Growing Tomatoes from Seed: Beginner’s Guide
Growing Tomatoes from Seed: Beginner’s Guide
Starting tomatoes from seed might sound hard, but it’s actually one of the most rewarding ways to grow your own food—especially if you want a specific or unusual tomato variety. Growing tomatoes from seed gives you more control over the varieties you grow and can save you money compared to buying nursery starts. Plus, it’s fun to watch your tomatoes go from tiny sprouts to sturdy little plants ready for the garden.
In this guide, we’ll walk through everything you need to know to start tomatoes from seed—indoors or out—up to the transplanting stage. Whether you’re brand new to this or just looking to up your game, you’re in the right place.
Choosing a Tomato Variety (and Why It Matters)
There are a lot of tomato varieties out there, and seed catalogs don’t exactly make it easier with all their options. The key is to match the variety to your space and goals. Some tomatoes are better for containers, others thrive in large gardens. Some are perfect for slicing and eating fresh, while others shine in sauces.
If you’re not sure where to start, head over to my full guide to tomato varieties. It breaks down the difference between determinate and indeterminate types, plus which tomatoes are best for beginners, cooking, snacking, and even indoor growing.
When to Start Tomato Seeds
Tomatoes are warm-season plants, which means you can’t just toss seeds in the dirt the minute the snow melts. Timing is everything.
- Indoors: Start seeds 6–8 weeks before your last expected frost date. If you’re not sure when that is, you can look it up by ZIP code on a frost date calculator.
- Outdoors: If you’re sowing directly outside (not typical, but possible in warmer climates), wait until all danger of frost has passed and the soil is consistently above 60°F.
Tomato seeds germinate best when the soil is between 70–85°F (21–29°C). Germination is slow or poor below 60°F, so if your house runs cool, a seedling heat mat can help speed things up.
Supplies You’ll Need
You don’t need fancy gear, but having the right basics will make things go a lot smoother.
- Tomato seeds (fresh is best)
- Sterile seed-starting mix (not regular potting soil—it’s too heavy and may carry diseases)
- Clean seed trays or small pots (like yogurt cups) with drainage holes
- Clear plastic dome or plastic wrap for heat and humidity
- Grow light or bright south-facing window
- Spray bottle or small watering can
- Labels (trust me, you’ll forget what you planted where—also jot down the sowing date)
How to Start Tomato Seeds Indoors
- Fill Your Trays or Pots
Use a pre-moistened, sterile seed-starting mix. It should feel like a wrung-out sponge—not soggy. - Plant the Seeds
Poke holes about 1/4 inch deep, drop in one or two seeds per cell or pot, and cover lightly with mix. - Label Everything
Once seedlings sprout, they all look the same. Include the variety name and sowing date so you can track growth. - Create a Greenhouse Effect
Cover with a humidity dome or plastic wrap until the seeds germinate. This helps maintain moisture and warmth. - Provide Warmth and Light
Tomato seeds germinate best at 70–85°F. Use a heat mat if needed. Once they sprout, they’ll need 14–16 hours of bright light daily. If your windows don’t get that much sun, a basic grow light will do wonders. - Water Gently
Keep the soil moist but not soggy. A spray bottle works well at first. - Encourage Airflow
Good air circulation helps prevent disease. A small fan nearby on low is often enough to keep fungal issues like damping off at bay.
Under ideal conditions, tomato seeds usually germinate in 5–10 days. Cooler soil? It might take longer.
If you’re looking for more tips for other plants, I’ve written a whole guide on starting seeds indoors.
What Happens After Sprouting?
Once your seeds germinate:
- Remove the humidity dome or plastic wrap.
- Move the seedlings to a slightly cooler spot (65–70°F) if you can—this keeps them from getting leggy.
- Keep the lights close—just a few inches above the seedlings—and adjust as they grow.
If you planted more than one seed per cell, thin the weaker one by snipping it at the soil line.
After the first true leaves appear (the ones that look like tomato leaves, not the rounded baby leaves), you can begin feeding your seedlings with a diluted, balanced liquid fertilizer once a week. This gives them a gentle nutrient boost to keep growing strong.
Potting Up (aka Transplanting to Bigger Pots)
When your seedlings outgrow their starter trays, it’s time to pot them up.
- Use a 3–4 inch pot with fresh, sterile potting mix.
- Bury the seedling deeper than it was before—up to the bottom leaves. Tomatoes grow roots along their stems, and this helps make a sturdier plant.
- Water well and return them to the grow light or sunny window.
You might need to pot up one more time if the seedlings outgrow their pots before your outdoor planting date.
Hardening Off
If your tomatoes have been living indoors, don’t just plop them outside one day and call it good. They need time to adjust to sun, wind, and fluctuating temps.
Start 7–10 days before transplanting:
- Put them outside in a sheltered, shady spot for a couple hours.
- Gradually increase their outdoor time and sun exposure each day.
- Don’t leave them out overnight until nighttime temps stay reliably above 50°F.
Transplanting
Once you’ve hardened off your seedlings, you can transplant them into your garden or a regular pot. I’ve written a complete guide to transplanting tomatoes to walk you through the process.
Direct Sowing Tomatoes (Outdoors)
Most gardeners start tomatoes indoors, but you can start growing tomatoes from seed directly outside in warm climates. Just wait until the soil is thoroughly warm and night temps stay above 55°F. Loosen the soil, plant the seeds about 1/4 inch deep, and keep the area evenly moist. This method works best with fast-growing, compact varieties.
To learn more tips, check out the full guide to starting seeds outdoors.
Happy Seed Starting!
Growing tomatoes from seed takes a little patience, but the payoff is huge. You get to pick exactly what you want to grow, avoid the sad leftover plants at the nursery, and set yourself up for a strong, healthy tomato season.
Starting tomato seeds is just the beginning of your tomato adventures! To learn more, I have a complete tomato care guide, along with lots more on…
