual Rainfall: 28-34 inches
- Common Soils: Silt loam, Clay loam, Sandy outwash
Plant Zone Range (zones 7-11)
Preferred Soil pH
Plant data: USDA PLANTS Database / Growable Ground plant catalog. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Sunn Hemp in Wisconsin
The frost window
Across Wisconsin, the last spring hard freeze (28°F) clears between Apr 17 and May 2, and the first fall hard freeze lands between Oct 9 and Oct 21 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 160-day window you can count on — up to 187 days on a mild site in a kind year. That marks the hard freeze, not the last light frost, so tender transplants usually wait another 2-3 weeks.
Frost tenderness
Sunn Hemp is frost-tender — its listed minimum temperature is 39.2°F (USDA PLANTS Database) — so set plants out two to three weeks PAST your local last hard freeze, not on it and not on the state's earliest date. The 28°F marker above is when killing freezes stop, not when light frost does.
When to start the seed
Sunn Hemp is direct-sown where it grows rather than started under cover, so there is no seed-tray head start to bank — Wisconsin's dates above are the whole calendar for it. That makes the last-freeze date the binding constraint, and a warmer pocket of your own yard is worth more to this crop than it is to a transplanted one.
Days to maturity vs. the window
At 90 days to maturity (USDA PLANTS Database), counting from the last hard freeze it ripens with 70 days to spare even in Wisconsin's tightest freeze scenario — room for a later start or a second sowing.
Timing tuned to sub-state frost dates — Florence County, not the statewide average.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: USDA PLANTS Database. Your site's own frost dates can run earlier or later than the state range — a parcel report pins them down.
Growing Season Fit
The zone check says sunn hemp does not survive a Wisconsin winter in the ground. The season figures below measure a different question — whether it could finish a season here at all — and they only bear on Wisconsin under protection.
Frost-free days
Sunn Hemp wants 60+ frost-free days; a typical Wisconsin site sees ~173 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves comfortable headroom for succession planting.
Growing degree days
Sunn Hemp needs ~2200 GDD (base 50°F) to ripen. The state median runs ~2102 GDD (from PRISM 1991-2020 climate normals), so Wisconsin's typical season runs short on heat — pick a south-facing site and consider season extension.
GDD and chill-hour figures are computed from PRISM 1991-2020 monthly climate normals; frost-free days from NOAA Regional Climate Centers ACIS, 1991-2020. Each is the state median across its counties (P25/P75 span the within-state spread). Hardiness-zone range is USDA PHZM 2023.
Soil + Drainage Fit
Sunn Hemp tolerates a wide pH range (pH 5-8.4) — 3.4 units, acid ground through alkaline — so soil acidity is rarely what limits it in Wisconsin. A soil test is still worth having, but for what else it tells you about this site, not to clear this plant. It will grow across that whole span, but it does best between pH 6 and 7 — worth knowing before you decide whether a Wisconsin soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Wisconsin site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Your land, not the state average
SSURGO maps about 19.9% of Wisconsin as poorly or somewhat-poorly drained, and the class flips parcel to parcel. Sunn Hemp sits mid-range for drought tolerance, so either extreme can cost you — your soil map unit, not the state average, is the real answer.
Check your parcel → Source: USDA NRCS SSURGO.
Plant pH and drainage requirements from USDA PLANTS Database. Wisconsin soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Sunn Hemp in Wisconsin — Quick Answer
- Verdict: Unlikely — Zone Mismatch
- Plant Zones: 7-11 (USDA PLANTS Database)
- State Zones: 3b-6a (USDA ARS PHZM 2023)
- Growing Season: 28°F hard freezes about Apr 17 - May 2 to Oct 9 - Oct 21 (NOAA Regional Climate Centers ACIS, 1991-2020)
- Days to Maturity: 90 days
What Else to Consider for Sunn Hemp in Wisconsin
Zone compatibility tells you about winter cold survival — but Wisconsin growers also need to think about:
Cold winters (-30F in northern WI)
Short growing season (110-140 frost-free days)
Sandy central soils drain too quickly
Growing sunn hemp here specifically
Sunn Hemp roots run medium and prefer pH 5.0–8.4, but drainage comes first here: SSURGO maps about 19.9% of Wisconsin as poorly or somewhat-poorly drained, and wet ground rots its roots before pH ever matters.
Plant sunn hemp on a raised, gravel-amended berm so water drains fast and the roots stays dry. How to handle it →
What to actually do about these — and the constraints that apply to every Wisconsin grower, not just sunn hemp — is covered on the Wisconsin growing guide.
Timing shifts within Wisconsin
Wisconsin isn't one climate. In Florence County, the last hard freeze (28°F) holds until about May 16 — roughly 21 days later than the recorded state median — so plant sunn hemp to your county's window, not the statewide date.
Sunn Hemp needs about 60 days of frost-free season to complete its cycle. Even Florence County — Wisconsin’s shortest season at 140 frost-free days — clears that by 80 days, so season length isn’t the constraint anywhere in Wisconsin. What varies is your own parcel.
Across Wisconsin’s 72 counties the frost-free season runs from 140 days in Florence County to 217 days in Milwaukee County — a spread of 77 days, which is why a single statewide planting date misleads. The median Wisconsin county sees 173 frost-free days, leaving sunn hemp about 113 days of slack on a typical parcel — room for a second sowing where the season runs long.
County last-freeze dates and frost-free day counts: NOAA Regional Climate Centers ACIS (1991-2020), county 28°F freeze (earlier than the folk 32°F "last frost"). Sunn Hemp's 60-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
Pollinator + Wildlife Value
Sunn Hemp draws honeybees, native bees, and butterflies (moderate value, Growable Ground Plant Database). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, sunn hemp can only work that bloom inside Wisconsin's frost window — roughly 173 days, opening after the last 28°F freeze (Apr 17 - May 2) and closing at the first (Oct 9 - Oct 21), per NOAA Regional Climate Centers ACIS, 1991-2020.
Good to Know Before You Plant Sunn Hemp
Sunn Hemp is listed as toxic to dogs, cats (seeds) at a moderate level (Cornell). Most listed plants only cause brief upset — a raised bed or a fenced corner usually keeps curious pets clear. Worth knowing for placement: in Wisconsin this is a one-season plant and the first hard freeze (Oct 9 - Oct 21) takes it down, so the exposure above is a summer-long one rather than something standing in the bed all year.
Wisconsin Cooperative Extension
For Wisconsin-specific cultivar recommendations, planting calendars, and pest pressure for sunn hemp, the canonical source is UW–Madison Division of Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Sunn Hemp native to Wisconsin?
No — the USDA PLANTS Database lists Sunn Hemp as introduced rather than native in the Lower 48, so it is not part of Wisconsin's native flora. It grows here as a garden plant; pairing it with a few Wisconsin natives keeps local pollinators fed too.
Looking for plants that belong here? The Wisconsin growing guide lists USDA-documented natives for the state.
Native-range data: USDA PLANTS Database state-distribution records, accessed 2026-07-01.
Common Questions About Growing Sunn Hemp in Wisconsin
When can I plant Sunn Hemp in Wisconsin?
Wisconsin's last spring hard freeze (28°F) clears between Apr 17 and May 2, and the first fall hard freeze lands between Oct 9 and Oct 21 (NOAA Regional Climate Centers ACIS, 1991-2020). Sunn Hemp is frost-tender — its listed minimum temperature is 39.2°F (USDA PLANTS Database) — so wait two to three weeks past your site's last hard freeze, when light frost has also stopped, before planting out.
Can Sunn Hemp mature before first frost in Wisconsin?
Yes — Sunn Hemp matures in 90 days (USDA PLANTS Database), and Wisconsin's dependable hard-freeze-free window runs 160 days (NOAA Regional Climate Centers ACIS, 1991-2020), leaving 70 days of margin. Plant once the last hard freeze has passed — two to three weeks later for anything tender — and it ripens ahead of the first fall freeze.
What hardiness zone is Sunn Hemp grown in across Wisconsin?
Wisconsin spans USDA hardiness zones 3b-6a (USDA ARS PHZM 2023). Sunn Hemp carries a range of zones 7-11, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Wisconsin site have?
A typical Wisconsin site sees ~173 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Sunn Hemp needs 60+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date. In cooler counties like Florence, the freeze-free season runs shorter than the state average, so verify your own county's window.
Is Sunn Hemp native to Wisconsin?
No — the USDA PLANTS Database lists Sunn Hemp as introduced rather than native in the Lower 48, so it is not part of Wisconsin's native flora. It grows here as a garden plant; pairing it with a few Wisconsin natives keeps local pollinators fed too.
How should I amend the soil for Sunn Hemp in Wisconsin?
Sunn Hemp tolerates pH 5-8.4 and well (dry spells) drainage (FAO ECOCROP). That is a 3.4-unit span, acid ground through alkaline, so pH is rarely what limits this plant on a Wisconsin site — amend for drainage and organic matter first.
Will Sunn Hemp actually grow on my specific land in Wisconsin?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores sunn hemp against your address's exact soil pH, drainage, sun, and frost-date data drawn from USDA SSURGO, NOAA, and PRISM — not state averages.
Check your specific parcel in Wisconsin
State-level data is a sketch. Your Growable Ground report scores sunn hemp against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Wisconsin sunn hemp averages can't show:
We read public map data around Wisconsin — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Wisconsin sunn hemp average
Explore More
Growing guides for sunn hemp
Analysis by the Growable Ground research team, grounded in USDA PLANTS, USDA NRCS SSURGO, NOAA Climate Normals (1991-2020), and named Cooperative Extension sources. How we know →
Cite this dataset
Kirchick, M. (2026). Growable Ground Plant Database [Data set]. Zenodo. https://doi.org/10.5281/zenodo.21435643
Licensed CC BY-NC 4.0. DOI 10.5281/zenodo.21435643
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