How this data is checked
Every agronomic number on this site is either checked against a named university extension publication or marked as an estimate. Nothing sits in between, and no figure here was invented to fill a gap.
Where the numbers come from
Crop spacing and planting dates come from US university extension services — the publicly funded outreach arms of land-grant universities, which publish home-garden guides for their own states. Compost carbon-to-nitrogen ratios come from the Cornell Waste Management Institute and the University of Nebraska-Lincoln. Lime rates come from the University of Kentucky, with limits from Penn State and Colorado State. Turf seeding rates come from Penn State, UMass, NC State, Cornell, Nebraska, Florida, Arkansas, Colorado State and Kansas State.
The crop guides cite these institutions:
- University of Illinois Extension — 18 crops
- University of Minnesota Extension — 10 crops
- Cornell Garden-Based Learning — 1 crop
- University of Wisconsin-Madison Extension — 1 crop
Magazine tables, gardening blogs and aggregator sites are not used, even where they agree with the extension figures. If two extension services disagree, both are shown rather than averaged: averaging a Kansas figure with a Colorado one produces a number that describes neither place.
Square foot gardening figures are a method, not research
The per-square plant counts you see quoted everywhere — 16 radishes, 9 spinach, 1 tomato per two squares — come from Mel Bartholomew’s Square Foot Gardening, a system for laying out intensively amended raised beds. Cornell CALS describes the method; it does not publish those counts as spacing research. They work well on their own terms and badly as general advice, so this site keeps them separate from the density a crop’s published row spacing implies, labels which is which, and never blends the two into one number.
Where the Cornell page does not name a crop — most herbs, sweet corn, the large vines — there is no square foot gardening figure here, and the planner falls back to spacing.
What “estimate” means on a page
A figure marked estimate is a typical published value that no extension source could be found for. It is kept because the calculator needs a number and removing it would leave a hole, but it has not been checked and should carry less weight than the figures around it. Most overseeding rates for turf are in this category, as are alfalfa meal and garden weeds in the compost tool.
Verification is recorded field by field rather than per crop, so a crop can have checked spacing and an unchecked row spacing. Of the 30 crop guides, 19 have every checked field confirmed and 11 have at least one that could not be. Yields, days to maturity, companion lists and common problems were never systematically checked — companion planting in particular is garden tradition rather than science, and the crop pages say so.
Dates assume a Northern Hemisphere spring
Planting offsets are counted in weeks from your average last spring frost, which transfers reasonably from the US to the UK and Canada. In the Southern Hemisphere the seasons are flipped: enter your own spring frost date and read autumn wherever a page says fall, or every date will be six months out.
Checking a figure yourself
Search for the crop name plus “extension” and your own state or province. In the United States and Canada every state and provincial service publishes home-garden guides free, and they will be tuned to your soils and season in a way this site cannot be. In the UK the RHS plays a similar role. Where their figure differs from the one here, theirs is the better number for your garden: this site had to pick one, and it picked the one with a citation rather than the one that sounded most typical.
Found an error?
Corrections are welcome, particularly with a source. See the contact page. The disclaimer covers why even a checked figure is a starting point rather than an instruction.