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

Planting zone for 21738 — Glenwood, MD

ZIP code 21738 (Glenwood, Howard County) is in USDA zone 7b: the coldest winter night averages 5 to 10 °F. The median last spring frost is April 14 and the first fall frost October 29.

Where ZIP 21738 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.

Zone7b5 to 10 °F
Coldest night−15 to −12.2 °C
Last frostApr 14
First frostOct 29
2012 zone7a

Was 7a 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 7b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Glenwood, 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 Glenwood, MD

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 14median — 50% chance of a later frost
Safe for tender plantsMay 2only 10% chance of frost after
First fall frostOctober 29median — 10% chance by Oct 13
Growing season196 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 2 and returns around November 8.

From the NOAA station Damascus 3 Ssw, MD — 11 miles away, elevation 699 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 21738

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 3 – Mar 17
SpinachSow outdoors · Mar 3 – Mar 24
Broccoli, cabbageStart indoors · Feb 3 – Feb 17
Broccoli, cabbageTransplant · Mar 24 – Apr 7
Onion setsPlant outdoors · Mar 17 – Mar 31
PotatoesPlant outdoors · Mar 24 – Apr 7
LettuceSow outdoors · Mar 17 – Mar 31
Carrots, beetsSow outdoors · Mar 24 – Apr 7
TomatoesStart indoors · Feb 17 – Mar 3
TomatoesTransplant · Apr 21 – May 5
Peppers, eggplantStart indoors · Feb 3 – Feb 17
Peppers, eggplantTransplant · Apr 28 – May 12
BeansSow outdoors · Apr 21 – May 5
Sweet cornSow outdoors · Apr 21 – May 5
Cucumbers, squashSow outdoors · Apr 21 – May 5
Fall lettuce, spinachSow outdoors · Sep 3 – Sep 17
GarlicPlant cloves · Oct 15 – Nov 19
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 14) and first fall frost (Oct 29) 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 21738 came on Mar 13 — 6 days earlier than the 1991–2020 average (Mar 19).

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 Glenwood, MD.

Soil in Glenwood, MD

The soil mapped at the center of ZIP 21738 is Glenelg — loam, well drained.

TextureLoama good balance for most vegetables
pH5.4strongly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter2.5%moderate
Hydrologic groupBwater soaks in moderately
Farmland classprime farmlandUSDA classification

Map unit: Glenelg loam, 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 21738; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Glenwood, MD

Summer high84 °Faverage high, July
Winter low25 °Faverage low, January
Hot days13days a year at 90 °F or more
Freezing nights98nights a year at 32 °F or less
Growing degree days3,528base 50 °F — the season’s usable heat
Longest day14 h 56 mindaylight on June 21
Shortest day9 h 24 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation47.9 inper year, rain and melted snow

Average high and low, by month

10°20°30°40°50°60°70°80°90°32 °FJanuary: average high 39 °F, low 25 °F 39°25° JanFebruary: average high 43 °F, low 26 °F 43°26° FebMarch: average high 52 °F, low 33 °F 52°33° MarApril: average high 64 °F, low 43 °F 64°43° AprMay: average high 73 °F, low 53 °F 73°53° MayJune: average high 80 °F, low 62 °F 80°62° JunJuly: average high 84 °F, low 66 °F 84°66° JulAugust: average high 83 °F, low 65 °F 83°65° AugSeptember: average high 76 °F, low 58 °F 76°58° SepOctober: average high 65 °F, low 47 °F 65°47° OctNovember: average high 53 °F, low 36 °F 53°36° NovDecember: average high 44 °F, low 30 °F 44°30° Dec

Precipitation by month inches

0″1″2″3″4″5″January: 3.38 in precipitation 3.4 JanFebruary: 2.91 in precipitation 2.9 FebMarch: 4.18 in precipitation 4.2 MarApril: 3.74 in precipitation 3.7 AprMay: 4.44 in precipitation 4.4 MayJune: 4.48 in precipitation 4.5 JunJuly: 4.46 in precipitation 4.5 JulAugust: 4.45 in precipitation 4.5 AugSeptember: 4.33 in precipitation 4.3 SepOctober: 4.23 in precipitation 4.2 OctNovember: 3.33 in precipitation 3.3 NovDecember: 3.97 in precipitation 4.0 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Damascus 3 Ssw, MD (11 mi away); precipitation: Sykesville 1.7 Sse, MD (6 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.

217377b
Glenelg, MD · 2 mi
210367b
Dayton, MD · 3 mi
217947b
West Friendship, MD · 3 mi
217237a
Cooksville, MD · 4 mi
217977b
Woodbine, MD · 4 mi
217657b
Lisbon, MD · 5 mi
208337b
Brookeville, MD · 5 mi
210417b
Ellicott City, MD · 5 mi

Maryland planting zones →

Questions about 21738

What planting zone is ZIP code 21738?

ZIP code 21738 (Glenwood, 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 Glenwood, MD?

The median last spring frost (32 °F) near 21738 is April 14; after May 2 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Damascus 3 Ssw station.

When is the first frost in Glenwood, MD?

The median first fall frost near 21738 is October 29, giving a median frost-free growing season of 196 days.

When should I plant tomatoes in 21738?

Start tomato seeds indoors around Feb 17 – Mar 3 and set transplants outside around Apr 21 – May 5, one to three weeks after the median last frost.

How much rain and snow does 21738 get?

About 47.9 inches of precipitation a year (NOAA 1991–2020 normals, Sykesville 1.7 Sse station).

How hot do summers get in Glenwood, MD?

The average high peaks at 84 °F in July, with about 13 days a year at 90 °F or more.

What kind of soil does 21738 have?

The USDA soil survey maps Glenelg at the center of ZIP 21738: loam, well drained, pH about 5.4 (strongly acidic).

Did spring come early in Glenwood, MD in 2026?

Spring leaf-out near 21738 came on Mar 13 in 2026, 6 days earlier than the 1991–2020 average (Mar 19), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Glenwood, MD?

Intermediate onions: the longest day near 21738 is 14 h 56 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

Did the zone for 21738 change?

Yes. The 2012 USDA map put 21738 in zone 7a; the 2023 map puts it in zone 7b. The newer map uses 1991–2020 data and more weather stations.