Tomatoes are a fan favorite of gardeners, and it’s easy to see why. Eating a sun-ripened tomato picked from the vine is a quintessential glory of summer gardening. But tomatoes take a long time to mature – and require a lot of planning.
When starting indoors, start tomato seeds 6 to 8 weeks before your region’s predicted Last Frost date. Transplant starts when nighttime temperatures predictably reach 50F (10C). When starting outdoors, plant tomato seeds a week or two after the threat of frost has passed. Always plant outside according to weather, and not the calendar.
However, it’s important to remember that these are only guidelines and not strict deadlines. If you start your tomatoes only four weeks ahead of the predicted Last Frost, your tomatoes still have a chance to ripen before the first frost of the year.
The tomato variety, intensity of light, and access to nutrients all affect how fast a tomato can grow. The length of your growing season also impacts how much wriggle room you have.
If you’ve got only a hundred days, then you have only a little bit of wriggle room. Your best bet is to buy or start seeds indoors.
If you’ve got more days, say 150 days, then you’ve got a lot more wiggle room. You could even save nursery space by starting them outdoors.
Above all, plant according to the weather and not according to the calendar. Calendars only give you an approximate date, which is handy when planning a garden, but nature has her own ideas.
With global warming, the weather has grown more unpredictable. Your last frost date may be weeks ahead of usual, or a killing frost can arrive a few weeks after you’ve planted your entire garden.
Keep an eye on the forecast and have sheets or row covers ready as a contingency plan. If the weather predictions seem to even get close to freezing temperatures, cover your cold sensitive plants.
If temperatures stay above freezing, your plants will be fine. But if they dip below, then you’ve just saved them. It pays to be cautious.
Ready? Let’s begin.
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
When To Plant Tomato Seeds
When Is The Best Time To Plant Tomato Seeds?
Indoors: start tomato seeds 6 – 8 weeks before the Last Frost date for your region. If you have a quick growing dwarf tomato, then you can start them up to 4 weeks before the Last Frost date.
Transplant seedlings a week or two after the last Frost Date (according to the weather), when nights stay consistently above 50F (10C). Tomatoes are sensitive to cold temperatures. If you plant them when temperatures drop below 50F (10C), cover them up at night to protect them.
Outdoors: start tomato seeds after the Last Frost date once the soil temperatures have reached 55F (13C). That’s the soil temperature, not the air temperature. Soil takes longer to warm up in the spring than soil does. You may also want to wait a week or two after the Last Frost date just to be sure that no unexpected frosts hit you. Keep an eye on the weather forecast.
How Do You Determine When Your Last Frost Date Is?
You can easily look up your first and last frost date with the Farmer’s Almanac, Dave’s Garden, or by downloading a seed starting calendar for your region from your local greenhouse or seed company. A seed starting calendar can also list when to start indoors, start outdoors, transplant, and cover for each vegetable.
The Farmer’s Almanac has a handy search bar to search by your City, State, Zip Code (United States) or Postal Code (Canada). The search works best when you use the abbreviation of the state (Missouri becomes MO) rather than the full state name. You can also browse their state and province directory.
So, if you’re in St Louis, Missouri, you’d search for St Louis, MO. The listing shows that the nearest climate station is Cahokia, IL with an altitude of 400 ft, a Last Spring Frost date of April 12, First Fall Frost as October 22, and that you can hope for a growing season of 192 days.
Underneath the table, it’ll list the frost dates probability – in St Louis, these dates have a 30% probability to happen.
You can get to the same page by scrolling down to the state listings, and clicking on Missouri, then St Louis.
If you’re in the US, you can also search by Zip Code on Dave’s Garden. Dave’s Garden shows data a bit differently. If you search for Zip code 63101, the listing tells you that you risk frost from October 31 through April 6, you’re almost guaranteed to get frost from November 17 to March 27, and you’re almost guaranteed not to get frost between April 17 through October 15. They say that you can hope for a growing season of 208 days.
This is handy information to have, so that you know that if you delay planting for a week, your garden is much less likely to suffer a killing frost.
Different sources may have different dates. There’s no exact method of predicting the last and first frost dates. Each source may use a different data source. The weather is changing as well, with more unexpected freezes or early thaws happening. You may get your last frost date earlier than later than usual.
Monitor the weather forecast when sowing or transplanting and/or wait a few weeks to plant to make sure all threat of frost is gone. A last frost date gives you a good idea for planning your garden, but you should plant according to the weather and not a calendar.
Hey – want to take my FREE 1-week seed starting email mini-course?
Learn about starting plants from seed – in just a few minutes per day.
What Is The Latest You Can Start Tomato Seeds?
How late you can still start tomato seeds and get a harvest depends on 4 things:
- The tomato variety’s days to maturity
- The length of your growing season
- The intensity of light the seedling receives
- The amount of nutrients available
Days to Maturity
Some tomatoes grow faster than others. Some tomato plants take an average of 110 days to grow, while others can grow and start producing fruit as early as 50 days (or even 42 days for dwarf varieties).
Days to maturity (DTM) tells you how long it takes for a plant to mature and begin producing vegetables, fruits, or flowers. Whether it’s days from sowing or from transplant depends on the plant as well.
Plants typically started indoors have a DTM starting from when they’re transplanted. In the tomato plant’s case, Days to Maturity is the number of days it takes for a tomato to ripen its first tomato since transplanting.
But like the predicted Last Frost date, days to maturity is just a prediction. Depending on your soil and light intensity, it could take shorter or longer.
But once you know how long one tomato plant needs to grow in your garden, you can adjust the DTM of other tomato varieties with a bit of math. If it takes 7 days longer for a 60 day tomato plant to mature, you simply put into your calculator 67 ÷ 60, which gives you 1.12. If another tomato variety lists 70 days to mature, then simply multiply 70 by 1.12 to figure out it’ll mature in around 78.4 days.
Growing Season Length
If you’re growing a tomato that takes 70 days to mature, and your growing season is only 100 days, then you know that you need a tomato ready to transplant by Day 20 at the latest (to give some time for more tomatoes to ripen). But if you have 150 days, then you could transplant up to 70 days after the Last Frost date.
Now, the longer a tomato plant has to produce and ripen, the more tomatoes you’ll get, especially if you’re growing an indeterminate tomato. These dates only show how close you can cut planting a transplant while still getting a few tomatoes – so long as it takes the average number of days to mature.
Tomato plants grow quicker the more light they get. If you’re starting a tomato plant in a window, then the seedling will grow slower than if it was grown under bright LED grow lights.
Likewise, a transplant in a full sun area will grow faster than one grown in a part shade area. How many sunny days you get and the length between sunrise and sunset will likewise affect a tomato plant.
So, if you’re in a rush to start tomato seeds indoors, use high quality LED grow lights. If that’s not an option, buy seedlings from a greenhouse.
Conversely, if your tomato plants grow to transplant size in 4 weeks instead of 6 because they’re under an intensely bright grow light, you’ll need to set your indoor start date back by 2 weeks.
Access To Nutrients
The amount of nutrients will also affect how fast a tomato grows. A tomato plant grown in fertile soil will grow faster than one in nutrient-poor soil. Tomatoes are hungry feeders. Once transplanted, they benefit from frequent topdressings of compost, worm castings, or liquid fertilizer.
What Happens If You Plant Tomato Seeds Too Early?
Tomatoes are heat-loving plants and hate the cold. If the night-time temperatures are too low, then tomatoes will wilt and turn brown, have stunted growth, and foliage necrosis (the leaf edge turns yellow or silver).
If soil temperatures are too low, then roots will struggle to grow and develop. Stunted root growth leads to stunted plant growth.
If temperatures drop below freezing, then the frost will likely kill your seedlings.
If you see a frost advisory, if night-time temperatures are expected to dip below 50F (10C), or if the forecast even looks close to dropping that low, always cover your tomatoes with a row cover or even an old sheet. A bit of cover to hold the heat in will help them survive the freeze. It pays to be cautious.
If you start tomato seeds indoors too early, then the tomato plant will outgrow its nursery pot and get rootbound. The fix is easy – just put them in a bigger pot. Starting them indoors too early will just mean they take up limited seed starting space for longer.
What Happens If You Plant Tomato Seeds Too Late?
If you plant tomato seeds too late, then they won’t have enough time to produce tomatoes before cold temperatures slow its growth and the first frost kills it. As the temperatures get colder, they’ll suffer the same problems as if you planted them too early.
A frost will destroy any tomatoes growing. In vineyards, grapes hit with frost are used to make the very sweet dessert Ice Wine. Tomatoes hit by frost also get very sweet, but they also get a very unpleasant sticky texture and they taste sickly sweet.
If you find yourself running out of time in the growing season, then either find a tomato variety that has a shorter date to maturity (such as dwarf or early tomatoes) and/or pick up a seedling or even a tomato bush from the greenhouse. You can also plant smaller varieties in a container to bring inside.
Like with planting too early, you can extend the growing season by covering up your tomatoes at night.
If your tomato plant has unripened tomatoes when expecting a frost, you have a few options:
- Learn to make green tomato chutney. Green tomato chutney is sweet and sour with a bit of heat. The blog Lovely Greens has a tasty recipe to try.
- Ripen tomatoes by leaving them in a sunbeam indoors. This is also a great way to extend the life of your tomatoes. Keep the green ones in a cool cupboard and set a few in the sun to ripen as needed.
- Cut the tomato plant off at the stem and hang the entire tomato plant upside down indoors. A lot more work and a lot more mess than simply leaving them in a sunbeam, but some people believe this funnels more nutrients into the ripening tomatoes.
The best time to start tomatoes indoors is 6 to 8 weeks before the predicted Last Frost Date, and transplant them outside when night-time temperatures stay above 50F (10C). But if you can’t make those dates, don’t worry, there are a few different ways to protect your plants and still get delicious tomatoes off the vine.
To find books, courses, seeds, gardening supplies, and more, check out The Shop at Greenupside!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.