Frost dates for 21792Unionville, MD
The average last spring frost in Unionville is April 22 and the average first fall frost is October 20, giving a typical growing season of about 179 days. Dates are based on NOAA 1991-2020 climate normals.
- Average last spring frost
- Apr 22
- Average first fall frost
- Oct 20
- Growing season
- 179 days
Safer after May 9 (1-in-10 risk)
Possible by Oct 7 (1-in-10 risk)
Last frost typically in 170 days
Frost risk by date
NOAA reports frost dates as probabilities. The middle date is the average (a 50% chance). For spring, the 10% date is a safer planting date: frost after it happens in about 1 year in 10. Light frost (36°F) can damage tender plants; a hard freeze (28°F) kills most annuals.
| Temperature | Last spring: 90% / 50% / 10% chance | First fall: 10% / 50% / 90% chance | Growing season (50%) | Years with at least one |
|---|---|---|---|---|
| Light frost (36°F) | Apr 20 · May 4 · May 22 | Sep 30 · Oct 10 · Oct 23 | 157 days | 100% |
| Frost / freeze (32°F) | Apr 7 · Apr 22 · May 9 | Oct 7 · Oct 20 · Nov 2 | 179 days | 100% |
| Hard freeze (28°F) | Mar 28 · Apr 9 · Apr 25 | Oct 13 · Oct 31 · Nov 13 | 203 days | 100% |
Planting calendar for Unionville
Planting windows from the University of Maryland Extension calendar, shifted to your average frost dates. Use them as a starting point and adjust for seed packet instructions and local conditions.
| Crop | Task | Window |
|---|---|---|
| Tomatoes | Start seeds indoors | Mar 22 – May 28 |
| Tomatoes | Transplant outdoors | May 8 – Jun 30 |
| Peppers | Start seeds indoors | Mar 22 – Apr 22 |
| Peppers | Transplant outdoors | May 22 – Jun 22 |
| Eggplant | Start seeds indoors | Apr 8 – May 7 |
| Eggplant | Transplant outdoors | May 22 – Jun 22 |
| Cucumbers | Sow outdoors | May 22 – Jul 10 |
| Summer squash | Sow outdoors | May 8 – Jun 22 |
| Winter squash & pumpkins | Sow outdoors | May 22 – Jun 22 |
| Snap beans | Sow outdoors | May 8 – Aug 2 |
| Sweet corn | Sow outdoors | May 8 – Jun 30 |
| Watermelon & cantaloupe | Sow outdoors | May 22 – Jun 22 |
| Okra | Sow outdoors | May 15 – Jun 7 |
| Peas | Sow outdoors (spring) | Mar 22 – May 7 |
| Potatoes | Plant outdoors (spring) | Mar 22 – May 22 |
| Onions | Start seeds indoors (spring) | Feb 8 – Mar 7 |
| Onions | Transplant outdoors (spring) | Apr 8 – May 7 |
| Broccoli | Start seeds indoors (spring) | Mar 4 – Apr 7 |
| Broccoli | Transplant outdoors (spring) | Apr 8 – May 7 |
| Broccoli | Start seeds indoors (fall) | Jun 20 – Jun 30 |
| Broccoli | Transplant outdoors (fall) | Jul 20 – Jul 31 |
| Cabbage | Start seeds indoors (spring) | Feb 22 – Mar 22 |
| Cabbage | Transplant outdoors (spring) | Mar 22 – Apr 22 |
| Cabbage | Start seeds indoors (fall) | Jun 10 – Jul 10 |
| Cabbage | Transplant outdoors (fall) | Jul 10 – Aug 10 |
| Leaf lettuce | Sow outdoors (spring) | Mar 22 – Jun 7 |
| Leaf lettuce | Sow outdoors (fall) | Jul 10 – Aug 26 |
| Spinach | Sow outdoors (spring) | Mar 15 – Apr 28 |
| Spinach | Sow outdoors (fall) | Jul 27 – Aug 31 |
| Radishes | Sow outdoors (spring) | Mar 22 – May 12 |
| Radishes | Sow outdoors (fall) | Jul 10 – Sep 10 |
| Kale | Sow outdoors (spring) | Apr 8 – Apr 28 |
| Kale | Sow outdoors (fall) | Jul 3 – Aug 10 |
| Carrots | Sow outdoors (spring) | Apr 22 – Jun 7 |
| Carrots | Sow outdoors (fall) | Jun 10 – Jul 26 |
| Garlic | Plant outdoors (fall) | Oct 10 – Nov 10 |
Where these dates come from
ZIP code 21792 is matched to the NOAA station Emmitsburg 2 Se, MD, 23.9 km away, at 403 ft elevation (37 ft lower than the ZIP code's center). Nearby stations for comparison:
| Station | Elevation | Avg last frost | Avg first frost |
|---|---|---|---|
| Emmitsburg 2 Se, MD (used) | 403 ft | Apr 22 | Oct 20 |
| Frederick Police Brks, MD | 380 ft | Apr 2 | Nov 6 |
| Damascus 3 Ssw, MD | 700 ft | Apr 14 | Oct 29 |
| Catoctin Mtn Park, MD | 1,610 ft | Apr 14 | Oct 30 |
Frost dates near Unionville
- Libertytown, MD 21762 · last Apr 22, first Oct 20
- New Windsor, MD 21776 · last Apr 24, first Oct 20
- Mount Airy, MD 21771 · last Apr 14, first Oct 29
- New Market, MD 21774 · last Apr 14, first Oct 29
- Woodsboro, MD 21798 · last Apr 22, first Oct 20
- Union Bridge, MD 21791 · last Apr 22, first Oct 20
- Keymar, MD 21757 · last Apr 22, first Oct 20
- Frederick, MD 21701 · last Apr 2, first Nov 6
Frequently asked questions
- When is the last frost in Unionville, MD?
- For ZIP code 21792, the average last spring frost (32°F) is April 22, based on NOAA 1991-2020 climate normals for the Emmitsburg 2 Se, MD weather station. There is a 1-in-10 chance of a frost after May 9.
- When is the first frost in Unionville, MD?
- The average first fall frost (32°F) is October 20. There is a 1-in-10 chance of frost by October 7, so tender plants are at risk from that point on.
- How long is the growing season in 21792?
- In half of years, the frost-free season (between the last and first 32°F frost) is at least 179 days long.
- Are these frost dates a forecast?
- No. They are 30-year averages and probabilities from NOAA weather station records. Use them to plan, then watch your local forecast before planting tender crops or leaving them uncovered.