Seed Starting Indoors: Zone-by-Zone Timing Guide
Winnie Astrid
Garden Design Editor
Starting seeds indoors 6–8 weeks before your last frost date is the most cost-effective way to extend the growing season. A single packet of tomato seeds costs less than two transplants at the garden centre and gives you twenty plants to choose from. But the advantage only materialises if you get the timing right. Start too early and you have leggy, root-bound seedlings that never recover. Start too late and you lose the entire head start you were trying to buy.
This guide covers everything from equipment to the zone-by-zone timing table, which crops belong indoors and which should never leave the ground they are sown into, and the hardening-off process that determines whether six weeks of careful indoor work survives its first week outside.
What You Need Before You Sow
Indoor seed starting does not require a purpose-built setup, but four pieces of equipment make the difference between robust transplants and a tray of stretched, floppy seedlings. Get these right and almost everything else is forgiving.
Seed Trays, Modules, or Jiffy Pellets
Cell trays (72-cell or 128-cell) are the most efficient for large sowings — easy to water uniformly, quick to pot up, and reusable for years. Jiffy pellets (compressed peat or coco coir discs that expand with water) eliminate potting up but cost more per plant. Root trainers are the best choice for crops with long taproots (sweet corn, beans) that are sometimes started indoors for transplanting. Avoid shallow half-trays for anything that will spend more than four weeks indoors — roots reach the bottom and stall growth.
Heat Mat
Critical for tomatoes, peppers, aubergine, and basil. These crops evolved in hot climates and germinate poorly below 70°F (21°C) soil temperature. A propagation heat mat raises the growing medium to 75–85°F and typically cuts germination time from 14–21 days down to 5–10 days. Remove seedlings from the mat once the majority have germinated — sustained heat after emergence encourages legginess. Cool-season crops (brassicas, lettuce, spinach) germinate well at ambient room temperature and do not need a mat.
Grow Lights vs. South-Facing Window
Seedlings need 14–16 hours of bright light per day to grow stocky rather than reaching toward the nearest photon source. A south-facing window in Zones 3–6 in January or February provides 3–4 hours of weak, low-angle light — enough to keep seedlings alive, not enough to prevent leggy growth. Full-spectrum LED grow lights (6500K colour temperature) positioned 2–4 inches above the seedling tops on a 16-hour timer reliably produce compact transplants anywhere in the country. In Zones 7–10 where winter days are longer and sun angle is higher, a south window can work for fast-germinating crops started in February or March, but grow lights remain the more reliable option.
Seed Compost vs. Multipurpose
Seed compost is finely textured, low in nutrients, and free-draining. Those three properties matter: fine texture allows small seeds to make good contact with the medium, low nutrients prevent fertiliser burn on roots that cannot yet handle rich soil, and drainage prevents the waterlogged conditions that cause damping off. Multipurpose compost is coarser, nutrient-rich, and retains more moisture. It is fine for potting up seedlings once they have their first true leaves, but poor for germination. Never use garden soil in trays — it compacts solid and carries pathogens.
One more thing: a small desk fan running 2–3 hours per day on the lowest setting dramatically reduces damping off (the fungal stem rot that collapses seedlings at soil level) by drying surface moisture and physically strengthening stems through movement. It costs nothing beyond the fan you already own.
Zone-by-Zone Indoor Seed Starting Timing
All dates are relative to the average last spring frost date for each zone. Find your zone at the USDA Plant Hardiness Zone Map by entering your zip code. For a full species list by zone, see the USDA Zone Plant Guide.
| Crop Category | When to Start Indoors | Weeks Before Last Frost | Calendar Window |
|---|---|---|---|
| Tomatoes, peppers | Late February | 12–14 weeks | Feb 20 – Mar 7 |
| Onions, leeks, celery | Late February to early March | 10–12 weeks | Feb 22 – Mar 14 |
| Brassicas (broccoli, cabbage, kale) | Early to mid-March | 8–10 weeks | Mar 1–21 |
| Squash, cucumbers | Late April to early May | 3–4 weeks | Apr 25 – May 10 |
| Basil, annual flowers | Late April | 4–6 weeks | Apr 20 – May 5 |
| Crop Category | When to Start Indoors | Weeks Before Last Frost | Calendar Window |
|---|---|---|---|
| Onions, leeks, celery | January to early February | 10–12 weeks | Jan 10 – Feb 7 |
| Tomatoes, peppers | Early to mid-February | 8–10 weeks | Feb 1–21 |
| Brassicas (broccoli, cabbage, cauliflower) | Late February to early March | 6–8 weeks | Feb 22 – Mar 10 |
| Squash, cucumbers, melons | Late March to early April | 3–4 weeks | Mar 25 – Apr 7 |
| Basil, annual flowers | Late March | 4–6 weeks | Mar 22 – Apr 3 |
| Crop Category | When to Start Indoors | Weeks Before Last Frost | Calendar Window |
|---|---|---|---|
| Onions, leeks | Early to mid-January | 10–12 weeks | Jan 5–20 |
| Tomatoes, peppers, aubergine | Mid to late January | 8–10 weeks | Jan 15–31 |
| Brassicas (broccoli, cabbage, kale) | Early to mid-February | 6–8 weeks | Feb 1–18 |
| Squash, cucumbers | Early March | 3–4 weeks | Mar 1–15 |
| Basil, annual flowers | Late February | 4–6 weeks | Feb 18 – Mar 5 |
| Crop Category | When to Start Indoors | Purpose | Notes |
|---|---|---|---|
| Tomatoes, peppers (spring crop) | October to November | Winter indoor start for Feb–Mar transplant | Avoids summer heat at transplant time |
| Tomatoes, peppers (fall crop) | July to August | Second summer crop for Oct–Nov harvest | Start in shade to protect from peak heat |
| Onions, leeks (cool-season) | September to October | Winter crop transplant | Cool-season crops grow through winter in Zone 9–10 |
| Brassicas (winter crop) | August to September | October transplant for winter harvest | Start in coolest part of day; shade cloth helps |
For month-by-month outdoor sowing and transplanting schedules, see the full Seasonal Planting Calendar which covers direct-sow and plant-out timing by zone for every month of the year.
Crops to Always Start Indoors
Some crops need 10–14 weeks to reach transplant size. In every zone except the warmest, direct-sowing these outside means the season ends before they produce. Start them indoors without exception.
Tomatoes
6–8 weeks indoors. Need soil temperatures above 60°F to germinate. Heat mat essential. The most commonly started crop β and the one most often started too early. In Zone 5, that means no earlier than mid-February for a mid-May last frost.
Peppers & chillies
8–10 weeks indoors. Germinate slowly and require consistent warmth (75–85°F) throughout. Never start peppers without a heat mat. They are the crop most punished by an early start β a root-bound pepper transplanted too early puts on no new growth for weeks.
Aubergine (Eggplant)
8–10 weeks indoors. Even more heat-dependent than peppers. In Zones 3–5, aubergine barely has time to ripen even with a head start. Choose compact varieties and start on time.
Celery & celeriac
10–12 weeks indoors. Tiny seeds with erratic germination. Surface-sow on damp compost, press lightly, and do not cover — celery needs light to germinate. Needs consistent moisture throughout the indoor period.
Leeks
10–12 weeks indoors. Direct-sown leeks are barely transplant-sized by the time they need to go out. Indoor-started leeks give you a much thicker stem at transplant time and larger finished plants by autumn.
Begonias & slow annual flowers
Wax begonias, petunias, lobelia, and snapdragons need 8–12 weeks from seed to a usable transplant. Buying trays from a garden centre is faster; growing from seed requires an early January start in most zones.
Planning where these crops will go before the transplants are ready is time well spent. See the Edible Garden Design guide for spacing, companion planting, and layout principles for productive vegetable beds.
Crops to Direct Sow Only — Never Start Indoors
Starting these indoors wastes time and produces inferior plants. They either resent root disturbance, bolt when transplanted, or establish so quickly from direct sowing that any head start is irrelevant.
| Crop | Why Not to Start Indoors | When to Direct Sow |
|---|---|---|
| Carrots | Taproot disturbance during transplanting causes forked, stunted roots | 2–4 weeks before last frost |
| Beets & parsnips | Same taproot issue as carrots; transplant shock causes misshapen roots | 4 weeks before last frost |
| Peas | Cold-tolerant and fast-germinating outdoors; roots resent disturbance | 6–8 weeks before last frost (as soon as soil is workable) |
| Beans | Germinate in 5–7 days outdoors in warm soil; no benefit to indoor start | After last frost, soil above 60°F |
| Squash & courgettes | Grow so fast they become root-bound within 3 weeks; direct sow beats a head start | After last frost, or start indoors for no more than 3 weeks |
| Corn | Needs block sowing for pollination; transplant shock causes poor establishment | After last frost, soil above 60°F |
| Radishes & turnips | Ready in 25–30 days from direct sowing; indoor start offers no advantage | 3–4 weeks before last frost |
If you want to extend the productive season for root vegetables and brassicas into autumn and winter, the key is a second direct sowing in late summer — not an indoor start. The Fall & Winter Edible Garden guide covers this in full, including which crops to sow in July and August for a November harvest.
Hardening Off: The Critical 7–10 Day Process
Hardening off is the acclimatisation period during which indoor-raised seedlings are progressively introduced to outdoor conditions — direct sun, wind, temperature fluctuation — before permanent transplanting. It cannot be skipped. Seedlings grown under grow lights have thin cell walls, tender foliage, and zero wind resistance. Moving them directly to a warm spring day causes sun scorch within hours and cold damage overnight.
1–2 hours in bright outdoor shade. No direct sun. Bring inside before temperatures drop below 50°F. The goal is wind exposure and temperature variation, not sun.
2–3 hours outdoors including 30–60 minutes of morning sun. Shade in the afternoon. Watch for wilting, which indicates the plant is struggling with water loss rates it cannot yet manage.
Half the day outdoors including direct morning sun. Bring in before evening temperatures fall below 45°F. Begin leaving out peppers and tomatoes only if overnight lows are reliably above 50°F.
Full days outdoors. Bring in only if frost is forecast. Leave cool-season crops (brassicas, lettuce, leeks) out overnight once temperatures stay above 35°F — they tolerate light frost once hardened.
Transplant after last frost has passed. Water transplants in well. If a late frost is forecast in the first week after transplanting, cover with fleece overnight.
Cold frames and cloches make hardening off easier in unpredictable spring weather. They allow you to leave seedlings outside overnight with the lid closed, opening during the day to control temperature. A cold frame also extends the transplanting window by 2–4 weeks in Zones 3–6 — placing broccoli transplants into a cold frame in March rather than waiting for outdoor planting conditions gives you a significantly earlier harvest.
Five Common Indoor Seed Starting Mistakes
Starting too early
The number one mistake. Growers see a warm February day and start tomatoes in mid-January. Six weeks later they have 12-inch, root-bound seedlings with nowhere to go for another four weeks. Pot-bound seedlings are stunted before they enter the ground. Follow the zone timing table above and resist the urge to add extra weeks.
Not enough light
A south window in January provides 3–4 hours of usable light. Seedlings need 14–16. The result is leggy, floppy plants that never produce well even after transplanting. Invest in grow lights and position them 2–4 inches above the foliage — most people hang them 12–18 inches away, which is not close enough.
Overwatering
Seed compost should be moist but never saturated. Lifting a tray to judge weight is more reliable than looking at the surface. Water from below by placing trays in a shallow dish — the compost wicks up what it needs and the surface stays drier, which prevents damping off.
Using the wrong growing medium
Garden soil in seed trays compacts, holds too much moisture, and carries fungal pathogens. Multi-purpose compost from an open bag does the same. Use purpose-made seed compost, or make a 50/50 blend of seed compost and fine perlite for extra drainage. Switch to standard compost only when potting up after the first true leaves appear.
Skipping or rushing hardening off
Moving seedlings directly from indoors to a sunny outdoor bed in April, even in Zone 7, causes immediate sun and wind damage. The 7–10 day process described above is the minimum. In unpredictable springs (late frosts, cold nights), extend it to 14 days. There is no shortcut that does not cost plants.
Hadaa Garden Design
Plan the Layout Before the Seedlings Go In
Six weeks into your indoor seed starting season, you have trays of tomatoes, peppers, brassicas, and leeks that all need to go somewhere specific. The common error is to plant without a plan and discover in July that the tall indeterminates are shading the peppers, or the squash has consumed three beds that were supposed to hold brassicas.
Hadaa generates photorealistic renders of your vegetable garden redesigned to a layout you specify — raised beds, in-ground beds, a kitchen garden — before you commit a single plant to the ground. Upload a photo of your current garden or yard, describe what you want to grow, and see 22 AI-generated views of how the finished layout looks. The planting guide that comes with each project includes spacing recommendations and zone-verified plant selections sized to your beds.
- →Visualise bed placement and path layout before breaking ground
- →Get plant quantities and spacing matched to your actual bed dimensions
- →Zone-verified species list so every crop is appropriate for your climate
- →Companion planting and rotation recommendations built into the layout
For raised bed ideas and construction formats, see the Raised Garden Bed Ideas guide.
Frequently Asked Questions
How many weeks before the last frost should I start seeds indoors?
It depends on the crop. Slow-growing crops like celery, leeks, and onions need 10–12 weeks indoors. Tomatoes and peppers need 6–8 weeks. Squash, cucumbers, and melons need only 3–4 weeks — they establish quickly and suffer more from being pot-bound than from a late start. Count back from your USDA zone’s average last frost date and mark the sow date on a calendar before you buy seeds.
Do I really need a heat mat for seed starting?
For tomatoes, peppers, aubergine, and basil, yes. These crops evolved in hot climates and germinate poorly below 70°F (21°C) soil temperature. A propagation heat mat raises the growing medium to 75–85°F and typically cuts germination time from 14–21 days down to 5–10 days. Cool-season crops like brassicas and lettuces germinate well at ambient room temperature and do not need a mat.
Can I use a south-facing window instead of grow lights?
Only if the window provides genuine direct sun for 6+ hours per day and you are in Zones 7–10 where winter days are longer. In Zones 3–6, a south window in January–March delivers 3–4 hours of weak, angled light — enough to keep seedlings alive but not enough to prevent leggy, floppy growth. Full-spectrum LED grow lights positioned 2–4 inches above the seedling tops, running 14–16 hours per day, reliably produce stocky transplants anywhere in the country.
Why do my seedlings fall over at soil level?
That is damping off — a fungal condition caused by Pythium or Rhizoctonia species. It is almost always triggered by overwatering combined with poor air circulation. Prevention: use sterile seed compost (not garden soil), water from below by placing trays in shallow water rather than watering overhead, and run a small fan on low for 2–3 hours per day to strengthen stems and dry the surface between waterings. Once damping off appears in a tray, remove affected seedlings immediately and treat the remainder with a chamomile tea drench or copper fungicide.
What does hardening off involve and can I skip it?
Hardening off is the 7–10 day acclimatisation period during which indoor-raised seedlings are progressively exposed to outdoor conditions. It cannot be skipped. Seedlings grown under grow lights have thin cell walls and no wind tolerance. Moving them directly outdoors causes sun scorch and wind damage within hours. The process: Days 1–2: 1 hour in bright shade. Days 3–4: 2–3 hours including morning sun. Days 5–7: half the day outside. Days 8–10: full days outside, bring in only if frost is forecast.
Plan Before You Plant
Design Your Vegetable Garden Layout This Season
You have the timing. Now you need a layout that works — beds in the right position, paths between them, and every crop in its correct place before your seedlings outgrow their trays. Hadaa generates 22 photorealistic renders of your garden redesigned to a layout you specify, plus a contractor-ready planting guide sized to your beds and verified to your USDA zone. Every Studio subscription includes a personal onboarding call so your first project starts right.
Upload a photo. See your garden redesigned. Results in minutes.