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How to Start Tomato Seeds Indoors and When to Plant Them Out

Lynn has been a professional organic farmer for the last 35 years and runs a 210-acre farm in Western Colorado with her husband.

Starting tomatoes indoors gives you the best chance of planting them out on time and getting a good crop.

Starting tomatoes indoors gives you the best chance of planting them out on time and getting a good crop.

3 Steps to Growing Perfect Tomatoes from Seed

So are you excited to get your tomato plants started but not sure when the best time is to get them going? Tomatoes can be picky—if you start them too early, your plants grow up leggy and weak, but start them too late and you won't get a crop at all.

On our farm in Western Colorado, we have 9,000 square feet of greenhouses and 11,000 square feet of outdoor gardens. You can imagine that we grow a lot of tomatoes. We've found that there are three simple steps to starting your tomatoes off right for the best growing success.

Find your area's average last frost date and count back 8 weeks on the calendar.

Find your area's average last frost date and count back 8 weeks on the calendar.

1. Sow Your Seeds Well Before Your Last Frost

Starting your tomato plants at the right time will keep them from getting too leggy and becoming weak and can get you the best harvest by getting them planted on time. Tomatoes do best if started eight weeks before your last frost date.

For example, I'm in zone 5, and our last average frost date is May 15. So if I count back eight weeks, that's around March 15th. I will plant my tomato seeds on March 15.

How Long Do Tomatoes Take to Germinate?

Your seeds will take about one week to germinate. Once germinated, I recommend growing them in a sunny window or under grow lights for about three weeks.

Around three weeks, they will be ready to transplant. Once your tomato plants get their first adult leaves, put one plant per 3-inch pot, and continue to grow them under lights or in a very sunny window for another three weeks.

Now we are around week 7 into the tomatoes growing, and it's time for the next step towards success, which is getting the plants ready to go into the garden.

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Read More From Dengarden

Hardening off your plants before planting them out in your garden is critical.

Hardening off your plants before planting them out in your garden is critical.

Step 2: Harden Off Your Plants

Tomato plants are just like most of us in the spring; when we go out into the sun for too long, we get sunburned, and it's the same for tomato plants. But if you expose them to a little sun and a little wind each day, they will be much stronger and more sun- and wind-tolerant when you plant them into the garden.

I've seen tomato plants that have been put into the garden without hardening off, and they turn white and die within a few days.

How to Harden Off Tomato Plants

Here's how I harden off my plants before they go out into the garden.

  • Days 1 and 2: Make sure that they are watered, then put them in a wind-protected spot in the sun for two hours. Then put them back into their normal growing location.
  • Days 3 and 4: Make sure they are watered, then put them in a spot with full sun and a gentle breeze for about four hours. Then put them back into their normal growing place.
  • Days 5 and 6: Make sure they are well watered, then put them in a spot where they can get six hours of sunlight and some more wind. (If you have warm nights, you can go ahead and leave them outside overnight. Just make sure that your temperatures are above freezing.)
  • Day 7: Your seedlings are ready to plant into the garden! Plant them out and put up a shade and wind block on the west side of the plant for about a week. That's going to cut a little sun and a little wind and protect them a little bit.
Cover your plants with blankets if a cold front blows in.

Cover your plants with blankets if a cold front blows in.

Step 3: Protect Your Plants From Frost

Once your seedlings are outside, you will need to protect them from frost. Just because you're past your average last frost doesn't mean that a cold front might not come in and freeze your plants, so you'll need to cover your tomato plants if a frost is predicted.

My general rule of thumb is if it's predicted to go below 36˚F or 2˚C, I cover the plants with a blanket or frost blanket for the night. I make sure that the blanket is up off the plants, and if it's predicted to rain or snow, I go ahead and cover the blanket with a sheet of plastic to keep the blanket dry.

Watch the Forecast for a Month After Planting

I usually watch the forecast for about four weeks after I plant my tomatoes out to be sure they will not get frozen. So doing these three steps (planting the seeds at the right time, hardening the tomatoes before they are planted outside, and protecting them from frost) will get your tomato patch off to a really great start.

More Tomato-Growing Tips

© 2022 Lynn Gillespie

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