Why fall planting dates disagree by a month or more

Published cooperative-extension fall planting dates for the same crops, converted to days before each region's own first frost, differ by 13 to 104 days depending on which region's extension service wrote them. That is the finding below, and it means a single national fall planting rule cannot be right: 12 of the 17 crops compared here diverge by more than a month even after the calendar difference between the regions has been taken out.

The dataset is 69 printed fall planting windows, transcribed from 6 extension publications covering 6 regions across the Midwest, the Gulf states and the Mountain West. Each printed date is measured against the region it was written for, using the 1991 to 2020 NOAA climate normals for that region's own median first freeze.⁶ What survives that conversion is a like-for-like number: how long before its own first frost each service tells a gardener to put that crop in.

Like for like is meant strictly. Some of these publications give a date for sowing seed in place and some give a date for setting out a started plant, which are different events separated by several weeks in a nursery. Every row below says which kind it is, and a crop's spread is measured only across dates of the same kind, so none of the figures on this page is a nursery period wearing a regional disagreement's clothes.

Where the disagreement is sharpest

The clearest case is not two distant states. Missouri Extension puts a fall turnip sowing 90 days before north Missouri's first frost.¹ Illinois Extension's Cook County calendar puts the same crop 110 days before Chicago's.² Those two regions sit inside the same region band this site uses, roughly the middle of the country, and they are 20 days apart on one of the simplest crops in the garden. Both are correct for where they were written.

The gaps widen from there. The widest one in the table sits on Collards: one publication puts it 149 days before frost for Cook County, Illinois, and another puts it 45 days before frost for Texas Region III, a difference of 104 days. Both are dates for the same kind of planting, so the gap is not a nursery period in disguise. The earlier of the two is worth reading carefully: University of Illinois Extension Cook County prints it on a row shared with Brussels sprouts, Cabbage and Kohlrabi, so it is that crop's published date without being specific to it.

Why one national rule cannot work

The usual shortcut in garden writing is to count backwards from first frost by days to maturity and call the result a fall planting date. That arithmetic gives one number for the whole country, and the table above is what it misses. What the table establishes is the disagreement itself. Why the services disagree is a separate question, and an open one.

There are plausible explanations and no way to choose between them from this data alone. A fall crop is bracketed at both ends, by the late-summer heat a young plant has to survive and by the frost that finishes it, and counting back from frost only models the second one. Services may also assume different days to maturity, or treat a light first freeze as the end of the season in one region and not in another. Each of those would move a published date by weeks, which is the order of much of this table, and none of them on its own accounts for the widest gaps in it. The honest summary is that what diverges, and by how much, is measurable, and that the cause is not settled.

So Row & Bed does not compute fall windows from one formula. Fall planting dates are authored per region band, from the extension calendar published for that region, and a region with no qualifying published source falls back to the arithmetic rather than borrowing another region's dates. The spread in the table is the reason that costs more work than a single formula and is worth it anyway. Species-level growing detail sits on the plant pages, and the frost dates the whole calculation hangs on are in typical first and last frost dates by state.

Every published date behind those spreads

Below is the full transcription, one row per printed window, grouped by crop. It includes published dates that produce no spread, because the crop has only one date of that kind in the set. The as-printed column is what the publication says, with a note where the publication prints that row for several crops at once. The right-hand column is the date expressed as days before the first frost of the region it was written for, which is the only form in which two regions can be compared at all.

How printed dates became day counts

Each publication was read for its fall column only, and the opening date of the printed window was recorded exactly as it appears. That opening date was then converted against the median first freeze for the region the publication writes for, taken from the 1991 to 2020 NOAA climate normals at a quality-controlled station in that region.⁶The published closing date is not used here, because closing a fall window involves an assumption about how long the crop takes to finish, and that assumption would be ours rather than the source's.

The publications do not all print the same shape, and the conversion respects the shape it was given. Texas A&M prints one date per crop per region rather than a range.⁴ UF/IFAS prints month names, so month boundaries are read literally rather than sharpened into a day.³ Utah State prints seeding icons on the first and fifteenth of a month, so those figures carry about a week of resolution either way.⁵ The as-printed text is reproduced with typography normalized, which means dashes are rendered as plain hyphens; the dates and wording are otherwise the publication's.

13 of the 69 rows come from a printed row that covers more than one crop, and each of those says so in its as-printed cell. The most shared of them is the fall row University of Illinois Extension Cook County prints for Brussels sprouts together with Cabbage, Collard and Kohlrabi. Every crop named on such a row takes the shared date, which is genuinely that crop's published date but is not specific to it. That matters when a grouped row sets one end of a spread, so the table flags them rather than leaving a reader to notice.

A figure below zero means the printed date falls after that region's median first frost rather than before it. That is not an error in the source. In the mildest regions covered here the median first freeze is a light one that a hardy crop grows straight through, so a service can reasonably print a planting date past it.

Read the spread, not any single row. No number in this table is the planting date for your garden. Each is what one service printed for one region, and the useful finding is the distance between them, because that distance is what any national rule of thumb is quietly averaging away. All 6 publications behind the table are named in it, and five of them are listed with links below. The sixth, the LSU AgCenter guide, falls outside the source-domain policy this site links under, so it is credited by name only. Its address, and the retrieval date for every one of them, are kept with the dataset in the site's source repository.