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

Planting zone for O Fallon, MO

O Fallon, MO is in USDA zone 6b across its 2 ZIP codes. The median last spring frost is April 9 and the first fall frost October 26.

O Fallon on the zone map

Outlined: its 2 ZIP code areas · 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.

Zone6b−5 to 0 °F

ZIP codes2

Last frostApr 9

First frostOct 26

Growing season198 days

6b 2 ZIPs

Frost forecast — next 7 nights

live from the National Weather Service

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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 O Fallon, MO

JanFebMarAprMayJunJulAugSepOctNovDec

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

Last spring frostApril 9median — 50% chance of a later frost

Safe for tender plantsApril 25only 10% chance of frost after

First fall frostOctober 26median — 10% chance by Oct 14

Growing season198 daysfrost-free, median

A hard freeze (28 °F) usually ends around March 31 and returns around November 5.

From the NOAA station St Peters 2 Se, MO, elevation 574 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 O Fallon

JanFebMarAprMayJunJulAugSepOctNovDec

PeasSow outdoors · Feb 26 – Mar 12

SpinachSow outdoors · Feb 26 – Mar 19

Broccoli, cabbageStart indoors · Jan 29 – Feb 12

Broccoli, cabbageTransplant · Mar 19 – Apr 2

Onion setsPlant outdoors · Mar 12 – Mar 26

PotatoesPlant outdoors · Mar 19 – Apr 2

LettuceSow outdoors · Mar 12 – Mar 26

Carrots, beetsSow outdoors · Mar 19 – Apr 2

TomatoesStart indoors · Feb 12 – Feb 26

TomatoesTransplant · Apr 16 – Apr 30

Peppers, eggplantStart indoors · Jan 29 – Feb 12

Peppers, eggplantTransplant · Apr 23 – May 7

BeansSow outdoors · Apr 16 – Apr 30

Sweet cornSow outdoors · Apr 16 – Apr 30

Cucumbers, squashSow outdoors · Apr 16 – Apr 30

Fall lettuce, spinachSow outdoors · Aug 31 – Sep 14

GarlicPlant cloves · Oct 12 – Nov 16

Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 9) and first fall frost (Oct 26) 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 63366 came on Mar 6 — 11 days earlier than the 1991–2020 average (Mar 17).

average
Mar 172026
Mar 6

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 6b in Missouri

525 plants native to the lower 48 states, recorded in Missouri, and rated by the USDA to survive −5 °F or colder — the bottom of zone 6b. 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 O Fallon, MO.

Soil in O Fallon, MO

The soil mapped at the center of ZIP 63366 is Weller — silt loam, moderately well drained.

TextureSilt loama good balance for most vegetables

pH6.8neutral; most vegetables like 6.0–7.0

DrainageModerately well drainednatural drainage class

Organic matter2.9%moderate

Hydrologic groupDwater soaks in very slowly

Farmland classprime farmlandUSDA classification

Map unit: Weller silt loam, 2 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 63366; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in O Fallon, MO

Summer high88 °Faverage high, July

Winter low21 °Faverage low, January

Hot days39days a year at 90 °F or more

Freezing nights103nights a year at 32 °F or less

Growing degree days4,056base 50 °F — the season’s usable heat

Longest day14 h 53 mindaylight on June 21

Shortest day9 h 27 mindaylight on December 21

Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully

Precipitation43.1 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: St Peters 2 Se, MO (9 mi away); precipitation: Stn Peters 1.3 Nne, MO (8 mi away). Daylight is calculated from latitude.

ZIP codes in O Fallon

Missouri planting zones →

Questions about O Fallon

What planting zone is O Fallon, MO?

O Fallon, MO is in USDA hardiness zone 6b on the 2023 map (6b: −5 to 0 °F).

When is the last frost in O Fallon, MO?

The median last spring frost is April 9 and the median first fall frost is October 26 (NOAA 1991–2020 normals, St Peters 2 Se station).

When can I plant a garden in O Fallon, MO?

Cool-season crops like peas and spinach go in around Feb 26 – Mar 19; tomatoes, peppers, beans and squash after about Apr 16, once frost risk has passed.

What kind of onions grow in O Fallon, MO?

Intermediate onions: the longest day is 14 h 53 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

How much rain and snow does O Fallon, MO get?

About 43.1 inches of precipitation a year (NOAA 1991–2020 normals).