A seed starting calendar is one of the most useful planning tools in the garden. After more than 30 years of year-round gardening in Sonoma Valley, I rely on a seasonal rhythm that tells me exactly when to start seeds indoors, when to sow outdoors, and how to stagger plantings for continuous harvests. With a little structure, you can build a seed starting calendar that works for your climate, your garden space, and your favorite crops.

Below is a step-by-step guide to building a seed starting calendar—plus sample timelines you can customize.

1. Start With Your Average Last and First Frost Dates

Every seed calendar begins here.

Last Frost Date: marks when you can begin planting tender warm-season crops outdoors.
First Frost Date: tells you when to start your fall and winter crops.

My example (Sonoma Valley):

Last frost: early March (often Feb 28–March 10)
First frost: early December

Your dates may differ, but the method is the same.

2. Work Backward From These Dates

Every seed packet lists days to maturity and often weeks to start indoors. Use these numbers to determine your planting windows.

Warm-season crops usually get:

Inside start: 4–8 weeks before last frost
Outdoor sow: After last frost when soil warms
Examples: tomatoes, peppers, eggplant, basil, squash

Cool-season crops are more flexible:

Indoor start: 4–6 weeks before last frost
Outdoor sow: 4–8 weeks before last frost
Examples: broccoli, cabbage, lettuce, kale, peas, radishes

3. Build Your Seed Starting Calendar by Crop

Below is a blog-ready calendar showing when to start seeds indoors vs outdoors. You can adjust the weeks based on your frost dates.

Seed Starting Calendar (Universal Template)

January

Start indoors: onions, leeks, celery
Optional: early cool-season greens under lights
Sow outdoors (mild climates): peas, fava beans

February

Start indoors: broccoli, cabbage, cauliflower, lettuce
Start indoors: early tomatoes and peppers (if using heat mats)
Sow outdoors: spinach, radishes, carrots (in mild climates)

March

Start indoors: tomatoes, peppers, eggplant, basil
Sow outdoors: lettuce, kale, beets, chard
After frost: potatoes, onions, peas (colder climates)

April

Transplant: tomatoes, peppers, brassicas (after frost)
Sow outdoors: beans, squash, cucumbers, corn
Sow herbs: dill, cilantro, parsley

May

Sow outdoors: melons, pumpkins, sunflowers
Succession sow: lettuce, beans, cucumbers
Transplant: basil, peppers, eggplant

June

Sow outdoors: heat-tolerant greens (Malabar spinach, New Zealand spinach)
Sow second rounds: cucumbers, summer squash, beans
Start fall crops indoors: broccoli, cauliflower (in hot-summer climates)

July

Start indoors: fall brassicas (broccoli, cabbage, kale)
Sow outdoors: carrots, beets, chard
Sow outdoors: pumpkins for fall display (early July)

August

Sow outdoors: peas for fall harvest
Start indoors: lettuce, mustard, chicories
Sow: radishes, turnips, cilantro

September

Sow outdoors: spinach, arugula, mache
Sow outdoors: garlic (late month or October)
Start indoors: winter lettuces for cold frames

October

Sow outdoors: garlic, shallots
Sow outdoors: fava beans, cover crops
Sow indoors: herbs like chives for winter windowsills

November–December

Sow indoors: microgreens
Sow outdoors (mild climates): lettuces, peas, radishes
Plan & order seeds for next year

4. Add Succession Planting Dates

To keep harvests coming all year:

Lettuce every 2–3 weeks
Radishes every 2 weeks
Beans & cucumbers every 4–5 weeks
Carrots every month in cool seasons

My year-round Sonoma garden depends on these staggered plantings.

5. Keep Notes Each Year

A simple notebook or spreadsheet helps track:

Dates started
Germination success
Transplant dates
Harvest windows
Best-performing varieties

Over the years, these notes become your most valuable tool.

6. Customize for Your Climate

If you’d like, I can create a personalized seed-starting calendar based on:

Your last/first frost dates
Your local climate
What crops you grow
Container or raised-bed gardening

Just tell me your ZIP code or frost dates.

Seed Starting Calendar by Crop (Indoor & Outdoor Timing)

Use this chart to decide when to start each crop indoors, when to sow outdoors, and when to transplant.

Warm-Season Crops (Frost Tender)

Tomatoes

Start Indoors: 6–8 weeks before last frost
Transplant: 1–2 weeks after last frost
Direct Sow: not recommended

Peppers

Start Indoors: 8–10 weeks before last frost
Transplant: 2+ weeks after last frost
Direct Sow: not recommended

Eggplant

Start Indoors: 8–10 weeks before last frost
Transplant: 2+ weeks after last frost

Basil

Start Indoors: 4–6 weeks before last frost
Direct Sow Outdoors: after soil warms

Summer Squash (Zucchini)

Start Indoors: 2–3 weeks before last frost
Direct Sow Outdoors: after last frost

Winter Squash

Start Indoors: 3–4 weeks before last frost
Direct Sow Outdoors: after last frost

Cucumbers

Start Indoors: 3–4 weeks before last frost
Direct Sow Outdoors: after last frost

Melons (Cantaloupe, Galia, Honeydew)

Start Indoors: 4 weeks before last frost
Direct Sow Outdoors: after weather warms reliably

Watermelon

Start Indoors: 4 weeks before last frost
Direct Sow: after soil reaches 70°F

Corn

Direct Sow Outdoors: 1–2 weeks after last frost
Indoor Start: not recommended

Beans (Bush or Pole)

Direct Sow Outdoors: after last frost
Indoor Start: not recommended

Cool-Season Crops (Frost Tolerant)

Broccoli

Start Indoors: 4–6 weeks before last frost
Direct Sow Outdoors: 2–3 weeks before last frost
Fall Crop: start indoors mid-summer

Cauliflower

Start Indoors: 4–6 weeks before last frost
Direct Sow Outdoors: 1–2 weeks before last frost
Fall Crop: start indoors mid-summer

Cabbage

Start Indoors: 6 weeks before last frost
Direct Sow Outdoors: 2–4 weeks before last frost
Fall Crop: start indoors mid-summer

Kale

Start Indoors: 4–6 weeks before last frost
Direct Sow Outdoors: 4–6 weeks before last frost
Fall Crop: sow late summer

Lettuce

Start Indoors: 4 weeks before last frost
Direct Sow Outdoors: 4–6 weeks before last frost; repeat every 2–3 weeks

Spinach

Direct Sow Outdoors: 6–8 weeks before last frost
Start Indoors: rarely necessary

Peas

Direct Sow Outdoors: 6–8 weeks before last frost
Fall Crop: sow late summer

Radishes

Direct Sow Outdoors: 4–6 weeks before last frost; repeat every 2 weeks

Carrots

Direct Sow Outdoors: 4–6 weeks before last frost
Fall Crop: sow late summer

Beets

Direct Sow Outdoors: 4 weeks before last frost
Fall Crop: sow late summer

Swiss Chard

Start Indoors: 3–4 weeks before last frost
Direct Sow Outdoors: 2–4 weeks before last frost

Long-Season Root & Bulb Crops

Onions (from seed)

Start Indoors: 10–12 weeks before last frost
Transplant: as soon as soil can be worked

Leeks

Start Indoors: 8–10 weeks before last frost
Transplant: early spring

Garlic

Direct Sow Outdoors: fall (September–November depending on climate)

Shallots

Direct Sow or Plant Sets Outdoors: fall or very early spring

Miscellaneous & Specialty Crops

Celery

Start Indoors: 10–12 weeks before last frost
Transplant: after frost danger passes

Parsley

Start Indoors: 8 weeks before last frost
Direct Sow Outdoors: after soil warms

Cilantro

Direct Sow Outdoors: spring and fall
Indoor Start: not recommended

Dill

Direct Sow Outdoors: when soil warms
Indoor Start: 2–3 weeks before planting

Seed Starting & Propagation Learning Hub

Start here — Seed Starting Basics: A Complete Beginner-to-Advanced Guide for Indoor and Outdoor Seed Starting

Seed Starting Fundamentals

Seed Starting Guide
Starting Plants from Seed
Vegetable Seed Starting Indoors and Out
How to Start Seeds Indoors Step-by-Step
Starting Vegetables Indoors: 15 Varieties for Success
Vegetable Seed Germination
How to Read a Seed Packet

Planning & Timing

Seed Starting Tools, Supplies & Setup

Seedling Care, Troubleshooting & Success Tips