When and How to Start Tomato Seeds Indoors

Starting your tomato seeds indoors provides quite a few different benefits. It allows gardeners in colder climates to get a head start on their spring gardens, while eliminating some early direct-sowing risks. In this article, organic farmer Jenna Rich walks through how and when to start seeding your tomatoes indoors this season.

As you may know, tomatoes are a tropical, heat-loving crop that will not survive any sort of frost so it’s important to time out their sowings. By sowing tomato seeds indoors, you can get an early jump on your gardening season.

When to start your seeds is dependent on the following:

  • Your USDA hardiness zone.
  • Your indoor growing setup.
  • The varieties you have selected.

Follow along to learn when best to start your tomato seeds indoors this year, no matter what setup you may have!

Contents

When to Start Sowing Seeds Indoors

Top view, close-up of tomato seedlings in a peat pot, held by a man's hand, against the background of seedlings in peat containers. The peat container is round, filled with potting mix. Tomato seedlings have small thin stems and several pairs of leaves of two types, smooth, oval and pinnately compound.
Start sowing tomato seeds 6-10 weeks before the expected last frost date in your area.

Sow seeds indoors no more than 6-10 weeks before the last potential frost date in your region. If you don’t know this information, you can search your area by zip code here. It breaks down sowing times by crop so keep this link handy.

The date you sow them will depend on your region and how long the tomato variety takes to mature. Keep in mind that you might even be in a microclimate (usually based on elevation) which can affect these dates by a bit on both ends.

Tomatoes are hardy plants and have a little more flexibility for when to start sowing compared to other vegetables you may sow indoors.

Starting Seeds by Hardiness Zone

The process of planting tomato seeds in plastic seed trays. Close-up of male hands planting tomato seeds in trays, with round cells filled with potting soil. There are also eco-friendly peat containers, a bag of soil, a trowel and a rake on the table.
Use the USDA Hardiness Zones map to start planting tomato seeds on time.

If you are new to gardening and aren’t sure of your zone, this map is a great place to start. The map divides the United States into zones based on minimum winter temperatures.

Tomatoes vary in days to maturity but you should plan to have 65-130 days to be on the safe side. Check your seed packet for more information.

Zones Start indoors Transplant outdoors Start outdoors in protected space
3a Early/mid – April Late May – early June Early May, heat required
3b-4 Late March – early April Mid-May – early June Early May, heat required
5-6 Early/mid – March Mid-April – early June Mid-April, heat required
7 February 15 –
early March
Early April – early June Late March – early April, access to heat recommended
8 Mid-January – mid-February Late March/April – early July March – April,
access to heat recommended
9 Mid-January – mid-February Mid-March – mid-April, again in August August
10+ Mid-January –  mid-February, again in September March-April, again in September-December September- December

Regarding the starting/transplanting times of year in the chart above: longer seasons offer longer transplant windows. This means you can sow and transplant multiple times resulting in several successions within the same season.

Selecting Region-Suitable Varieties

When it comes to garden planning, the first thing most of us will do is select seeds. When flipping through seed catalogs, you’ll want to pay close attention to a few things to set yourself up for success.

Days to Maturity

Close-up of different types of tomato seedlings in a garden center. A lot of dark green pots with seedlings and plates with images and inscriptions of tomato varieties.
The ripening of the crop depends on the climate of a particular season and on the date of planting.

This very important number will tell you how many days, under ideal conditions, that a crop will take to fully mature and be ready for harvest. It may be shorter or longer depending on the particular season’s climate. Most seed companies list this number list from the day direct sown.

However, since transplanting tomatoes is generally recommended, many times the days to maturity will be listed from the date of transplant. Just be sure you know how your selected seeds company formulates the days to maturity, otherwise, you could be off a few weeks in your planning.

If you live in a region that has a shorter growing season, try a variety labeled “early season” so you can enjoy tomatoes sooner!

Pro tip: If days to maturity is formulated for direct sowing but you plan to start indoors and transplant, as a general rule of thumb, you can subtract 10-14 days from this number to give you a more accurate estimated first harvest date.

Zone Type of tomato recommended Days to maturity Notes
3-5 Early season or those marked “cooler season” 65 days or less Extra frost barriers and protected space are recommended
5-8 Mid-season 65-80 days You can push the boundaries with protected space, heaters, and row cover
8-11+ Late season 80+ Only grow these in cooler climates if you have a greenhouse

Disease Resistance Packages

Close-up of a farmer's hand planting tomato seeds in the soil. The starting tray for sowing seeds is black, plastic, has square deep cells filled with fresh soil. Next to the tray is a package of tomato seeds with a picture of ripe tomatoes.
Choose disease-resistant tomato varieties.

Pay attention to disease resistance packages. If you live in a particularly humid or in an area with long, wet springs, you may want to choose something with resistance to various fungal tomato diseases.

Indoor Seed Starting Benefits

It is highly recommended that you start tomatoes in pots or cell trays and allow them to grow for a few weeks rather than direct sowing them into your soil. Here are a few reasons why.

You Can Select the Healthiest Plants

Close-up of farmer's hands holding tomato seedlings in a plastic container, ready to be transplanted into the garden. There is a large box with tomato seedlings next to the gardener. Tomato seedlings have upright hairy stems and pinnately compound leaves with pale green lobed leaflets. There are many raised beds in the blurred background.
When growing seeds indoors, you have the option of choosing seedlings that are well-developed and healthy for transplanting into your garden.

When you start your seeds safely indoors, you can select the best-looking ones when it comes time to transplant them into your garden plot. If a variety has a germination rate that is not particularly high and you direct sowed them outdoors for instance, you may end up with lots of gaps in your garden where seeds did not sprout properly.

Occasionally, you’ll have a tomato seedling that is a bit stunted. When this happens, you can choose to skip on transplanting it or simply give it more time to grow before doing so. You lose this control when you direct sow.

Direct-Sown Seeds May Rot or Freeze

Close-up of a woman's hand showing a damaged young tomato plant in the garden. The plant has a tall, upright, slightly hairy stem and sluggish, brown, rotten, pinnately compound leaves.
Your seeds may start to rot if the soil is still damp and cold.

If your soil is still pretty damp and cold from the winter snow melt and spring rain, your seeds may rot in the soggy environment. A few weeks could pass before you even know something is awry, leaving you behind in your sowing schedule.

In addition to possible rot, late spring frost is not uncommon in particular zones which could damage or even kill your delicate tomato seeds.

Just be sure you use a seed planting calendar which allows you to enter your last frost date. If you aren’t sure when your last frost date is, check out this helpful Farmer’s Almanac chart.

Seeds Can Focus on Germination

Top view, close-up of sprouted tomato seeds in black plastic containers, on a light windowsill. The seedlings have slender, hairy stems and two pairs of leaves, one with oval, smooth, pale green leaves, and the other with pinnately compound leaves of lobed leaflets.
It is important to create optimal conditions for seeds to focus their energy on the germination process.

The healthiest plants start out as healthy seedlings. Any added stress takes away energy meant to be spent on germinating. Seeds only have so much stored energy for the germination process, so do what you can to allow them to focus that energy on the first part of their journey.

When optimal conditions are provided for seeds, especially heat-loving ones like tomatoes, the seedlings should thrive, giving you a long season of healthy plants that produce lots of delicious fruit.

How to Sow Tomato Seeds Indoors

Before sowing seeds indoors, you’ll want to make sure that you’ve prepared properly for the task. Here are a few checklist items you’ll want to have aligned before you start sowing.

Purchase Seeds from a Trusted Source

Close-up of many different packages of tomato seeds in boxes, in several rows, on a shelf in a garden center. Packages of seeds are green, with inscriptions and images of tomato varieties.
Make sure your tomato seeds are from a reputable source.

This is important no matter where you are starting your seeds or what crop it is. Always be sure your source is reputable. Remember that sometimes the more expensive seeds are as such due to additional testing and safety precautions taken before they arrive on your doorstep.

Use a High-Quality Seed Starting Mix

Top view, close-up of a seed germination tray, some of the cells filled with fresh seed starter mix. A small garden shovel with a wooden handle is inserted into the soil.
The seed starter mix should not have a lot of nutrients and should be fluffier and lighter than potting soil.

The seed-starting mix should be a bit fluffier and lighter in weight than potting soil. This is because seeds need to be able to push through to the surface and you don’t want them working extra hard on this.

In addition, the mix should not be high in nutrients because in general, seeds contain what they need to germinate. When it comes time to “step up” your seedlings into larger pots, a higher nutrient content mix should be used so they can continue to mature before you plant them outside.

Use Cell Trays with Good Drainage

Close-up of an empty seed germination tray in a garden, on a concrete floor. The tray has 6 deep holes with many oblong drainage holes.
Use trays and containers with proper drainage holes to prevent root rot and provide airflow.

Not all cell trays are created equally. Grab something that features some sort of hole or slit at the bottom to allow for proper drainage. Otherwise, the roots can rot. Also, be sure not to pack the soil in too tightly to allow for good airflow

Pro tip: If you are reusing plastic from a past season, disinfect it before using it to decrease any risk of spreading disease that may be lingering.

Use Heating Mats

Close-up of a seed germination starter tray with a transparent wet dome standing on a heating black mat with green lettering. The tray is black, plastic, has deep square cells filled with soil.
Use a heating mat to maintain the proper temperature for germinating tomato seeds.

Tomatoes will germinate best with temperatures between 65°-85° so unless you have an area that remains this temperature consistently, I recommend getting a heat mat or two to be safe.

We have used one before without a thermostat that stayed around 72° which worked just fine. You can also use humidity domes to keep some extra heat and humidity in.

You should remove heat mats and humidity domes once the seeds have germinated and it’s time to put them under a light.

Properly Use Artificial Light

Lots of tomato seedlings in clear plastic containers under an LED lamp as artificial light. The lamp illuminates young plants. Tomato seedlings consist of upright pale green stems and pinnately compound leaves of slightly hairy lobed leaflets.
LED lights are a good artificial light option for your tomatoes.

Within a few hours of germinating, you should provide light for your seedlings. If you wait too long, they will become leggy, stretching their little necks toward any bit of light they can find. It is hard to come back from this so it’s best to provide light a few inches directly above them as soon as you notice any germination. The others shouldn’t be long behind.

It’s best to have the ability to move your light up and down easily so that as your plants grow, you can distance the light appropriately. Tomatoes grow fairly quickly so you’ll likely need to adjust the light every 1-2 days.

In my opinion, LED lights are the way to go. They tend to last longer, cost less to run, and be more environmentally friendly when it comes time to dispose of them.

Provide Regular Clean Water

Close-up of a woman's hand spraying tomato seedlings. Seedlings are in black, square containers filled with moist soil. The tomato plant consists of thin stems and pinnately compound leaves, slightly hairy.
Use a mister to water young seedlings and let the soil dry out between waterings.

Water is very important to new seedlings but keep in mind, overwatering is very easy to do. I use a mister on most new seedlings because although water is needed, overwatering can cause damping off or rotting. Your soil should be slightly dried out in between waterings.

Plan For Proper Airflow

Close-up of small tomato sprouts in peat containers with soil. Sprouts has small thin hairy stems and two pairs of pinnately compound leaves of lobed leaflets, and oval, smooth, pale green leaves.
Good airflow helps reduce disease and prepares the plant for outdoor life.

This helps keep disease pressure down but the added bonus is that running a lightly blowing, oscillating fan on your plants, even lightly, helps bulk up the stems, making them strong and preparing them for rain and high winds when it comes time to live outside. The breeze causes the plant to release a hormone that triggers it to thicken up its stem.

Hardening Off

Close-up of tomato seedlings outdoors, in direct sun for hardening. Seedlings have tall hairy pale green stems and pinnately compound leaves of bright green lobed leaflets.
Be sure to harden off your seedlings by taking them outdoors for a few hours a day.

When seeds are sown indoors, be sure you account for the time you’ll need to allow them to “harden off” outdoors before fully transplanting them into the ground. Hardening off is simply getting your plants acclimated to the real outdoors without the shelter or warmth of your home or greenhouse.

This can start slowly by setting your trays of seedlings outdoors for just a few hours a day, then bringing them back inside at night for a few days. Then allow them to spend a whole night outside and perhaps cover them with row cover.

Then eventually, leave them uncovered for a few nights before planting them out. This allows them to slowly get used to being outside without being shocked and stressed.

Pro Tip: If it’s especially sunny, wait another day. Harsh, direct sunlight can cause unnecessary stress or sunburn on the leaves.

Final Thoughts

Tomatoes are heat-loving crops that are 100% intolerant to frost. While each year is different and you should keep your eye on weather patterns as well as your last frost date according to your growing zone, a lot of the timing to sow tomatoes is dependent upon your growing setup.

If you have the time and space to set up an indoor growing space to start your tomatoes, I highly suggest doing so. Your tomatoes will appreciate the extra warmth and attention.

Remember, the healthier the seedlings, the healthier the plants so doing what you can for them in the early stages will make for a long, productive season. Happy tomato sowing!

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