Your garden’s success starts well before you dig into the soil. Starting seeds indoors gives you control over the environment, ensures stronger plants, and lets you extend your growing season like a pro.
Whether you’re aiming for sweet peppers, crisp greens, or juicy tomatoes, timing is everything. This month-by-month guide breaks it all down—what to sow, when to sow, and how to prepare your seedlings for a smooth transition outdoors.
- Starting seeds indoors gives your veggies a head start before the growing season.
- Following a month-by-month guide keeps your garden organized and productive.
- Seedlings are happiest with the right temperature, light, and care (don’t worry, I’ll explain).
- Timing is everything—transplant your seedlings outdoors when the conditions are perfect.
January: Start Slow but Strong
What to Sow Indoors
- Aubergines (Eggplants): These guys need a loooong growing season. Start early!
- Chillies and Peppers: Heat-loving plants that need a head start to reach their potential.
- Tomatoes (Greenhouse Varieties): Early sowing = more tomatoes for sandwiches. 🍅
When to Move Outdoors
- Wait until after the last frost, when nights are consistently warmer than 13°C (55°F). Late May or early June is usually perfect.
💡 Tip: Keep soil warm (between 21–29°C / 70–85°F) by using a heat mat. Happy soil = happy seeds.
💡 Tip: Light is your seedlings’ best friend. Aim for 12–16 hours of light daily with grow lights. If you’re using a sunny windowsill, rotate the pots so your seedlings don’t lean to one side like sleepy toddlers.
| Vegetable | Germination Temperature (°C) | Days to Sprout | Outdoor Planting Time |
| Aubergines | 21–29 | 7–14 | Late May or June |
| Chillies & Peppers | 21–29 | 10–20 | Late May |
| Tomatoes | 18–29 | 6–12 | Late May (after frost) |
February: The Real Action Begins
By February, the days are getting longer, and so is your list of seeds to plant.
What to Sow Indoors
- Broccoli and Cauliflower: These cold-hardy champs can handle a head start.
- Leeks: Slow and steady wins the race—leeks love a little extra time indoors.
When to Move Outdoors
- You can transplant these guys about 4–6 weeks before the last frost, usually when daytime temperatures hover around 10°C (50°F).
💡 Tip: Use biodegradable pots so you can plant the whole thing without disturbing the roots. Your seedlings will thank you!
💡 Tip: Harden off seedlings by taking them outside for a few hours a day. Think of it as plant boot camp—they need to toughen up for the real world.
| Vegetable | Germination Temperature (°C) | Days to Sprout | Outdoor Planting Time |
| Broccoli | 18–24 | 7–10 | 4–6 weeks before last frost |
| Cauliflower | 18–24 | 8–10 | 4–6 weeks before last frost |
| Leeks | 16–21 | 10–14 | 4–6 weeks before last frost |
March: Greens, Glorious Greens 🌸
March is the perfect time to sow your leafy greens and some hardy staples.
What to Sow Indoors
- Lettuce and Spinach: Fast-growing and perfect for cool weather.
- Onions: Start them from seeds, and you’ll have a steady supply all summer.
- Cabbage: Early planting ensures a healthy spring harvest.
When to Move Outdoors
- Leafy greens can handle cooler temps, so move them out when daytime temps reach 5–10°C (41–50°F). Onions and cabbage can go out 2–4 weeks before the last frost.
💡 Tip: Don’t overcrowd your seedlings—thin them out to give each one room to grow.
💡 Tip: If frost threatens, use fleece covers or cloches to protect your tender babies.
| Vegetable | Germination Temperature (°C) | Days to Sprout | Outdoor Planting Time |
| Lettuce | 13–24 | 7–14 | Late March or April |
| Spinach | 7–24 | 6–12 | Early to mid-April |
| Onions | 18–29 | 7–10 | Early April |
April: The Warmth Lovers Are Coming
Now’s the time to start preparing for those crops that thrive in summer heat.
What to Sow Indoors
- Cucumbers: Start these early so they’re ready to explode with growth outdoors.
- Zucchini: Another prolific producer that loves a head start.
- Melons: Sweet, juicy, and perfect for summer—start them indoors for best results.
When to Move Outdoors
- Wait until nighttime temps are consistently above 13°C (55°F) and the soil is toasty warm (18°C / 65°F). Usually late May or early June.
💡 Tip: Transplant seedlings when they have 2–3 sets of true leaves. Anything smaller might struggle outdoors.
💡 Tip: Use coir pots for easy transplanting and minimal root disturbance.
May: The Grand Finale of Indoor Sowing
By May, you’re wrapping up your indoor sowing and focusing on transitioning your seedlings outside.
What to Sow Indoors
- Herbs: Basil, parsley, and dill love a cozy indoor start.
- Sweet Corn: This fast-growing crop benefits from a little indoor TLC.
When to Move Outdoors
- Transplant herbs and sweet corn after the last frost. Basil, in particular, needs nighttime temps of 15°C (59°F) or warmer to thrive.
💡 Tip: Water seedlings well before transplanting—they need the hydration to settle into their new home.
💡 Tip: Gradually increase their outdoor exposure to prevent shock.
| Herb/Crop | Germination Temperature (°C) | Days to Sprout | Outdoor Planting Time |
| Basil | 18–29 | 5–10 | Late May |
| Parsley | 16–21 | 14–21 | Late May |
| Sweet Corn | 18–24 | 7–14 | Late May |
How to Know Your Seedlings Are Ready to Move Outdoors
- They Have True Leaves: Seedlings should have at least 2–3 sets of true leaves.
- They’re Sturdy: The stems should be strong, not floppy or spindly.
- Their Roots Are Healthy: Check for a well-developed root system that isn’t root-bound.
- Outdoor Conditions Are Right: Temperatures should match the plant’s needs.
Common Mistakes to Avoid 🚫
Starting seeds indoors can be incredibly rewarding, but even seasoned gardeners sometimes make mistakes. Avoiding these common pitfalls can save you time, effort, and frustration:
Starting Too Early: It’s tempting to get a head start, but starting seeds too early often leads to leggy, overgrown plants that outgrow their containers before it’s time to transplant. Always check the seed packet for the recommended sowing time and plan around your area’s last frost date.
Using Regular Garden Soil: Garden soil is heavy, can compact in containers, and often contains pests or diseases. Instead, use a light, well-draining seed-starting mix. This creates the perfect environment for your seeds to sprout and grow healthy roots.
Skipping the Hardening-Off Process: Transplant shock is real! Moving seedlings directly outdoors without acclimating them to sunlight, wind, and temperature changes can stunt their growth or even kill them. Take 7–10 days to gradually introduce your seedlings to outdoor conditions.
Overwatering or Underwatering: Overwatering can cause root rot, while underwatering dries out the fragile roots. Keep the soil consistently moist but not soggy. A spray bottle is great for maintaining the right balance.
FAQs about When and How to Start Vegetable Seeds Indoors
How do I calculate the best transplant time for seedlings?
Check the seed packet for recommended timing based on your last frost date. Calculate 6–8 weeks from sowing to allow seedlings to grow.
What should I do if seedlings outgrow their pots before transplanting?
Repot seedlings into larger containers with fresh soil to give them more space to grow.
How do I protect seedlings from transplant shock?
Harden off plants by exposing them to outdoor conditions gradually and use biodegradable pots to avoid disturbing roots during transplanting.
