Seed Starting Date Calculator
Counts back from your last frost date through indoor weeks and hardening off to the date to sow each crop indoors.
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Transplant Date Calculator
Dates the move: crop-specific offset from last frost gives transplant and hardening days plus seedling age at setting.
Transplant Date Calculator
Chlorova · updated for 2026
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Offsets are conventional extension windows measured from the average last frost, and a single year's frost can land two weeks either side, so pair the verdict with your forecast and keep cover for the hardy rows. Seedling age assumes sowDate was actual germination-to-setting material; a rescued windowsill plant or a bought flat with an unknown bench date makes the readiness band only as good as your estimate. Soil temperature is the hidden third variable for peppers and eggplant, which recover from frost far less slowly than they recover from thirty-eight-degree ground.
Transplanting is a date fight run from two directions: the weather will allow a move on one day, and the seedling is ready on yet another day. This calculator reconciles the two, offsetting your last frost date by each crop's hardiness, subtracting a week of hardening off, and measuring how many days old the plant actually will be on the day it moves. The readiness verdict is the part that saves seasons, because overgrown seedlings stall in ways no fertilizer ever fixes.
Dating the Move in Steps
- Choose the crop you are moving outdoors this spring.
- Enter your local last spring frost date, from the almanac or the frost tool.
- Enter the day the seeds actually sprouted or you bought the flat.
- Read the transplant date at the top of the results panel.
- The hardening-off line sits exactly one week before that move.
- Check the seedling age in days against the calendar you entered.
- Read the readiness verdict band before ordering your Saturday.
- If the age reads young, sow later or hold the transplant a week.
- If it reads overgrown, plan on deep or sideways planting at the move.
Three Date Subtractions and One Age
The transplant target is the last frost date shifted by the crop's offset, negative for the hardy brassicas and onions that can go out two weeks early and positive for the tender fruitings that need a grace period after the freeze. Broccoli lands fourteen days before an April 25 frost while eggplant waits two weeks past it.
Hardening off always begins seven days ahead of the move, and the seedling's age is the difference between the setting date and the sowing date, both plain day counts under the hood. The readiness band reads that age against the sizes where transplants actually perform, from too-young to overgrown, with prime roughly at six to nine weeks depending on the crop's pace.
Worked Example: March-Sown Broccoli for Early Spring
- Broccoli seeds sprouted around March 1 on the heat mat.
- Your last spring frost date is April 25.
- Broccoli carries a minus-14-day offset, hardy at setting.
- The transplant day therefore lands on April 11.
- Hardening off begins exactly a week earlier, April 4.
- Age at setting is 101 minus 60, so the seedling is 41 days old.
- Forty-one days sits squarely inside the prime transplant-size band.
- A light frost after that setting date is survivable for broccoli.
- Running the same dates with pepper would land a checked, stunted start.
- A March 15 sowing instead would read 27 days, the young band worth waiting out.
Transplant Offsets by Crop
Days before or after the average last frost that each crop class can be set out, negative meaning early, the same numbers the formulas add. Hardy crops shrug off a sprinkle at establishment; the tender group stalls or dies from the same weather.
| Crop | Offset | Typical prime age | Cold behavior |
|---|---|---|---|
| Broccoli | -14 d | 6-7 wk | Survives light frost |
| Cabbage | -14 d | 6-7 wk | Very hardy |
| Lettuce | -7 d | 4-6 wk | Hardy, cover at freeze |
| Onion | -7 d | 7-8 wk | Early and cool |
| Tomato | +3 d | 7-8 wk | Tender, needs warm nights |
| Pepper | +10 d | 8-9 wk | Hates cold soil |
| Eggplant | +14 d | 8-9 wk | Heat-picky |
Key Terms
- Offset
- Days a crop's safe setting window sits before or after the average last frost.
- Hardening off
- The week of gradual outdoor acclimation before the real move.
- Seedling age
- Days from sowing to transplant, the number the readiness band scores.
- Checked growth
- A permanent stall from cold or transplant shock, worst in peppers and eggplant.
- Circled roots
- Roots spiraling in an over-potted cell pack, needing teasing or sideways planting.
What Nursery Growers Time Their Splits Around
- A seven-week tomato and a nine-week pepper are the same calendar decision viewed from different ends: growers sow to the transplant date, not to packet weeks, and this page inverts the same equation for home gardeners.
- The overgrown band is where most home crops fail quietly, because root-bound cells that sit two weeks past prime recover with three to four weeks of lost yield that never comes back.
- Cold soil hurts more than cold air below fifty degrees, and the pepper and eggplant offsets exist largely for ground temperature, which is why black plastic or a warm south wall can legitimately buy a week.
- Hardy crops set early often beat spring-planted siblings, since broccoli and cabbage established in cool March weather grow into the summer rather than trying to establish through it.
- The computed date is the center of a few days either side, so experienced growers set the calendar date and then let the five-day forecast pick the actual Saturday.
Common Mistakes & Fixes
- Setting tender crops on the frost date because the calendar said so and the week looked warm.
- Nursing an overgrown cell pack for another week of nice leaves instead of transplanting.
- Forgetting the hardening week and dropping full-light plants into full sun.
- Assuming bought flats know their own sow date and trusting a guessed age.
- Planting eggplant in fifty-degree soil because air temperatures looked fine.
- Skipping the deep or sideways planting that rescues every tall, leggy tomato.
When to Use This (and When Not To)
Bring it out the week seedlings hit their second true leaves, when the real question is which Saturday the plants move outdoors. It earns its keep for mixed hardy and tender trays where one flat cannot follow the other's calendar. To build the whole chain from the frost date backward instead, use the seed-starting date calculator, and let the frost-date tool pin your local average before trusting either set of dates.
Transplant Date Calculator: Frequently Asked Questions
When should I transplant tomatoes after last frost?
Around the frost date itself through a few days past, with nights holding above the mid-fifties, which the plus-three-day offset here encodes. An early-set tomato survives but stalls, and the lost week of root warmth costs more than the calendar risk.
Can I transplant broccoli before the last frost?
Yes, two weeks early is standard, because broccoli handles a light frost at establishment and sizes up better from cool spring growth than from a summer start. The same logic covers cabbage, onions and hardy lettuce.
How old should seedlings be when transplanted?
Roughly six to nine weeks for most vegetables, which this page scores as the prime band against your actual dates. Younger plants survive but crawl, and overgrown cells carry root circling that costs weeks of recovery.
What if my seedlings are overgrown for the date?
Transplant anyway and fix the geometry: tease out or slice the circled root mass, set tomatoes deep to the top leaves, and plant brassicas slightly sideways so roots branch. Waiting further almost always costs more than the rescue does.
Do I really need to harden off for a week?
Plan on it, because unhardened seedlings wilt for days under full sun and wind, and a checked pepper may never fully recover. Six to eight days of increasing outdoor exposure is the standard, which is why the hardening date sits exactly a week ahead.
Is soil temperature or air temperature the real limit?
Below the frost question, soil wins for the tender crops, since roots cannot take up water in cold ground regardless of a warm afternoon. That is why peppers and eggplant carry the biggest after-frost offsets on the whole page.
Sources & Further Reading
- Transplant Timing for Home Vegetables — University of Minnesota Extension. Source of crop-by-crop frost offsets.
- Hardening Off and Transplant Shock — Texas A&M AgriLife Extension. Basis for the hardening-week and age-band guidance.
- Soil Temperature and Tender Crop Establishment — Cornell Cooperative Extension. Supports the soil-versus-air temperature caution.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated