Planting Date Calculator
Work out when to sow indoors, transplant and expect a harvest from your frost dates.
Last updated · View algorithm, constants and default inputs (JSON)
Results
About this calculator
Everything in a vegetable calendar hangs off two dates: the last frost in spring and the first frost in autumn. Once you have those, each crop can be pinned to a fixed number of weeks before or after, which turns seed packets into actual dates.
Worked example
calculated with the default values below
Last spring frost 2027-05-31 · First autumn frost 2027-10-31 · Crop hardiness Tender (tomato, corn, bean) · Starting method Start indoors, then transplant · Days to maturity 70 days Safe sowing / transplant date 2027-06-07 · Start seeds indoors 2027-04-12 · Move outside 2027-06-07 · Harvest begins 2027-08-16 · Harvest ends 2027-09-06 · Growing days available 70 days · Days to spare before frost 76 days · What this means Comfortable margin of 76 days for a slow start.
General background references
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
D_sow = D_frost + 7w; D_ripe = D_sow + m; spare = (D_autumn − D_ripe) ÷ 86400000
- D_sow
- Safe sowing date
- D_frost
- Last frost date
- w
- Crop offset (weeks)
- m
- Days to maturity
- spare
- Margin (days)
w = −6/−4/−2/+1/+2 by hardiness; indoor start 4/6/8 weeks earlier; harvest end = min(first frost, ripe + 21 d).
Frequently asked questions
Does this account for local weather and crop variety?
No. It uses the frost dates, hardiness category and days-to-maturity value you enter, plus fixed indoor-start offsets. Local microclimates, variety, soil and forecast conditions can shift safe dates; use local extension or seed-packet guidance.
Why does the calculator count backward from the autumn frost for harvest?
The harvest end is the first autumn frost minus the crop's days to maturity, so a 70-day tender crop must be sown early enough to finish before cold kills it. The safe sowing date is pinned to the last spring frost, shifted by hardiness, and the transplant method starts seeds indoors weeks ahead. The days-to-spare output is simply growing days available minus days to maturity; if that goes negative the tool warns the season is too short.