When To Start Tomato Seeds Indoors

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Updated 2.19.24

When you start tomato seeds indoors or in a greenhouse before planting season, you can save money. And you have a greater selection than what’s offered at you local garden center. That means you can grow tomatoes that may not be readily available where you live – as well as more resistant types of tomatoes or less common varieties.

And by starting seeds indoors in staggered batches you can ensure a continuous supply of ripe tomatoes throughout the growing season. That’s much preferable to harvesting a glut of fruit all at once! Succession planting is particularly useful if you’re growing determinate tomatoes to preserve in canning, freezing, or drying.

Plus, when you start tomato plants indoors, you have control over your seedlings’ growing conditions. You can make sure they have the optimal temperature, light, and humidity to grow strong and healthy, and you can protect baby tomato plants from pests and diseases. When spring comes, seedlings will be more ready to plant outdoors. You’ll grow a more successful crop than if you sow them directly in the garden.

Clearly, starting tomato seeds indoors has plenty of advantages. The question is when. What’s the optimal date for you to drop those precious little seeds into a bit of potting soil?

Timing, as they say, is everything.

When to start tomato seeds: how to choose a start date with Tomato Dirt #GrowingTomatoes #HomeGardening

Why it matters when you start tomato seeds indoors

By mid-winter, most gardeners are itching to get growing. Have patience to not start too early … and intentionality so as to not put it off until a late date.

Start your tomato seeds too early and you may face …

  • Overgrowth. Your plants will outgrow their pots quickly and become leggy, root-bound, and weak.  They’ll spend their first weeks in the garden regaining strength, rather than producing new leaf growth and flowers.
  • Crowded space. Your mature seedlings require more space, light, and resources than you can provide.
  • Diseases. Overcrowded or stressed seedlings are more susceptible to damping-off fungi and other diseases that thrive in warm, moist conditions. Plus, seedlings kept indoors for too long without proper air circulation or hardening off become more vulnerable to fungal diseases and pests once transplanted outdoors. In short, they have transplant shock.

Start your tomato seeds late and you may face …

  • A delayed harvest. Tomatoes need a certain amount of time to grow, flower, and set fruit before they can be harvested. If seedlings are not well-established or mature enough to be transplanted into the garden at the appropriate time, they may take longer to produce ripe fruit, especially in regions with shorter growing seasons.

Planting tomato seeds indoors at the wrong time means wasted time, effort, and resources. If seedlings become overgrown or fail to thrive due to being started too early or too late, you may need to start over with new seeds or purchase nursery transplants. That means expense and delay in your gardening plans.

Fortunately, there’s a way to calculate the best time for you to start tomato seeds where you live.

How to calculate when to start tomato seeds indoors

Start dates differ from region to region. Timing your tomato seed starting is not a one-size-fits-all. Much depends on your climate. Try these 2 easy steps to calculate the best time to start your seeds where you live. 

1. Determine the last frost date for your region.

If you’re not sure, contact your local extension office. You can also view the freeze/frost information provided by your local Department of Agriculture, on gardening websites, or through gardening apps. In the USA, check the USDA Plant Hardiness Zone map. I’ve also found the Farmer’s Almanac to be a reliable source. 

It’s crucial for you to identify the average last frost date in your region because it marks the point at which it's generally safe to transplant tender seedlings outdoors without risking frost damage.

2. Count back 4-8 weeks from the last frost date for your region.

To determine when to start your tomato seeds indoors, count backward 4-8 weeks from your average last frost date.

Oops. That’s a big window, right? Drill down a bit further. Different tomato varieties have different growth rates and maturity times. Determinate varieties typically mature earlier and have a more compact growth habit, while indeterminate varieties continue to grow and produce fruit throughout the season. Check the seed packet or catalog description for information on the days to maturity for the specific tomato varieties you plan to grow.

Here's a general guideline:

  • For indeterminate varieties: Start seeds indoors 6-8 weeks before the average last frost date. Indeterminate tomatoes take longer to mature and benefit from a longer growing season, so starting them earlier allows for larger, more established plants at transplanting time.
  • For determinate varieties: Start seeds indoors 4-6 weeks before the average last frost date. Determinate tomatoes have a more compact growth habit and shorter maturity time, so they can be started a bit later than indeterminate varieties.

If you’re growing tomato varieties that have a wide variance in days to maturity, then you may need to start tomato seeds accordingly. Be sure to label your varieties clearly to track progress. But on average, start tomato seeds indoors 6 weeks before you want to set seedlings in the garden.

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When to start tomato seeds indoors: examples

When to start tomato seeds indoors USDA zone 5
Last frost date: May 15
Average tomato seed starting date: April 3
When to plant tomato seedlings: May 25-June 8

When to start tomato seeds indoors USDA zone 6
Last frost date: May 1
Average tomato seed starting date: March 20
When to plant tomato seedlings: May 11-May 25

When to start tomato seeds indoors USDA zone 7
Last frost date: April 15
Average tomato seed starting date: March 4
When to plant tomato seedlings: April 25-May 9

When to start tomato seeds indoors USDA zone 8
Last frost date: April 1
Average tomato seed starting date: February 19
When to plant tomato seedlings: April 10-24

Another factor to consider when starting tomato seeds

Keep in mind that the timing recommendations are general guidelines. Other factors may influence your decision on when to start tomato seeds indoors: your microclimate, specific weather patterns, and personal preferences for plant size at transplanting time.

Maybe you’re having a cold, wet spring. If so, it will take your garden soil longer to warm so you might choose to delay seed starting a week or two. On the other hand, perhaps bulbs and blooms are a couple of weeks ahead of schedule this year. Adjust your plans accordingly.

Once you’ve got healthy tomato seedlings underway …

Be sure to harden off tomato plants. That is, several days before planting, start to give them short periods outdoors in a protected area. Gradually increase the time plants spend outside. Allow at least a week to 10 days for hardening off.

Then, plant your tomato seedlings outdoors 1-2 weeks after the last frost date. Waiting longer to plant after the last frost date will reduce the chance of losing your tomato crop to freeze and will allow the ground to warm – which is the kind of environment in which tomatoes do best!

From other Tomato Dirt-ers

Dear Tomato Dirt,

Why should I wait a couple of weeks after the last frost date to plant my seedlings in the garden? I want to get my tomato plants in the ground as soon as I can so I can have the first tomato on the block.

Itchy Garden Gloves

Dear Itchy,

Tomatoes like hot weather. They don’t like wet feet and they don’t like to be cold. Give the ground a chance to warm up and your tomatoes will reward you. Go with the calendar even if you're impatient.

If you absolutely must plant early, then use the “Black Plastic Trick.” Two to three weeks before setting tomatoes in the garden, cover your planting area with black garden plastic. Black absorbs heat from the sun and will warm your soil more quickly than if it is left to nature’s devices.

Happy gardening!
Tomato Dirt

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