Free tool

Seed Starting Calculator

Enter the last frost date where you garden. Get the week to stratify, sow and transplant each of 64 flowers, herbs and vegetables — and a calendar file that remembers it for you.

By Sohail Akhtar · Updated 14 September 2026

Your last spring frost date

The average date of the last frost where you garden. If you do not know it, your local extension service or a neighbour who grows vegetables will. Everything below counts backwards from this one date.

Showing a mid-April default. Change it to your own date.

What are you growing?

Your first job is Jan 7 — start cold stratification for lavender. Everything else on this list can still be caught up on; this one cannot.

Seed starting schedule for 10 species, calculated from a last frost date of Apr 15
Species	What to do	When
CalendulaDirect sowing after frost works just as well and is less work.	Mar 11Sow indoors (or wait and direct sow)5 weeks before your last frost	Mar 11
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
California PoppyTaproot — never transplant. Sow as soon as soil is workable; it prefers cool soil.	Mar 18Direct sow outdoorsAbout 4 weeks before your last frost — it wants cool soil	Mar 18
ChicoryDeep taproot — direct sow only. Sow once soil has warmed.	Apr 15Direct sow outdoorsOnce frost has passed and soil has warmed	Apr 15
EchinaceaStratify first. Flowers in year two.	Jan 28Start cold stratification4 weeks in the refrigerator, barely damp	Jan 28
	Feb 25Sow indoors7 weeks before your last frost	Feb 25
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
Evening PrimroseBiennial. Direct sow; flowers in year two, not year one.	Apr 15Direct sow outdoorsOnce frost has passed and soil has warmed	Apr 15
FeverfewSurface sow. Sown this early it usually flowers in year one.	Feb 25Sow indoors7 weeks before your last frost	Feb 25
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
German ChamomileSurface sow. Needs light, and the surface must not dry out.	Mar 11Sow indoors (or wait and direct sow)5 weeks before your last frost	Mar 11
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
LavenderStratify first, surface sow, and expect slow uneven germination.	Jan 7Start cold stratification5 weeks in the refrigerator, barely damp	Jan 7
	Feb 11Sow indoors9 weeks before your last frost	Feb 11
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
MarshmallowStratify first, then sow. Wants consistently damp ground.	Jan 28Start cold stratification4 weeks in the refrigerator, barely damp	Jan 28
	Feb 25Sow indoors7 weeks before your last frost	Feb 25
	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22
YarrowSurface sow. Expect foliage only in year one.	Feb 25Sow indoors7 weeks before your last frost	Feb 25
YarrowSurface sow. Expect foliage only in year one.	Apr 22Harden off and transplantAbout a week after your last frost date	Apr 22

Put these dates in your calendar

The first job is months away and easy to forget. This downloads all the dates above as calendar entries, each one carrying the instructions and a link to that species’ guide. Works with Google Calendar, Apple Calendar and Outlook.

No email required. Nothing is sent anywhere — the file is built in your browser.

These are general horticultural ranges for each species, not results from our own garden. Local conditions vary, and an average frost date is an average — a late cold snap beats any calendar.

How the dates are worked out

One date drives everything: the average last spring frost where you garden. Every job the calculator gives you is a fixed number of weeks before or after it.

A tomato that wants six weeks indoors is sown six weeks before that date. An echinacea that wants four weeks of cold stratification and then seven weeks indoors goes into the refrigerator eleven weeks before it — which, for an April frost date, means late January. That eleven-week lead is the reason seed catalogues arrive in December, and the reason the single most common seed-starting mistake is discovering in March that a species needed a month of cold you did not give it.

The species that need that cold period are flagged automatically here. If you want the reasoning rather than the dates, our guide to cold stratifying seeds covers the refrigerator method step by step, and the list of which seeds need cold stratification gives a yes, no or optional for sixty-eight species.

What the calculator covers

64 species, grouped the way people actually shop for seed. The categories above the table switch between them; the full list is here so you can see whether yours is included before you bother entering a date.

Medicinal herbs (10)

Calendula, California Poppy, Chicory, Echinacea, Evening Primrose, Feverfew, German Chamomile, Lavender, Marshmallow, Yarrow

Perennial flowers (21)

Anise hyssop, Aster, Bee balm / wild bergamot, Black-eyed Susan / rudbeckia, Blanket flower, Butterfly weed, Canterbury bells, Columbine, Coreopsis, Delphinium, Forget-me-not, Foxglove, Hardy hibiscus, Hollyhock, Hosta, Hyssop, Lupine, Milkweed, Scabiosa, Shasta daisy, Swamp milkweed

Annual flowers (20)

Bachelor's buttons, Celosia, Cleome, Cosmos, Dahlia, Gomphrena, Larkspur, Marigold, Morning glory, Nasturtium, Pansy / viola, Petunia, Poppy, Snapdragon, Strawflower, Sunflower, Sweet alyssum, Sweet pea, Verbena, Zinnia

Culinary herbs (12)

Basil, Borage, Chives, Cilantro / coriander, Dill, Lemon balm, Oregano, Parsley, Rosemary, Sage, Strawberry, Thyme

Vegetables & fruit (1)

Tomato

Indoors or straight into the ground?

This is decided by the root, not by the weather. Anything that makes a taproot — California poppy, chicory, larkspur, poppies, cilantro, dill, lupine — resents being moved and does better sown where it is to grow, however tempting a head start looks. Slow, expensive or low-percentage seed like lavender, petunia and rosemary earns its tray space. Plenty of things work either way, and the calculator shows both dates for those.

The one rule worth keeping: a transplant date is not a sowing date. Hardening off takes the better part of a week, and seedlings moved straight from a windowsill into open ground in a cold week are the other half of the losses people blame on seed quality.

Frequently asked questions

How does a seed starting calculator work?

Everything counts backwards from one date: the average last spring frost where you garden. A tomato wanting six weeks indoors is sown six weeks before that date; an echinacea wanting four weeks of cold stratification before a seven-week indoor start goes into the refrigerator eleven weeks before it. The calculator does that arithmetic for every species at once so you get a dated list rather than a set of instructions to work out yourself.

What if I do not know my last frost date?

Your local extension service publishes it, and so does any neighbour who grows vegetables. In the US it typically falls between mid-March in the south and early June in the far north. Use an approximate date rather than none — a schedule that is a week out is still far better than sowing everything in April because that is when the weather turned nice.

Should I start seeds indoors or sow them directly?

It depends on the root, not on the weather. Anything with a taproot — California poppy, chicory, larkspur, poppies, cilantro, dill — resents transplanting and does better sown where it is to grow. Slow plants that need a head start, like lavender, petunia and tomato, earn their tray space. The calculator shows whichever method suits each species, and both dates where either will work.

Which seeds need to go in the refrigerator first?

Roughly a third of the perennials here. Milkweed, echinacea, lavender, bee balm, columbine and anise hyssop all want a cold, moist period before sowing, and the calculator schedules it automatically as a separate dated job. Our full list of which seeds need cold stratification covers sixty-eight species with the length each one needs.

Does the calendar download cost anything or need an email?

No to both. The .ics file is built in your browser and downloads straight to your device — nothing is uploaded, and no address is asked for. It works with Google Calendar, Apple Calendar and Outlook, and each entry carries the instructions for that job rather than just a line of text.

Are these dates guaranteed?

No, and be suspicious of any tool that says otherwise. These are general horticultural ranges for each species, and an average frost date is an average — a late cold snap beats any calendar. Treat the output as a schedule to plan around and then watch the actual weather in the fortnight before you plant out.

Found this useful on your own site or in a gardening group? A link back is genuinely appreciated and is the only thing keeping tools like this findable. If something here is wrong for your region, tell us at the contact page — regional corrections are the most useful mail we get.