How to Start Tomato Seeds Indoors

Tomatoes are a favorite among gardeners, not only because they’re easy to grow but also because they can be used in a variety of dishes.

But have you ever wondered how to give your tomato plants the best possible start?

Starting your tomato seeds indoors gives you a head start on the growing season, which is especially helpful if you live in an area with a short growing window.

By growing them indoors, you can control the environment, allowing your plants to develop strong roots and sturdy stems before moving them outdoors.

In this article, I’ll guide you through the steps for starting tomato seeds indoors, covering when to start, what you’ll need, and how to care for your seedlings.

Let's get into it!

When to Start Tomato Seeds Indoors

You’ll want to start tomato seeds about 6 to 8 weeks before the last frost in your area.

For example, if your last frost is around mid-May, you’ll want to start your seeds indoors in late March or early April.

This gives the plants time to develop strong roots and leaves before being moved outside).

Starting too early can result in leggy plants, while starting too late may leave your seedlings underdeveloped (they won’t have enough time to grow properly before being transplanted).

You can find your local frost dates by checking online or asking at local gardening centers (many websites provide tools that estimate the best planting time based on your location).

Keep in mind that frost dates can vary slightly each year, so it’s best to wait until the danger of frost has truly passed before transplanting (even if the calendar says the frost season is over, the weather can still be unpredictable).

What You’ll Need for Growing Tomato Seeds

To get started, gather these items:

• Tomato seeds
• Containers (seed trays, small pots, or recycled items like yogurt cups with drainage holes)
• Seed-starting mix
• Watering Can or Spray Bottle
• 3- to 4-inch pots with drainage holes (for transplanting seedlings)
• Grow lights (optional) (if natural light is insufficient)
• Fan (optional) (for air circulation and stronger seedlings)
• Liquid fertilizer (optional)

Containers for Starting Tomato Seeds

You don’t need to invest in expensive, purpose-made seed trays.

In fact, many everyday items can serve as great containers for starting your tomato seeds (and reusing materials helps reduce waste!).

If you’re up for a “little creativity”, you can repurpose items you might already have at home.

Seed Trays or Cell Packs

These are the go-to option for many gardeners because they’re designed specifically for starting plants, with multiple small compartments that help keep seeds separated. They’re especially useful if you’re starting a lot of seeds.

Small Pots

If you have small, unused pots lying around (with drainage holes), these can work well. If you don't, even disposable cups can serve as temporary containers as long as you make sure they have drainage.

Recycled Plastic Containers

Items like yogurt cups, takeout containers, or small plastic tubs are great options (just remember to poke holes in the bottom to allow water to drain and avoid root rot).

Egg Cartons

If you’re in a pinch, egg cartons can be used to start seeds (though they’re a bit small for growing tomatoes to full size).

They’re ideal for getting seeds started before transplanting them into larger containers.

If you use cardboard egg cartons, be mindful that they might break down when wet, so handle them carefully.

If you’re reusing containers, make sure to clean them thoroughly with warm, soapy water. This helps remove any dirt or bacteria that could harm the seedlings.

Choosing the Right Tomato Varieties for Indoor Growing

Some tomatoes do better in small spaces or containers, so here are a few things to consider when picking the right type of tomato:

Small or compact varieties

If you're growing tomatoes indoors or in a small space, choose smaller varieties like cherry or grape tomatoes.

These plants don’t require much space and are easier to manage indoors.

Indeterminate varieties

These tomatoes keep growing and producing fruit all season long. They can grow tall and require extra support, but if you have sufficient light and space, they can be an excellent choice for indoor growing.

Determinate varieties

These tomatoes grow to a specific size, produce fruit all at once, and then stop growing.

They’re a great option if you have limited space, as they stay smaller and don’t need as much support.

Popular determinate types include Roma and Patio Princess tomatoes.

How to Start Tomato Seeds Indoors in 5 Easy Steps

Now that you’ve gathered everything you need (like choosing the right container and picking your tomato variety), it’s time to get started with the steps to grow healthy tomato plants indoors. 

I’ve broken the process down into simple, easy-to-follow steps so you can feel confident as you begin.

Step 1: Prepare Your Containers

Before planting your seeds, you need to prepare your containers (just like setting up a home for your new plants).

Fill each container with a light seed-starting mix (don’t use regular garden soil, as it’s too heavy).

Leave about half an inch of space at the top of each container to prevent the soil from spilling over when watered.

Once filled, gently tap the sides of the container to settle the mix, but don’t pack the soil down tightly.

The soil should remain loose enough to allow water and air to move through, giving the roots plenty of room to grow.

Step 2: Plant the Tomato Seeds

Next, you’ll want to plant your tomato seeds.

Take one seed at a time (they’re tiny, so handle them gently!) and press it about a quarter of an inch into the soil—roughly the size of a pencil eraser.

Cover the seed lightly with soil, but don’t press it down too much. Make sure the seed is covered, but leave enough space for it to sprout.

If you’re planting multiple seeds in the same container, try to give each one some space to grow. A little breathing room will help them develop without crowding.

Step 3: Water the Soil

Once your seeds are planted, it’s time to water them.

Use a small watering can or a spray bottle to moisten the soil gently (a spray bottle is especially useful because it won’t disturb the seeds or wash away the soil).

The soil should feel damp but not wet. If the soil becomes soggy, it could lead to seed rot, so ensure there’s good drainage in your containers to allow excess water to escape.

Keep an eye on the moisture level and water the seeds as needed (but remember, less is often more in this case).

Step 4: Create the Right Environment

Tomato seeds need warmth to sprout, so placing your containers in a warm spot is essential.

A good location might be the top of your refrigerator, where it’s cozy, or a sunny window that gets plenty of light (just be cautious of drafts, as cool air can slow down the germination process).

If your home tends to be cooler, you can use a seedling heat mat—a small electric mat that gently warms the soil.

Maintain a temperature between 70 and 75°F, which is ideal for tomato seed germination.

Some people even wrap their containers with a plastic cover to help retain heat, but be sure to check on the seedlings regularly—they don’t need to get too hot.

Step 5: Give Them Adequate Light

Once your seeds sprout (usually within 5–10 days), it’s time to move them to a bright location.

If you’re using a grow light, keep it about 2–3 inches above the seedlings—close enough so they don’t stretch toward the light, but not so close that it burns them.

As the seedlings grow taller, you can raise the light further away. If you’re not using grow lights, ensure the seedlings are in a spot that gets plenty of natural light (ideally 12–16 hours of light per day).

Without enough light, the seedlings might grow weak or leggy (which isn’t what you want).

You can rotate the containers daily to help the seedlings grow straight and avoid them leaning to one side.

When to Transplant Tomato Seedlings Outdoors

The best time to transplant tomato seedlings outdoors is when they have well-established roots and the danger of frost has passed. This usually occurs about 6 to 8 weeks after starting the seeds indoors.

You’ll also want to ensure the temperature is warm enough for your tomatoes to thrive.

Before moving your seedlings outside, consider transferring them to a larger container about four weeks beforehand.

Wondering how big your container should be?

Choose one that provides the roots with plenty of space to grow. A container that’s too small can stunt the plant’s growth, while one that’s too large may make it harder for the plant to stay healthy.

Hardening Off Your Seedlings

Before you transplant, I suggest getting your seedlings used to being outside.

How do you do that? Start by putting them outside for a few hours each day, gradually increasing the time over a week.

This helps them adjust and reduces the chance of shock when they’re moved outdoors.

Once they’re in the ground, be sure to follow best practices for watering transplants to ensure they settle in properly.

Tips to Care for Your Tomato Seedlings

I’ve put together some practical tips to help your seedlings stay healthy and grow strong, so I suggest you take notes on these.

1. Water regularly, but don't overdo it.

Water when the top of the soil feels dry, and ensure there’s good drainage in your containers to avoid water buildup.

Overwatering can lead to issues like yellow leaves. If you notice yellowing on your tomato leaves, it could be a sign of too much water or a lack of nutrients.

2. Rotate containers to encourage even growth.

Turn the containers every few days to ensure your seedlings grow straight and evenly, rather than leaning toward the light.

3. Thin out weak seedlings.

If you have multiple seedlings in one container, remove the weaker ones to prevent overcrowding and give the stronger ones enough space to grow.

4. Feed with a mild fertilizer once true leaves appear

After your seedlings develop their first true leaves, start feeding them with a diluted liquid fertilizer (about once a week or every other week).

5. Check for pests

Regularly inspect your plants for pests like aphids. If you find any, gently remove them with water or wipe them off with a damp cloth.

Just follow these simple steps and tips, and you’ll be all set to successfully grow your tomato seeds indoors!

Frequently Asked Questions (FAQs)