Unlikely — Zone Mismatch
sunn hemp (zones 7-11) does not overlap with Iowa's zone range (4b-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 Iowa is zone 6 — 1 zone short. Planted in open ground it will not survive a Iowa winter. To grow it here, grow sunn hemp in a hoop house or unheated greenhouse — its modeled 1.5-zone lift covers this gap.
A hoop house or unheated greenhouse is the lift this needs. How a greenhouse changes what you can grow →
Zone math from 7-11 (plant) against 4-6 (Iowa). Lift modeled by crop_engine.GREENHOUSE_ZONE_MODIFIER_BY_SUBTYPE['hoop_house'] (+1.5 zones) via api/fast_sections.py Unlock Engine v2.
Sunn Hemp in Iowa — the short answer
Sunn Hemp in Iowa: hardy in zones 7–11, full sun, against a state median of ~189 frost-free days.
- Plant zones
- 7-11
- Manual horticultural correction (source not retained); NC State Extension and Missouri Botanical Garden, with the remaining rows from other published sources or Growable Ground compilation
- Iowa zones
- 4b-6a
- USDA PHZM 2023
- Frost window
- Apr 10 - Apr 17 – Oct 18 - Oct 23
- NOAA RCC ACIS 1991-2020, 28°F
- Shortest county season
- 168 days
- NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze
- Days to maturity
- 90 days
- Growable Ground Plant Database
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS and university Extension sources — per-field provenance varies and is published with the dataset.
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 Iowa site runs ~189 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.
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
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Iowa is not one climate
A single Iowa answer averages 99 counties whose growing seasons are 48 days apart — from 168 frost-free days in Howard County to 216 in Lee County. Which end of that your land sits nearer changes what sunn hemp can finish before the first hard freeze.
The middle half of 99 counties runs 183–197 days, with a median of 189 days — a 48-day span end to end.
Measured across 99 Iowa counties. NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze. Quartiles are order statistics — every figure shown is a value a real county recorded, not an interpolation. Your own site sits somewhere inside this range; a parcel report resolves where.
Where Sunn Hemp Stands in Iowa
Sunn Hemp is rated for zones 7–11 — a cold floor USDA puts at 0 to 10°F. No Iowa county sits inside it, so in open ground this is a grown-under-cover plant here rather than a bed plant — which is a statement about Iowa winters, not about sunn hemp.
Below the zone 7 floor
99 counties
- Clayton County · zone 4b
- Howard County · zone 4b
- Winneshiek County · zone 4b
3 of 99 shown
Winter cold is the limit here — the same plant works in a container that comes under cover.
County hardiness zones: USDA Plant Hardiness Zone Map (PHZM) 2023, per county. A zone is a 10°F band of average annual extreme minimum temperature, so the floor above is the band the plant's rating denotes, not a single measured kill point. Slope, shelter and cold-air drainage routinely move a site half a zone either way from its county figure — which is the part only your address resolves.
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
Iowa Has
- USDA Zones: 4b-6a
- Last Hard Freeze (28°F): Apr 10 - Apr 17
- First Hard Freeze (28°F): Oct 18 - Oct 23
- Annual Rainfall: 26-36 inches
- Common Soils: Prairie loess, Silt loam, Clay loam
Plant Zone Range (zones 7-11)
Preferred Soil pH
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS, and university Extension sources — per-field provenance varies and is published with the dataset. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Sunn Hemp in Iowa
The frost window
Across Iowa, the last spring hard freeze (28°F) clears between Apr 10 and Apr 17, and the first fall hard freeze lands between Oct 18 and Oct 23 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 184-day window you can count on — up to 196 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.
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 — Iowa'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 (Growable Ground Plant Database), counting from the last hard freeze it ripens with 94 days to spare even in Iowa's tightest freeze scenario — room for a later start or a second sowing.
Timing tuned to sub-state frost dates — Howard County, not the statewide average.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: Growable Ground Plant Database — frost-free days from FAO ECOCROP, days to harvest from university Extension production guides, and growing-degree-day and chill-hour minimums that are compilation estimates rather than published figures. 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 Iowa 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 Iowa under protection.
Frost-free days
Sunn Hemp wants 60+ frost-free days; a typical Iowa site sees ~189 (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 ~2730 GDD (from PRISM 1991-2020 climate normals), so Iowa's typical season clears that easily.
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 Iowa. 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 Iowa soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Iowa 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 42.9% of Iowa 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 requirements: FAO ECOCROP and USDA PLANTS, with the remaining rows from other published sources or Growable Ground compilation. Drainage: FAO ECOCROP, with the remaining rows from other published sources or Growable Ground compilation. Iowa soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
What Else to Consider for Sunn Hemp in Iowa
Zone compatibility tells you about winter cold survival — but Iowa growers also need to think about:
Cold winters reaching -20F or below
Variable spring weather delays planting
Wind exposure on open prairies desiccates plants
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 42.9% of Iowa 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 Iowa grower, not just sunn hemp — is covered on the Iowa growing guide.
Timing shifts within Iowa
Iowa isn't one climate. In Howard County, the last hard freeze (28°F) holds until about Apr 24 — roughly 10 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 Howard County — Iowa’s shortest season at 168 frost-free days — clears that by 108 days, so season length isn’t the constraint anywhere in Iowa. What varies is your own parcel.
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 Iowa's frost window — roughly 189 days, opening after the last 28°F freeze (Apr 10 - Apr 17) and closing at the first (Oct 18 - Oct 23), 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 Iowa this is a one-season plant and the first hard freeze (Oct 18 - Oct 23) takes it down, so the exposure above is a summer-long one rather than something standing in the bed all year.
Iowa Cooperative Extension
For Iowa-specific cultivar recommendations, planting calendars, and pest pressure for sunn hemp, the canonical source is Iowa State University Extension and Outreach. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Sunn Hemp native to Iowa?
No — the USDA PLANTS Database lists Sunn Hemp as introduced rather than native in the Lower 48, so it is not part of Iowa's native flora. It grows here as a garden plant; pairing it with a few Iowa natives keeps local pollinators fed too.
Looking for plants that belong here? The Iowa 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 Iowa
When can I plant Sunn Hemp in Iowa?
Iowa's last spring hard freeze (28°F) clears between Apr 10 and Apr 17, and the first fall hard freeze lands between Oct 18 and Oct 23 (NOAA Regional Climate Centers ACIS, 1991-2020). Time outdoor planting to after the last hard freeze for your specific site — a further two to three weeks for anything tender — and count back from those dates for transplant scheduling.
Can Sunn Hemp mature before first frost in Iowa?
Yes — Sunn Hemp matures in 90 days (Growable Ground Plant Database), and Iowa's dependable hard-freeze-free window runs 184 days (NOAA Regional Climate Centers ACIS, 1991-2020), leaving 94 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 Iowa?
Iowa spans USDA hardiness zones 4b-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 Iowa site have?
A typical Iowa site sees ~189 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 Howard, the freeze-free season runs shorter than the state average, so verify your own county's window.
Is Sunn Hemp native to Iowa?
No — the USDA PLANTS Database lists Sunn Hemp as introduced rather than native in the Lower 48, so it is not part of Iowa's native flora. It grows here as a garden plant; pairing it with a few Iowa natives keeps local pollinators fed too.
How should I amend the soil for Sunn Hemp in Iowa?
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 Iowa site — amend for drainage and organic matter first.
Will Sunn Hemp actually grow on my specific land in Iowa?
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 Iowa
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 Iowa sunn hemp averages can't show:
We read public map data around Iowa — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Iowa sunn hemp average
Explore More
Growing guides for sunn hemp
Explore Iowa county by county
Iowa carries 99 counties with their own USDA zone — a sample of the growing pages for sunn hemp there:
Where else sunn hemp grows
Sunn Hemp's zone band crosses state lines — these are the states closest to Iowa in winter climate where it also has a page, plus the far ends of its range. Each one re-answers the question with that state's own record.
More to grow in Iowa
Iowa has a growing page for 806 plants — a rotating sample of the rest of the shelf.
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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