How the Almanac works

How to read the numbers in this Almanac

Frost dates, climate trends, and grower results each answer a different question. Here is what each one can tell you and where to be cautious.

Frost dates

What it uses: Thirty years of gridded daily low temperatures for your area, plus the frosts you report
How much history: 30 complete years for the nearby weather grid, with elevation considered
How to use it: Usual date and the 1-in-10 late date shown side by side

Your account does not use the zone-wide rule of thumb. Once we have your ZIP or location, we use 30 years of gridded daily lows for the surrounding area, estimate the last spring freeze and first fall freeze from every year, and keep the whole spread rather than one average. This is an area-level estimate, not a temperature reading in your garden. Report a frost you actually saw and your calendar shifts toward your ground. You can still set your own dates, and nothing recalculates over them.

Zone drift

What it uses: Long-term weather history near your garden, using the most local reading available
How much history: Last ten years compared against a thirty-year normal
How to use it: Your own area once your location is set; a broad zone view otherwise. Always a trend, never a new zone assignment

Once your location is set, the comparison uses 30 years of weather history near your garden, so coastal and inland growers in the same zone can see different trends. Without a location, we show a broad example for the zone and label it clearly. We compare the recent decade with the longer record for last frost, first frost, season length, and hot days. A shift of a few days is a direction of travel, not a promise about next April. Your official zone stays the planning baseline.

Grower results

What it uses: Plantings and harvests recorded by Harvestead members in that zone
How much history: Published only once enough growers have logged the same crop
How to use it: Median shown; the grower count is printed next to every row

Sowing weeks, days to first harvest, and yield per planting come from real member records. The data refreshes automatically every night, but a result only appears once enough growers have contributed. That is why some zones and crops stay blank rather than showing a weak number. We show the middle result so one exceptional or disastrous season cannot distort the picture. Individual records are never shown, and no result can be traced back to a member.

How this beats a zone table

A zone table answers one question: roughly how cold does it get here in winter. That is useful for choosing a perennial and close to useless for deciding when to sow.

It shows what worked, not what should work. Seed-packet days to maturity are grown out under trial conditions. The Almanac reports what growers in your zone actually recorded, including the weeks nothing came up.
It admits its gaps. A crop with too few growers is left blank instead of filled with a plausible guess. Blank is information.
It moves. The drift comparison tells you whether your area has been running ahead of or behind the book values over the last decade, which a static table structurally cannot do.
It gets more specific the more you log. Your own recorded dates override the zone seeds, so the calendar converges on your ground rather than a band on a map.

What we will not claim: Harvestead does not model your microclimate, and it does not read a station in your town. Any page that says otherwise is wrong and should be reported.

What this cannot tell you, said plainly

We would rather lose a sale than have you plan around a number we quietly rounded. These are the real limits, and what we do about each one.

No frost date is a guarantee. The usual last-frost date is a coin-flip year — frost lands later than it about one spring in two. That is why every date we show carries the "1 year in 10" safer date beside it. Plant tender crops by the safe date, not the usual one.
We measure your area, not your yard. The finest weather history covers roughly 5-6 miles, with a broader 15-17 mile reading used where needed. We account for your elevation, but an area that wide still blends ridges, valleys, water, and city blocks. Nothing on the market sees your yard; your own frost reports are how this calendar learns it.
Cold air runs downhill, and broad weather history cannot catch every pocket. Frost forms first where cold air pools — the bottom of a slope, a hollow, a hedge-bound low corner, the floor of a valley under a clear, calm sky. Two gardens a quarter-mile apart at the same elevation band can differ by one to three weeks if one sits in a drainage basin and the other on open slope. This is the single largest error source for frost specifically, and elevation correction does not fix it: elevation says how high you are, not whether cold air drains onto you. If your low spot frosts before the rest of the yard, trust your own eyes and log it — those reports are exactly what shifts your calendar.
A ZIP covers more than your address. Rural ZIP codes can span thirty miles. If your garden differs from the middle of your ZIP, share your location once for a reading closer to the ground you actually grow in.
"Frost-free" does not mean tender crops are safe. Basil, tomatoes, and peppers suffer on clear, calm nights a few degrees above freezing — technically no frost at all. Cold nights in the mid-30s still warrant row cover for tender crops.
First seasons are the least personal. Before you have logged a frost, your calendar is based on the area estimate. Every report you file shifts it toward your ground — that calibration is the part no static table or weather app offers at all.
One odd report will not swing your calendar. Calibration is smoothed on purpose: a single reported frost nudges your dates a little, three consistent seasons move them most of the way, and recent years count for more than old ones. Reports more than about six weeks off the measured record are capped as likely mis-logged rather than taken at face value. A fluke year or a typo cannot run away with your planting dates.
This is history, not a forecast. The Almanac answers "how has my season been shifting" from 30 years of record. It does not tell you whether frost is coming this Thursday — that is the separate bed-risk alert, which reads a live forecast. Do not plan a cover night off an Almanac date.
Drift is a direction, not a prediction. A zone running nine days earlier over a decade tells you which way the climate is moving. It does not promise next April.

Even with every one of those limits stated, this is more useful than a single zone-table date: a 30-year area-level weather record, elevation considered, recalibrated by your own reports, refreshed automatically, and overridden by you whenever you know better. It is not an exact measurement of your garden, and no zone table, seed packet, or weather app can see every garden-scale condition. We mitigate everything that can be mitigated — and we would rather tell you the rest than pretend it away.

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