July 21, 2026
Your last-frost date is probably wrong for your yard
You look up your last-frost date, count back on the calendar, and set out your tomatoes. The problem: that date is an area-wide average, and your yard has a mind of its own. The real date can be a week or two off — enough to lose a tray of seedlings, or to waste weeks waiting for a frost that already passed.
Where the standard date comes from — and why it isn't yours
Published last-frost dates are calculated from a weather station that may sit miles away, often at an airport: open, breezy, and not much like a sheltered backyard. They're also probabilities, not deadlines — roughly the date after which the chance of frost drops below a threshold, averaged across a broad region. It's a fine starting point, but it was never measured in your yard.
Why your yard runs colder (or warmer) than the forecast
Two gardens on the same street can have last frosts a week apart. A few things drive it:
- Cold-air drainage. Cold air is denser than warm air, so on calm nights it flows downhill and pools in low spots — the classic "frost pocket." A dip of a few feet can sit several degrees colder than the slope just above it.
- Clear, calm nights. With no clouds to hold heat in and no wind to mix the air, the ground radiates warmth straight to the sky. Open, low, still spots frost first — and those are exactly the nights a regional forecast is hardest to trust locally.
- What's around you. Elevation, which way your slope faces, tree canopy, walls, and pavement all nudge your overnight low up or down from the regional number.
How to find your real offset — free, no signup
Instead of guessing, you can measure it. Siltwell's free microclimate checker compares the forecast low each night to what nearby home weather stations actually recorded over recent nights, and tells you — in one sentence — how much colder or warmer gardens near you have been running than the forecast. It shows how many stations it used and a confidence level, so you know how solid the read is. Enter your town; there's no account and nothing to install.
Be clear about what it is: an honest indicator of how your area has been running lately, not a guaranteed frost date for your exact spot — the hyperlocal factors above still apply. But it turns "the internet says May 15" into "gardens near me have been running a few degrees colder than forecast, so I'll treat that date as optimistic."
What to do with it
If nearby gardens run colder than the forecast on clear nights, treat your published last-frost date as early — hold tender transplants like tomatoes, peppers, and basil a week or two longer, or be ready to cover them. If your area runs warmer, you may have a genuine jump on the season.
A free Siltwell account lets you enter that offset in Settings, so it's reflected in your frost dates and planting plan. And if you own an Ecowitt, Ambient, Tempest, or Davis station, Pro can learn it automatically from your own readings instead — here's how that works.
The same idea applies at the other end of the season: our when-to-plant calculator shows the last date to sow each crop before your area's first fall frost, so nothing gets caught out when the season turns.
One honest caveat: no forecast or offset is a guarantee. On a marginal, clear, calm night, the safest move is still to throw a cover over anything tender. This narrows the guesswork — it doesn't repeal the weather.
Common questions
- Why is my garden colder than the weather forecast?
- Forecasts come from a coarse grid or a weather station that can be miles away. Your yard's elevation, how cold air drains and pools in low spots, and how much heat it radiates to a clear sky on calm nights can leave it several degrees colder than the forecast — especially on the exact nights frost matters.
- How do I find the last frost date for my specific yard?
- Start with your area's published average, then adjust for your microclimate. Siltwell's free microclimate checker shows how much colder or warmer gardens near you have actually been running versus the forecast low, so you can shift your date accordingly — no account needed.
- Is a microclimate offset a guaranteed frost date?
- No. It's an indicator of how your area has been running on recent nights, with a confidence level — not a promise. Your exact spot still varies, so on a marginal, clear, calm night, cover tender plants to be safe.
Related guides
Freezing Garden Green Beans: How to Time the Last Frost-Beating Pick at Your Yard
Blanching and bagging is the easy part. The real question is when to strip the plants — before the first freeze hits your specific garden, not the regional date.
Can Tomato Plants Survive Frost? The Kill Temperature vs. Whether Your Yard Even Reached It
Tomatoes take damage near 0°C and die in a hard freeze — but the real question is whether your yard actually hit that low, not whether the region did.
Frost Damage on Plants: How to Tell If Your Yard Actually Got Cold Enough — and What Recovers
Frost damage shows up a day late, and half the time the yard never froze. How to tell whether cold or something else wilted your plants — and what recovers.
Free planting tools
- When to plant — the last date to sow each crop before your fall frost
- Microclimate checker — how much colder your yard runs than the forecast
- GDD estimator — whether a crop will ripen where you are
