Seed starting calendar from your frost date

The idea

Every vegetable gardener runs the same maths each spring: count back six weeks from the last frost for tomatoes, four for brassicas, direct-sow beans after it, and remember to re-sow lettuce every fortnight. Most do it badly, from seed-packet fine print and half-remembered rules, then miss the succession windows entirely and wonder why there is no lettuce in August. GrowVeg sells a USD 35/year garden planner successfully, but it is a bed-layout tool; the recurring pain is timing, not geometry.

The wedge is making the calendar itself the product. Frost dates in, crops ticked, and out comes every dated action for the season, including the succession re-sows and the backwards-computed autumn plantings that even diligent gardeners fumble. The weekly what-to-do-now email does the remembering, which is the part spreadsheets and packet maths never did.

Who pays

The committed home vegetable gardener, who already spends GBP 100+ a season on seeds and compost and treats the hobby as identity. GBP 16 a year is two packets of seeds. The buying moment is January-to-March planning fever, so distribution is gardening YouTube and seed-starting content, r/vegetablegardening, and SEO on “when to start seeds [location]”, a huge seasonal query currently answered by generic zone charts. Seed sellers make natural affiliate partners.

MVP scope

• Frost date lookup by zip/postcode with manual override
• Crop database of ~60 vegetables with offset-based timing rules
• Generated calendar: indoor sowing, transplant, direct sow, succession, autumn crops
• Month grid and this-week views
• Sunday weekly action email, free 5-crop tier, Stripe annual billing
• Skip for v1: bed layout planning, harvest tracking, photos or journals, and weather-forecast-based adjustments

Build SowWhen, a web app that generates a personalised seed-starting calendar from a gardener's frost dates. Stack: Next.js (App Router), Supabase (Postgres + auth + cron), Stripe.

Core flow: user enters a zip/postcode, the app looks up average last spring frost and first autumn frost from a bundled lookup table (compile from NOAA station normals; let the user override both dates). They tick the crops they grow from a database of about 60 vegetables and herbs, and the app generates every dated action: sow indoors (e.g. tomatoes, frost minus 6 weeks), pot up, harden off, transplant out, direct sow, and succession re-sows (lettuce every 2 weeks until frost minus 8, bush beans every 3 weeks until first-frost minus days-to-maturity minus 2 weeks). Autumn crops compute backwards from first frost.

Pages: onboarding (location, crop picker with search), calendar view (month grid plus a this-week list), crop detail (its full timeline and notes), and email settings.

Implementation details: store crop timing as offsets relative to the two frost anchors, plus succession interval and cutoff rules, so one data table drives everything and adding a crop is a row, not code. The retention engine is a Sunday-morning email via Supabase cron: exactly what to sow, transplant, and start this week, with nothing listed meaning a genuinely free week.

Pricing: free for 5 crops with the calendar only; GBP 16/year via Stripe unlocks unlimited crops, succession scheduling, and the weekly email.