When to Start Seeds Indoors in Zone 3
Your month-by-month seed-starting calendar, timed to a last frost around Mid May – early June.
- Min Winter Temp
- -40 to -30 °F / -40 to -34 °C
- Last Spring Frost
- Mid May – early June
- First Fall Frost
- Early September – early October
- Growing Season
- 100–130 days
- Annual Rainfall
- 15–35 in
Seed starting in Zone 3
Starting seeds indoors gives heat-loving crops the head start they need to ripen within Zone 3's season. Because everything is timed to your last spring frost — around Mid May – early June in Zone 3 — the calendar below shows exactly which month to start each crop. Count back from your transplant date, sow under bright light, and harden seedlings off before they go outside.
Zone 3 seed-starting calendar
January
All January tasks →
February
All February tasks →Start tomatoes indoors (10–12 weeks before last frost)
Sow ¼ inch deep in 72-cell trays; keep at 70–75°F until germination in 7–10 days.
Tomatillos
Sow ¼ inch deep at 70°F; transplant after last frost, 3 feet apart.
Start celery and celeriac indoors
Surface-sow under lights; needs 10–12 weeks indoors and consistent moisture.
Celeriac
Surface-sow, press lightly into mix; very slow — start 10–12 weeks early.
March
All March tasks →Start cucumbers, squash, and melons indoors (4 weeks before last frost)
Sow 2 seeds per cell ½ inch deep; thin to one seedling; transplant carefully to avoid root disturbance.
Sow 1 inch deep, 1 seed per large cell; transplant at true-leaf stage — roots are sensitive.
Sow 1 inch deep in individual pots; direct sow is preferred once soil hits 60°F.
Sow ½ inch deep in 4-inch pots; transplant gently 2–3 weeks after last frost.
Start brassica transplants indoors
Sow ¼ inch deep; transplant at 4–6 weeks when 4–5 inches tall; space 18 inches apart.
Sow ¼ inch deep; harden off well before transplanting; space 12–18 inches apart.
Sow ¼ inch deep; needs consistent cool temperatures; space 18 inches apart.
Sow ¼ inch deep directly or in trays; very cold-hardy; direct sow works well in cool soil.
July
All July tasks →Start kale and chard transplants for fall
Sow ¼ inch deep directly or in trays; very cold-hardy; direct sow works well in cool soil.
Sow 1 inch deep, thin to 6 inches; very productive cut-and-come-again crop.
How to start seeds indoors
- 1
Count back from your last frost. Most warm-season crops start 6–8 weeks before; peppers and eggplant 8–10 weeks.
- 2
Sow into a clean seed-starting mix, at the depth on the packet, and keep it consistently warm (a heat mat speeds germination).
- 3
Give seedlings strong light as soon as they sprout — a sunny window is rarely enough; a shop light a few inches above works well.
- 4
Pot up if seedlings outgrow their cells before it’s warm enough to plant out.
- 5
Harden off over 7–10 days — gradually expose them to outdoor sun and wind — before transplanting after your last frost.
Seed starting in Zone 3: common questions
When should I start seeds indoors in Zone 3?＋
In Zone 3, the indoor-sowing months are January, February, March, and July, timed to a last spring frost around Mid May – early June. Start the longest-season crops first and quicker crops closer to your transplant date.
What seeds can I start indoors in Zone 3?＋
Common crops to start indoors in Zone 3 include Onions, Leeks, Tomatoes, Tomatillos, Peppers, Eggplant, Celery, Celeriac. Warm-season crops like tomatoes and peppers benefit most from an indoor head start.
How many weeks before the last frost should I start seeds in Zone 3?＋
It depends on the crop: peppers and eggplant want 8–10 weeks, tomatoes 6–8 weeks, and quicker crops like cucumbers or squash just 3–4 weeks before transplanting. Count back from Zone 3's last frost (around Mid May – early June) to find each start date.
Zone 3 overview →
Frost dates, climate, and everything to grow here.
Monthly calendars →
What to sow, transplant, and harvest each month.
Plant growing guides →
Full care guides for every crop in the calendar.