Tools · Interactive

Roselle planting-date calculator

Roselle's calendar is set by two clocks: frost on one hand, day length on the other. Enter where you are and when you plan to plant — the model works out your safe sowing window, the short-day flowering trigger, and the calyx harvest calendar for your exact spot.

Works in any latitude · no data leaves your browser · guidance, not guarantees — confirm with local extension

The two clocks Frost: roselle is frost-tender and hates cold soil — planting starts only after the last spring frost, once the soil has warmed past roughly 60 °F / 15 °C (figure ~2 weeks after last frost). Day length: roselle is a short-day plant — bloom is induced when days shrink below a threshold in the ~12–12.5 h range (Morton 1987; Rutgers NJAES). In the tropics the gate never fully closes and flowering is age-driven (~4–5 months); away from the tropics the trigger date decides everything — and autumn frost can beat the harvest.

1 · Where are you growing?

Location & frost window
Negative = south of the equator. The model is calibrated for −60° to +60°.
Month and day only — year is ignored.
Southern hemisphere: autumn frosts (e.g. June) count as "first frost".

Quick presets (typical dates — confirm locally)

2 · When do you plan to plant?

The model checks your date against the safe window it computes.
Direct seeding or transplanting both count as "planting".

3 · Model settings (optional — sensible defaults)

Photoperiod & phenology assumptions
Roselle is induced to flower as days fall below this. Sources cite ~12–12.5 h; some cultivars respond a little earlier or later.
Days of short days until flower buds appear after the trigger date.
Used where the day-length gate never closes (equatorial tropics): flowering after ~4–5 months of growth.
Time from first flowers to calyces worth picking.
Warm-season days a plant needs before the trigger to build a worthwhile canopy. Sets the latest sensible planting date.

These are approximations from the agronomic literature — tweak them only if you have local experience or trial data.

Your calendar

Day length through the year at your latitude. Shaded bands: safe planting window and expected calyx harvest. Dashed outlines mark events that fall in the next calendar year (southern hemisphere).
What the model assumes

Builds and prints a clean one-page plan with your dates, window, verdict and recommendations.

How the model works

The day-length curve is computed from your latitude with the standard solar-declination approximation — day length on a given date is 24/π · arccos(−tan φ · tan δ), where φ is your latitude and δ is the sun's declination for that date. The short-day trigger is the first date (after each hemisphere's longest day) when day length falls to the flowering threshold. Below roughly 8–9° of latitude the longest day of the year never reaches the threshold, so the gate never closes — the model switches to age-driven flowering automatically, using the adjustable ~135-day default.

The safe sowing start is ~2 weeks after the last spring frost (warm-soil rule); the latest sensible planting leaves the adjustable minimum grow time before the trigger; and the harvest window runs from ~30 days after first flowers until either the first fall frost or ~90 days of picking, whichever comes first.

This is a transparent approximation of a genuinely variable plant: cultivars, microclimate, plant population and water all shift real dates by days to weeks, and in the subtropics the flowering response itself is the least certain number in the model. Use it to frame your calendar, then confirm with local extension guidance — the regional playbooks are a good starting point for your area.

Reality check The calculator turns agronomic ranges into specific dates, which feels precise — treat the dates as working estimates. Nothing here replaces a soil thermometer, a weather history for your exact site, and a test row in year one. Frost dates in particular vary hugely over even short distances (valleys frost earlier, urban and coastal spots later).
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