When To Start Tomato Seeds Indoors – Easy Rule Of Thumb

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Last Updated: February 8, 2024

Tomato plants are long-season crops, requiring many months of growth before setting their first harvest. To maximize yields, most gardeners plant seeds indoors to elongate the growing season.

However, timing is very important. Plant tomatoes too early, and you may end up with overgrown seedlings. Plant too late, and your overall yields will suffer.

After growing tomatoes for many, many seasons, I have an easy rule of thumb to follow. In this article, I’ll share my advice on when to start tomatoes seeds based on your particular climate.

Quick answer: Plant tomato seeds indoors 3-4 weeks before your local last frost date. Transplant outdoors after the soil has warmed up, usually about 2-3 weeks after the final frost.

Tomato seedlings in seed starter tray in the shade
Young tomato seedlings in small tray.

When To Start Tomato Seeds

Tomatoes are fast-growing plants. For this reason, I believe it is important that you do not start tomato seeds too early! I have made this mistake more than once, and the result is overgrown, tall and lanky seedlings. Not good!

Root bound plants, early flowers, and leggy seedlings are not ideal. Instead, it is best to start your tomato seeds at the right time so they have a seamless transition outdoors in the spring.

In general, I recommend planting tomato seeds about 3-4 weeks before your average last frost date. When the weather begins to warm up, your plants should just be outgrowing their indoor containers.

Tomato seedlings on grow shelf
Healthy tomato seedlings planted indoors.

When is my last frost date?

Learning your average last frost date is an incredibly valuable step towards a successful garden. Most crops are planted based on this date.

To find your average last frost date, use this tool. Enter your postal code and make a note of your area’s average last frost date.

Remember that this is an average date. It represents the average time of year when each region experienced its last frost over the previous years. So it is always best to use this as a guideline, but it is more important to pay attention to the weather forecast each season.

For example, here in Southern New England, my last frost date is right around May 1st. However, we sometimes have a late freeze in mid-May, so we have to be cautious when planting frost-sensitive crops outdoors.

You may end up with a warmer-than-average spring, allowing you to plant outdoors sooner. Or, you could have the odd frost late into the season, requiring you to protect your tomatoes.


Timing Adjustments

While 3-4 weeks before your last frost date is generally best practice, there are a few cases where you may want to adjust your planting time forward or backward.

Indoor lighting

Grow light vs window tomatoes
Tomatoes planted on the same day with window light (left) vs. grow light (right).

In order to help your tomatoes grow as quickly as possible, I highly recommend using grow lights. While you can use a sunny window, it will likely take your plants more time to grow to transplanting size.

If you will rely on a sunny window, I recommend planting about 2 weeks earlier, or about 4-6 weeks before your last frost date. This extra time in the sunlight should be enough for your tomatoes to grow strong enough to move outside.

Using cold protection outdoors

Young tomato plant under row cover in raised bed
Tomato planted in raised bed under row cover fabric.

If you want to truly maximize your gardening season, there are ways to plant tomatoes outdoors earlier in the spring. By covering your plants with an insulating barrier, you can keep them warm in the colder spring weather.

If you use cold protection outdoors, you can plant tomato seeds up to 10 weeks before your last frost date! This will give your plants a huge head start in the garden, establishing outdoors ahead of schedule.

Here in zone 6, I use protective coverings to harvest my first ripe tomatoes in early July. Without protection, the plants don’t begin producing until August.

Warmer and colder climates

If you live in an especially warm climate, you may be able to plant your tomato seeds at any time of year. However, you want to time it so that your plants are in production during the milder season, while temperatures are around 70-75°F.

Warm season gardeners also have the luxury of succession planting tomatoes. For example, you may plant a round of tomato seeds every 1-2 weeks. Later in the season, your plants will produce their yields at staggered intervals, giving you a continual harvest.

On the flip side, colder climates have to be very careful to plant early enough. It may be necessary for growers in zones 4 and below to plant a few weeks early, about 6-8 weeks before the last frost. This will allow time for the plants to grow a bit larger indoors to make the most of the warm season.

Staked tomato plant on pole trellis

Germinating Tomato Seeds

When you’re ready to plant your tomato seeds, the process is fairly simple. I have a couple simple tricks to get them to sprout quickly, but it is pretty hard to mess up!

  • Fill pots with pre-moistened potting soil. For tomato seeds, I simply use normal potting soil with nutrients added in. My favorite brand is Happy Frog.
  • Plant 2-3 seeds per container. Create a small hole in the soil, about 1/4″ deep, in the center of each pot. Add 2-3 seeds (to ensure at least one germinates). Then, cover the seeds with the surrounding soil.
  • Mist the soil thoroughly to wet the seeds. Use a spray bottle to spray the soil surface with water. This ensures the seeds are well-moistened, which is critical for germination.
  • Keep warm and humid until seeds sprout. Tomato seeds germinate best in temperatures between 75-85°F. I use a seed heating mat to achieve this, but you can also just place your trays in a warm location. Cover the pots with a humidity dome or other covering to keep it humid. Mist regularly to avoid the seeds drying out.
  • Provide light to seedlings. After 4-7 days, your first seedlings should emerge. Place the plants under grow lights or in the sunniest window in your home.

For a more detailed guide, read my article on germinating tomato seeds here.

Pushing tomato seeds into soil with pen
Planting tomato seeds.

After gardening for over 10 years, I understand just how important timing is. With a solid plan in place for when to plant tomatoes, you’ll feel confident when starting this year’s garden. Good luck!

Cherry tomato trusses from Super Sweet 100

Calvin

Hi, I’m Calvin, creator of Tomato Geek. I have over a decade of gardening experience and I love helping others grow healthy plants!

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