Growing Tomatoes From Seed. It's Easy!

Growing tomatoes from seed is a great way to get tomato seedlings for your backyard garden.

And learning how to plant tomato seeds is easy.

Most of the general information you need for starting seeds is available on another page of this website. If you haven’t read that page yet, I’d recommend starting there for detailed instructions on starting seeds indoors.

But each crop has its own unique quirks and preferences, and tomatoes are no exception. So on this page, I’ll fill you in on those specifics about growing tomatoes from seed.

 When to Plant Tomato Seeds

Deciding when to plant any crop can be a tough decision because of the many variables you have to weigh. But perhaps the decision is just a bit tougher for tomatoes.

Why?

Well, let’s fact it: tomatoes are among the most popular of backyard garden crops. And you’re really looking forward to your first bite of that first juicy, summer savory tomato that you pick out of your garden, aren’t you?

But the longer you wait to start your tomato seeds, the longer the wait for that first tomato!

Unfortunately, though, tomatoes are highly sensitive to frost and freeze damage.

So you’ll either play it safe, and plant your tomato seed late enough that there’s little risk of frost by the time the tomato seedlings are ready to plant, or do some gambling and plant early.

Nothing wrong with either strategy, but only you can decide which you’re comfortable with.

If you don’t mind going to the trouble of growing tomatoes from seed in two different batches, you might want to plant an early batch and a late batch. Play it safe AND gamble.

Either way, though, you’ll want to be prepared to protect your plants from a cold snap.

Once you know your last frost date, and have decided upon a target date for transplanting your seedlings, counting back about six weeks from that date will give you the date to start your seeds.  

Heat Requirements For Growing Tomatoes From Seed 

The ideal temperature for germinating tomato seeds is 78 degrees F. If you’re using a seedling heat mat (recommended), then you’ll be able to maintain the temperature precisely where you want it.

Otherwise, find an area where you can approximate the desired 78 degrees as closely as possible.

Be careful, though, not to let the seeds get too warm. Temperatures higher than 85 degrees F can actually kill the seeds.

And temperatures below 72 degrees F will substantially delay the germination of the seeds, and slow the rate of initial growth.

Maintain the target temperature of 78 degrees (or as close as possible) for 1 week. By that time, all of the seeds should have germinated if the temperature has been close to the target temperature. (Or at least, all of the seeds that are going to germinate will have germinated. There will always be some that don’t germinate, through no fault of your own. That’s the reason to always plant extra.)

After that, maintain the daytime temperature at 78 F, but lower the nighttime temperature to around 62 – 64. Having a lower nighttime temperature will help prevent the tomato plants from becoming ‘leggy.’ The ideal growth characteristic for tomato seedlings is for them to be at least as wide as they are tall.

Now, in case you’re thinking that you won’t possibly be able to control the temperature for your seedlings with such precision – don’t worry.

The temperature recommendations I’ve given you are the ideal to shoot for. And if you’re using a heating mat, you should be able to come very close to these ideal temperatures.

But as long as you don’t let the seedlings get too hot, or too cold, they’ll do fine. They may not grow at quite the optimum rate, but they’ll be OK.

Do your best, though, not to let the temperature fall much below 60 F for an extended period. Otherwise, you likely will have some problems. One indication that the plants are getting too cold is a bit of a purplish hue to the leaves.

Light Requirements For Growing Tomatoes From Seed 

The light requirements for tomatoes is pretty simple. They don’t need any light at all to germinate, so cover the seed completely when you plant it (about ¼ “ deep). Once the seedlings have emerged, 16-18 hours of light each day is ideal.

If you’re not using grow lights, you probably won’t be able to provide 16-18 hours of light each day, and that’s OK. As with the temperature, just come as close as you can to the ideal.

But be sure not to grow the seedlings in a location where they will receive only partial sun. If you’re not using grow lights, your seedlings need every minute of the natural light that is available each day. 

Transferring to Larger Containers 

If you didn’t start your seeds in the containers that they will grow in until transplanting, plan on transferring the seedling from the initial container or flat to a larger container before the leaves of the seedlings are crowding each other.

Once a seedling’s leaves begin to touch those of its neighbors, it will go into a competitive growth mode. It will begin stretching for light, resulting in an undesirable, spindly growth characteristic.  

Seedling Fertilizer For Tomatoes

Tomato seedlings don’t need a lot of fertilizer. And they don’t need any at all until after they’ve germinated.

Probably the best thing would be to buy a fertilizer blended for tomatoes, and follow the label directions for mixing for seedlings. The label should also give directions for frequency of application.

Generally, though, a commercial blend mixed at half strength and applied once per week will be suitable for tomato seedlings.

Fertilizers mixed specifically for tomatoes are available in both organic and synthetic forms.  

Don’t Forget Hardening Off

After you’ve gone to the work of growing tomatoes from seed, and carefully nurturing your seedlings for so many weeks, be sure to finish the job of getting your seedlings ready for the garden by hardening them off.

It’s the last step before transplanting your seedlings, and it’s important!  

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