Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 29, 2026

Planting zone for 76831 — Castell, TX

ZIP code 76831 (Castell, Llano County) is in USDA zone 8b: the coldest winter night averages 15 to 20 °F. The median last spring frost is March 18 and the first fall frost November 13.

Where ZIP 76831 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.

Zone8b15 to 20 °F

Coldest night−9.4 to −6.7 °C

Last frostMar 18

First frostNov 13

2012 zone8a

Was 8a 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 8b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Castell, TX…

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 Castell, TX

JanFebMarAprMayJunJulAugSepOctNovDec

Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free

Last spring frostMarch 18median — 50% chance of a later frost

Safe for tender plantsApril 7only 10% chance of frost after

First fall frostNovember 13median — 10% chance by Oct 30

Growing season241 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 4 and returns around November 26.

From the NOAA station Mason, TX — 16 miles away, elevation 1,532 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 76831

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Feb 4 – Feb 18

SpinachSow outdoors · Feb 4 – Feb 25

Broccoli, cabbageStart indoors · Jan 7 – Jan 21

Broccoli, cabbageTransplant · Feb 25 – Mar 11

Onion setsPlant outdoors · Feb 18 – Mar 4

PotatoesPlant outdoors · Feb 25 – Mar 11

LettuceSow outdoors · Feb 18 – Mar 4

Carrots, beetsSow outdoors · Feb 25 – Mar 11

TomatoesStart indoors · Jan 21 – Feb 4

TomatoesTransplant · Mar 25 – Apr 8

Peppers, eggplantStart indoors · Jan 7 – Jan 21

Peppers, eggplantTransplant · Apr 1 – Apr 15

BeansSow outdoors · Mar 25 – Apr 8

Sweet cornSow outdoors · Mar 25 – Apr 8

Cucumbers, squashSow outdoors · Mar 25 – Apr 8

Fall lettuce, spinachSow outdoors · Sep 18 – Oct 2

GarlicPlant cloves · Oct 30 – Dec 4

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Mar 18) and first fall frost (Nov 13) 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 76831 came on Jan 30 — about the same time as the 1991–2020 average (Jan 31).

average
Jan 312026
Jan 30

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 8b in Texas

620 plants native to the lower 48 states, recorded in Texas, and rated by the USDA to survive 15 °F or colder — the bottom of zone 8b. Each links to its USDA PLANTS profile.

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 Castell, TX.

Soil in Castell, TX

The soil mapped at the center of ZIP 76831 is Castell — sandy loam, well drained.

TextureSandy loamdrains and warms fast; dries out and needs more feeding

pH6.3slightly acidic; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter0.8%low — add compost

Hydrologic groupCwater soaks in slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Castell sandy loam, 1 to 5 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 76831; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Castell, TX

Summer high96 °Faverage high, July

Winter low34 °Faverage low, January

Hot days113days a year at 90 °F or more

Freezing nights51nights a year at 32 °F or less

Growing degree days6,330base 50 °F — the season’s usable heat

Longest day14 h 08 mindaylight on June 21

Shortest day10 h 09 mindaylight on December 21

Onions to growShort-daysummer days stay under about 14 h 20 min, so long-day varieties won’t bulb

Precipitation30.1 inper year, rain and melted snow

Average high and low, by month

Precipitation by month inches

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Mason, TX (16 mi away); precipitation: Castell 0.1 E, TX (4 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 8b.

768208b

Art, TX · 5 mi

768858b

Valley Spring, TX · 9 mi

Texas planting zones →

Questions about 76831

What planting zone is ZIP code 76831?

ZIP code 76831 (Castell, TX) is in USDA hardiness zone 8b on the 2023 map: the coldest night of an average winter reaches 15 to 20 °F (−9.4 to −6.7 °C).

When is the last frost in Castell, TX?

The median last spring frost (32 °F) near 76831 is March 18; after April 7 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Mason station.

When is the first frost in Castell, TX?

The median first fall frost near 76831 is November 13, giving a median frost-free growing season of 241 days.

When should I plant tomatoes in 76831?

Start tomato seeds indoors around Jan 21 – Feb 4 and set transplants outside around Mar 25 – Apr 8, one to three weeks after the median last frost.

How much rain and snow does 76831 get?

About 30.1 inches of precipitation a year (NOAA 1991–2020 normals, Castell 0.1 E station).

How hot do summers get in Castell, TX?

The average high peaks at 96 °F in July, with about 113 days a year at 90 °F or more.

What kind of soil does 76831 have?

The USDA soil survey maps Castell at the center of ZIP 76831: sandy loam, well drained, pH about 6.3 (slightly acidic).

Did spring come early in Castell, TX in 2026?

Spring leaf-out near 76831 came on Jan 30 in 2026, about the same as the 1991–2020 average (Jan 31), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Castell, TX?

Short-day onions: the longest day near 76831 is 14 h 08 min, so summer days stay under about 14 h 20 min, so long-day varieties won’t bulb.

Did the zone for 76831 change?

Yes. The 2012 USDA map put 76831 in zone 8a; the 2023 map puts it in zone 8b. The newer map uses 1991–2020 data and more weather stations.