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

Planting zone for 21791 — Union Bridge, MD

ZIP code 21791 (Union Bridge, Carroll County) is in USDA zone 7a: the coldest winter night averages 0 to 5 °F. The median last spring frost is April 22 and the first fall frost October 20.

Where ZIP 21791 is

Outlined: the ZIP code area · 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.

Zone7a0 to 5 °F
Coldest night−17.8 to −15 °C
Last frostApr 22
First frostOct 20
2012 zone6b

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

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Union Bridge, 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 Union Bridge, MD

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 22median — 50% chance of a later frost
Safe for tender plantsMay 9only 10% chance of frost after
First fall frostOctober 20median — 10% chance by Oct 7
Growing season179 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 9 and returns around October 31.

From the NOAA station Emmitsburg 2 Se, MD — 11 miles away, elevation 404 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 21791

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 11 – Mar 25
SpinachSow outdoors · Mar 11 – Apr 1
Broccoli, cabbageStart indoors · Feb 11 – Feb 25
Broccoli, cabbageTransplant · Apr 1 – Apr 15
Onion setsPlant outdoors · Mar 25 – Apr 8
PotatoesPlant outdoors · Apr 1 – Apr 15
LettuceSow outdoors · Mar 25 – Apr 8
Carrots, beetsSow outdoors · Apr 1 – Apr 15
TomatoesStart indoors · Feb 25 – Mar 11
TomatoesTransplant · Apr 29 – May 13
Peppers, eggplantStart indoors · Feb 11 – Feb 25
Peppers, eggplantTransplant · May 6 – May 20
BeansSow outdoors · Apr 29 – May 13
Sweet cornSow outdoors · Apr 29 – May 13
Cucumbers, squashSow outdoors · Apr 29 – May 13
Fall lettuce, spinachSow outdoors · Aug 25 – Sep 8
GarlicPlant cloves · Oct 6 – Nov 10
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 22) and first fall frost (Oct 20) 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 21791 came on Mar 15 — 7 days earlier than the 1991–2020 average (Mar 22).

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).

Soil in Union Bridge, MD

The soil mapped at the center of ZIP 21791 is Catoctin — channery loam, well drained.

TextureChannery loama good balance for most vegetables
pH5.8moderately acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1.25%low — add compost
Hydrologic groupBwater soaks in moderately
Farmland classFarmland of statewide importanceUSDA classification

Map unit: Catoctin-Spoolsville complex, 3 to 8 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 21791; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Union Bridge, MD

Summer high85 °Faverage high, July
Winter low22 °Faverage low, January
Hot days18days a year at 90 °F or more
Freezing nights114nights a year at 32 °F or less
Growing degree days3,329base 50 °F — the season’s usable heat
Longest day14 h 58 mindaylight on June 21
Shortest day9 h 22 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

10°20°30°40°50°60°70°80°90°100°32 °FJanuary: average high 40 °F, low 22 °F 40°22° JanFebruary: average high 43 °F, low 23 °F 43°23° FebMarch: average high 52 °F, low 30 °F 52°30° MarApril: average high 64 °F, low 40 °F 64°40° AprMay: average high 72 °F, low 51 °F 72°51° MayJune: average high 81 °F, low 60 °F 81°60° JunJuly: average high 85 °F, low 64 °F 85°64° JulAugust: average high 84 °F, low 62 °F 84°62° AugSeptember: average high 77 °F, low 55 °F 77°55° SepOctober: average high 66 °F, low 44 °F 66°44° OctNovember: average high 54 °F, low 33 °F 54°33° NovDecember: average high 44 °F, low 27 °F 44°27° 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: Emmitsburg 2 Se, MD (11 mi away); precipitation: Westminster 1.0 W, MD (9 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

217927b
Unionville, MD · 4 mi
217627a
Libertytown, MD · 5 mi
217767a
New Windsor, MD · 5 mi
217577a
Keymar, MD · 6 mi
217987a
Woodsboro, MD · 6 mi
217597a
Ladiesburg, MD · 6 mi
217757a
New Midway, MD · 6 mi
217937a
Walkersville, MD · 9 mi

Maryland planting zones →

Questions about 21791

What planting zone is ZIP code 21791?

ZIP code 21791 (Union Bridge, MD) is in USDA hardiness zone 7a on the 2023 map: the coldest night of an average winter reaches 0 to 5 °F (−17.8 to −15 °C).

When is the last frost in Union Bridge, MD?

The median last spring frost (32 °F) near 21791 is April 22; after May 9 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Emmitsburg 2 Se station.

When is the first frost in Union Bridge, MD?

The median first fall frost near 21791 is October 20, giving a median frost-free growing season of 179 days.

When should I plant tomatoes in 21791?

Start tomato seeds indoors around Feb 25 – Mar 11 and set transplants outside around Apr 29 – May 13, one to three weeks after the median last frost.

How much rain and snow does 21791 get?

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

How hot do summers get in Union Bridge, MD?

The average high peaks at 85 °F in July, with about 18 days a year at 90 °F or more.

What kind of soil does 21791 have?

The USDA soil survey maps Catoctin at the center of ZIP 21791: channery loam, well drained, pH about 5.8 (moderately acidic).

Did spring come early in Union Bridge, MD in 2026?

Spring leaf-out near 21791 came on Mar 15 in 2026, 7 days earlier than the 1991–2020 average (Mar 22), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Union Bridge, MD?

Intermediate onions: the longest day near 21791 is 14 h 58 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

Did the zone for 21791 change?

Yes. The 2012 USDA map put 21791 in zone 6b; the 2023 map puts it in zone 7a. The newer map uses 1991–2020 data and more weather stations.