Maybe you started tomato seeds indoors this year, or perhaps you bought established seedlings at a local garden center. Either way, you probably want to know exactly when to transplant your seedlings into bigger pots and eventually outdoors.
So, when do you transplant tomato seedlings? Tomato seedlings can be transplanted into larger pots (repotted) after they have their first set of true leaves, when they are 2 to 4 inches (5.1 to 10.2 centimeters) tall. Tomato seedlings can be transplanted into the garden 5 to 7 weeks after planting from seed, or when they are 6 to 10 inches (15.2 to 25.4 centimeters) tall.
Of course, there are some steps to take along the way to ensure that your tomato seedlings have a healthy transition from indoors to outdoors when you transplant them. We’ll get into things like thinning seedlings, fertilizing, hardening off, and staking.
First, let’s start by looking a little closer at when to repot and transplant tomato seedlings.
When to Transplant Tomato Seedlings
Tomato seedlings can be transplanted more than once, depending on your situation (whether you start them from seed or buy them).
The first (optional) transplanting is done when you repot a tomato seedling, or move it into a larger pot. The second transplanting is done when you move the established tomato seedling from its pot into your garden.
Let’s start with transplanting tomato seedlings into larger pots.
When to Transplant Tomato Seedlings into Larger Pots (When to Repot Tomato Seedlings)
According to the University of Maryland Extension, tomato seedlings in small cells or containers should be transplanted into larger pots when they are 2 to 4 inches (5.1 to 10.2 centimeters) tall, once they have their first set of true leaves.
The first set of leaves on a tomato plant is made up of cotyledons, or embryonic leaves that feed the young plant. This is the set of leaves that appear when the tomato seed germinates.
The next set of leaves to appear is the first true set of leaves. This is what you are waiting for to repot your tomato seedlings into a larger pot.
When transplanting, you will need some extra potting mix for the larger individual containers. You will also want to avoid watering right before transplant.
This ensures that the soil will be lighter and less sticky, reducing the chance of damaging the plant.
This is no time to be impatient! Don’t repot your tomato seedlings too soon, or else you could damage the tiny, sensitive root system or the stem of the plant itself.
Do I Need To Repot My Tomato Plants?
No, you do not need to repot your tomato plants. However, if the container is too small, tomato seedlings may become root bound.
This will stunt their growth and delay their development. It also makes them more susceptible to diseases, and can lead to a poor harvest.
To avoid this problem and save yourself the trouble of repotting, start your tomato seeds in individual containers. Make sure that the containers are large enough to hold the tomato plants up until they are 5 to 7 weeks old.
Generally, a container that is 3 inches (7.6 centimeters) in diameter will be large enough to hold tomato seedlings until they can be transplanted into the garden.
When to Transplant Tomato Seedlings into Your Garden
According to the University of Minnesota Extension, tomato seedlings should be transplanted into your garden 5 to 7 weeks after planting seeds, or when they are about 6 to 10 inches (15.2 to 25.4 centimeters) tall.
Of course, you will need to pay attention to the weather when transplanting tomato seedlings into the garden.
According to the Old Farmer’s Almanac, you should transplant seedlings outdoors into the garden after the last spring frost, when the soil has warmed up.
To find the last spring frost date for your area, check out this frost date calculator from the Old Farmer’s Almanac.
As you can see, if you are starting from scratch, you will need to plan ahead to decide when to start your tomato seeds. Let’s go through an example to see how it works.
Where I live, the last spring frost date is May 8. The Old Farmer’s Almanac suggests starting tomato seeds indoors 6 to 8 weeks before the last spring frost date.
Working backwards 6 weeks (42 days) from May 8, I can see that the latest I should start my tomato seeds is March 27 (4 days in March, plus 30 days in April, plus 8 days in May = 42 days or 6 weeks).
Working backwards 8 weeks (56 days) from May 8, I can see that the latest I should start my tomato seeds is March 13 (18 days in March, plus 30 days in April, plus 8 days in May = 56 days or 8 weeks).
So, I should start my tomato seeds indoors between March 13 and March 27 so that my seedlings are ready to transplant after the last spring frost date on May 8.
Simply use the last spring frost date in your area and work backwards to get a general idea of when to start your seeds.
Remember that if you transplant your tomato seedlings outside too early, there is the danger of cold and frost damage. You can mitigate this to some extent with cloches or row cover to keep plants warm.
For more information, check out my article on protecting tomato plants from cold and frost.
On the other hand, if you wait too long to transplant, your tomato seedlings will become large and root bound in their pots, making transplant more difficult. In addition, your tomato plants may not have a long enough growing season in some areas.
So, pay attention to frost dates and transplanting timelines to ensure a good harvest!
Timeline for Tomato Seed Planting, Germination, Thinning, Repotting, Hardening Off, and Transplanting
To give you a more complete timeline for tomato plants, from seed to transplant, let’s take a look at what you should do at the various stages of plant development.
Starting Tomato Seeds (Day 1 – 8 weeks before last spring frost)
If starting from seed, you should plant your tomato seeds 5 to 7 weeks before the last spring frost date. This will give the seedlings enough time to grow before being transplanted outside.
Sow your tomato seeds 0.25 inches deep in sterile potting mix (purchased online or from a garden center). Using old potting mix or soil from outdoors can lead to damping off, which is caused by pathogens that will kill your tomato seeds and seedlings.
For more information, check out my article on damping off of seedlings.
If starting in a large tray without cells, plant your tomato seeds at a rate of 4 per inch (leaving the seeds 3 inches or 7.6 centimeters apart). These trays can make it much easier to transplant all of your plants at once.
For more information, check out my article on seed trays.
Tomato Seed Germination (Day 1 to Day 8 – 7 to 8 weeks before last spring frost)
The ideal soil temperature for tomato seed germination is 65 to 85 degrees Fahrenheit (18.3 to 29.4 degrees Celsius). At these soil temperatures, tomato seeds should germinate in 6 to 8 days.
For more information, check out this article from the University of California on soil temperature and time for seed germination.
Make sure to keep the soil moist (but not wet) for seed germination. Also, keep the air humid – a humidity dome can be helpful here, although it is not necessary.
For more information, check out my article on humidity domes and my article on how long tomato seeds take to germinate.
Thinning Tomato Seedlings (Day 14 – 6 weeks before last spring frost)
After your tomato seeds germinate, you may find that they are overcrowded, or that you have more than you can possibly grow in your garden.
Either way, it is a good idea to thin your tomato seedlings at this point. This will prevent overcrowding, which reduces competition between plants and makes it less likely that diseases will appear and spread.
Basically, you choose the healthiest tomato seedlings and leave them to grow, pulling up the rest and disposing of them.
For more information, check out my article on thinning seedlings.
Repotting Tomato Seedlings
You should repot your tomato seedlings when they are 2 to 4 inches (5.1 to 10.2 centimeters) tall. The time this occurs will vary depending on the tomato variety and growing conditions.
Hardening Off Tomato Seedlings (Day 42 to Day 56 – 0 to 2 weeks before last spring frost)
The transition from indoors to outdoors can be a shock for tomato seedlings (and for other plants as well). To avoid this, gardeners often “harden off” seedlings to help them gradually transition to the outdoor conditions: wind, sun, rain, insects, and diseases.
For about two weeks, put your tomato seedlings outside for longer periods of time. Start with an hour on the first day, and work up to a full day at the end of the 2nd week.
Start them off in indirect sunlight, and work your way up to partial and then full sunlight over the course of two weeks. Avoid putting them out in a storm, or if a frost threatens.
For more information, check out my article on hardening off seedlings.
Transplanting Tomato Seedlings (Day 56 – last spring frost)
You made it! Your patience is paying off, and it is time to permanently transplant your tomato seedlings into the garden.
Aim to transplant in the late afternoon on a cool, cloudy, calm day if possible. The last frost date should have passed already, and temperature should be consistently over 50 degrees Fahrenheit (10 degrees Celsius).
You can see my illustration of ideal temperature ranges for tomato plants here.
Before transplanting your seedlings, mix some compost and fertilizer into the soil where they will be planted. According to the University of New Hampshire, use 2.5 pounds of 10-10-10 per 100 square feet of garden.
Also, put in stakes (for taller, indeterminate tomato varieties) or cages (for shorter, determinate tomato varieties) before you plant the seedlings. This will prevent the stakes and cages from disturbing the roots of your plants.
For more information, check out my article on supporting tomato plants.
Now you have a good idea of when to repot tomato seedlings and when to transplant them into the garden. You also know when to thin seedlings and when to harden them off.
You might also be interested in reading about when you can direct sow tomato seeds – and when you might be better off starting seeds indoors.
You can learn more about everything you need to grow tomatoes in my article here.
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