Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated

Planting zone for 21792 — Unionville, MD

ZIP code 21792 (Unionville, Frederick County) is in USDA zone 7b: the coldest winter night averages 5 to 10 °F. The median last spring frost is April 2 and the first fall frost November 6.

Where ZIP 21792 is

tap the map for the zone at any spot · scroll or pinch to zoom

Not the official USDA Plant Hardiness Zone Map. Zones come from the 2023 USDA hardiness-zone data by the PRISM Group, Oregon State University; we re-mapped and recolored them. The official map is at planthardiness.ars.usda.gov.

Zone7b5 to 10 °F
Coldest night−15 to −12.2 °C
Last frostApr 2
First frostNov 6
2012 zone6b

Was 6b on the 2012 map — a full zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 7b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Unionville, MD…

Forecast data: National Weather Service (api.weather.gov), public domain. This is not a warning service — for official frost and freeze warnings see weather.gov.

Frost dates in Unionville, MD

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 2median — 50% chance of a later frost
Safe for tender plantsApril 15only 10% chance of frost after
First fall frostNovember 6median — 10% chance by Oct 23
Growing season218 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 21 and returns around November 21.

From the NOAA station Frederick Police Brks, MD — 14 miles away, elevation 381 ft. NOAA NCEI U.S. Climate Normals 1991–2020, 32 °F threshold. Low spots and valleys can freeze later in spring than the station.

When to plant vegetables in 21792

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Feb 19 – Mar 5
SpinachSow outdoors · Feb 19 – Mar 12
Broccoli, cabbageStart indoors · Jan 22 – Feb 5
Broccoli, cabbageTransplant · Mar 12 – Mar 26
Onion setsPlant outdoors · Mar 5 – Mar 19
PotatoesPlant outdoors · Mar 12 – Mar 26
LettuceSow outdoors · Mar 5 – Mar 19
Carrots, beetsSow outdoors · Mar 12 – Mar 26
TomatoesStart indoors · Feb 5 – Feb 19
TomatoesTransplant · Apr 9 – Apr 23
Peppers, eggplantStart indoors · Jan 22 – Feb 5
Peppers, eggplantTransplant · Apr 16 – Apr 30
BeansSow outdoors · Apr 9 – Apr 23
Sweet cornSow outdoors · Apr 9 – Apr 23
Cucumbers, squashSow outdoors · Apr 9 – Apr 23
Fall lettuce, spinachSow outdoors · Sep 11 – Sep 25
GarlicPlant cloves · Oct 23 – Nov 27
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 2) and first fall frost (Nov 6) with the rules of thumb U.S. extension services publish. Cool-season crops take a light frost; warm-season crops do best once the soil is above about 60 °F.

When spring arrived in 2026

In 2026, spring leaf-out near 21792 came on Mar 13 — 7 days earlier than the 1991–2020 average (Mar 20).

JanFebMarAprMayJun

The Spring Index marks the day when accumulated warmth is enough for lilac and honeysuckle to leaf out — a widely used signal that the growing season has started. Source: USA National Phenology Network, www.usanpn.org — Spring Indices, first leaf, 2026 (data as of 2026-09-08) and 30-year average 1991–2020 (CC BY 4.0).

Native plants for zone 7b in Maryland

618 plants native to the lower 48 states, recorded in Maryland, and rated by the USDA to survive 5 °F or colder — the bottom of zone 7b. Each links to its USDA PLANTS profile.

Trees 83

American basswood100 ft · blooms late spring · low shade tolerance · to −46 °FAmerican beech80 ft · blooms mid spring · low shade tolerance · to −44 °FAmerican chestnut115 ft · blooms early summer · high shade tolerance · to −33 °FAmerican elm120 ft · blooms early spring · medium shade tolerance · to −46 °FAmerican sycamore100 ft · blooms spring · medium shade tolerance · to −34 °Farborvitae50 ft · blooms mid spring · medium shade tolerance · to −33 °F

Shrubs 127

alderleaf buckthorn3 ft · blooms late spring · low shade tolerance · to −33 °FAllegheny serviceberry35 ft · blooms early spring · low shade tolerance · to −38 °Falternateleaf dogwood25 ft · blooms mid spring · low shade tolerance · to −47 °FAmerican beautyberry6 ft · blooms early summer · medium shade tolerance · to −8 °FAmerican black currant3 ft · blooms early spring · to −46 °FAmerican hazelnut10 ft · blooms spring · medium shade tolerance · to −33 °F

Wildflowers & perennials 206

American bur-reed3 ft · blooms indeterminate · medium shade tolerance · to −33 °FAmerican eelgrass1 ft · blooms mid summer · high shade tolerance · to −33 °FAmerican lotus3 ft · blooms mid summer · high shade tolerance · to −36 °FAmerican pokeweed7 ft · blooms mid summer · low shade tolerance · to −33 °FAmerican speedwell0.8 ft · blooms mid summer · low shade tolerance · to −38 °FAmerican spongeplant0.5 ft · blooms mid spring · high shade tolerance · to −24 °F

Grasses 176

American beachgrass2.5 ft · blooms spring · high shade tolerance · to −28 °FAmerican cupscale3 ft · blooms early summer · high shade tolerance · to −18 °Farctic brome3 ft · blooms early summer · low shade tolerance · to −33 °FAtlantic mannagrass3 ft · blooms summer · low shade tolerance · to −8 °Fautumn bluegrass1.5 ft · blooms late spring · medium shade tolerance · to −8 °Fawlfruit sedge4 ft · blooms mid spring · low shade tolerance · to −38 °F

Vines 12

American bittersweet15 ft · blooms spring · low shade tolerance · to −38 °FAmerican wisteria50 ft · blooms spring · low shade tolerance · to −3 °Fdevil's darning needles15 ft · blooms mid summer · medium shade tolerance · to −38 °Ffrost grape83 ft · blooms late spring · medium shade tolerance · to −23 °Fgraybark grape50 ft · blooms late spring · medium shade tolerance · to −8 °Fheartleaf peppervine2 ft · blooms late spring · low shade tolerance · to −8 °F

Source: USDA, NRCS. 2026. The PLANTS Database (plants.usda.gov), National Plant Data Team — state distribution, native status and the “Temperature, Minimum” characteristic. Check light, moisture and soil needs before buying; local nurseries and your state native plant society can confirm what thrives in Unionville, MD.

Soil in Unionville, MD

The soil mapped at the center of ZIP 21792 is Hatboro — silt loam, poorly drained.

TextureSilt loama good balance for most vegetables
pH5.9moderately acidic; most vegetables like 6.0–7.0
DrainagePoorly drainedconsider raised beds
Organic matter3%moderate
Hydrologic groupB/Dmoderate when drained; wet otherwise
Farmland classNot prime farmlandUSDA classification

Map unit: Hatboro-Codorus silt loams, 0 to 3 percent slopes.

Soil Survey Staff, Natural Resources Conservation Service, USDA. Soil Survey Geographic (SSURGO) Database, via Soil Data Access, accessed 2026-10-29. The soil at the center point of ZIP 21792; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Unionville, MD

Summer high89 °Faverage high, July
Winter low27 °Faverage low, January
Hot days37days a year at 90 °F or more
Freezing nights84nights a year at 32 °F or less
Growing degree days4,174base 50 °F — the season’s usable heat
Longest day14 h 57 mindaylight on June 21
Shortest day9 h 23 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation46.6 inper year, rain and melted snow

Average high and low, by month

20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 43 °F, low 27 °F 43°27° JanFebruary: average high 48 °F, low 29 °F 48°29° FebMarch: average high 55 °F, low 36 °F 55°36° MarApril: average high 69 °F, low 45 °F 69°45° AprMay: average high 77 °F, low 55 °F 77°55° MayJune: average high 85 °F, low 63 °F 85°63° JunJuly: average high 89 °F, low 68 °F 89°68° JulAugust: average high 87 °F, low 66 °F 87°66° AugSeptember: average high 80 °F, low 59 °F 80°59° SepOctober: average high 69 °F, low 47 °F 69°47° OctNovember: average high 56 °F, low 38 °F 56°38° NovDecember: average high 47 °F, low 31 °F 47°31° Dec

Precipitation by month inches

0″1″2″3″4″5″January: 3.36 in precipitation 3.4 JanFebruary: 2.82 in precipitation 2.8 FebMarch: 4.08 in precipitation 4.1 MarApril: 3.57 in precipitation 3.6 AprMay: 4.05 in precipitation 4.0 MayJune: 4.16 in precipitation 4.2 JunJuly: 4.27 in precipitation 4.3 JulAugust: 3.94 in precipitation 3.9 AugSeptember: 4.88 in precipitation 4.9 SepOctober: 4.33 in precipitation 4.3 OctNovember: 3.34 in precipitation 3.3 NovDecember: 3.83 in precipitation 3.8 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Frederick Police Brks, MD (14 mi away); precipitation: Westminster 1.0 W, MD (11 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

8 of the 8 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

217627a
Libertytown, MD · 3 mi
217917a
Union Bridge, MD · 4 mi
217767a
New Windsor, MD · 5 mi
217717a
Mount Airy, MD · 6 mi
217747a
New Market, MD · 7 mi
217987a
Woodsboro, MD · 7 mi
217017a
Frederick, MD · 8 mi
217757a
New Midway, MD · 9 mi

Maryland planting zones →

Questions about 21792

What planting zone is ZIP code 21792?

ZIP code 21792 (Unionville, MD) is in USDA hardiness zone 7b on the 2023 map: the coldest night of an average winter reaches 5 to 10 °F (−15 to −12.2 °C).

When is the last frost in Unionville, MD?

The median last spring frost (32 °F) near 21792 is April 2; after April 15 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Frederick Police Brks station.

When is the first frost in Unionville, MD?

The median first fall frost near 21792 is November 6, giving a median frost-free growing season of 218 days.

When should I plant tomatoes in 21792?

Start tomato seeds indoors around Feb 5 – Feb 19 and set transplants outside around Apr 9 – Apr 23, one to three weeks after the median last frost.

How much rain and snow does 21792 get?

About 46.6 inches of precipitation a year (NOAA 1991–2020 normals, Westminster 1.0 W station).

How hot do summers get in Unionville, MD?

The average high peaks at 89 °F in July, with about 37 days a year at 90 °F or more.

What kind of soil does 21792 have?

The USDA soil survey maps Hatboro at the center of ZIP 21792: silt loam, poorly drained, pH about 5.9 (moderately acidic).

Did spring come early in Unionville, MD in 2026?

Spring leaf-out near 21792 came on Mar 13 in 2026, 7 days earlier than the 1991–2020 average (Mar 20), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Unionville, MD?

Intermediate onions: the longest day near 21792 is 14 h 57 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

Did the zone for 21792 change?

Yes. The 2012 USDA map put 21792 in zone 6b; the 2023 map puts it in zone 7b. The newer map uses 1991–2020 data and more weather stations.