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When To Start Tomato Seeds: Do’s and Don’ts

If you know when to start tomato seeds indoors, you can grow your own tomato seedlings and save money. You can also choose tomato varieties that may not be available at your local garden center. And in the process, you may find you have extra tomato seedlings to share with family and friends.

Tomatoes don’t thrive when they’re directly sown into garden soil, as do radishes or green beans or spinach. They need a bit of TLC at the beginning of their lives. Meaning a smaller container (versus the entire garden), consistent warmth, regular watering, and protection from the elements like wind.

Once they’ve got several sets of leaves, tomato seedlings are ready to expand their root systems and put on lots of foliage. That’s why tomatoes do best when they’re started ahead of time and can strengthen before moving to their summer home in your garden plot or in a patio container.

When should you start tomato seeds in your specific area to get the best result for your garden and your crop?

Fortunately, other home gardeners have had some experience in answering the question. Here are some helpful Do’s and Don’ts about when to start tomato seeds.

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When to start tomato seeds: Do's

1. Do choose tomato seed varieties that are most suitable for your area

Cultivars that flourish well in the short, cool season in Ottawa, Canada will struggle in the heat of Tucson, AZ.

2. Do read the seed packet

It will tell you how much time to allow for seeds to grow indoors before hardening them off and setting them in the garden. Some varieties are ready as soon as 4-6 weeks. Most take 6-8 weeks indoors before they’re ready for their new home outdoors. Follow the vendor’s advice.

3. Do check identify the last frost date in your area

Follow your local garden extension’s recommendation. You can also view the freeze/frost information provided by the National Climatic Data Center. 

4. Do study the calendar

Count back from the last frost date for your region to determine when to start your tomatoes.

5. Do treat the last frost date as a guideline

Tomatoes like to be warm, so if the extended forecast is cool and wet then err on the side of waiting a little longer to start your tomato seeds.

When to start tomato seeds: Don’ts

6. Don’t sow tomato seeds directly in the garden

Start tomato seeds indoors or in a greenhouse. You’ll avoid late spring frosts. And you’ll help seedlings be strong enough to withstand transplanting and the inevitable weather swings that accompany early season gardening. The exception for this rule is if you live in a very stable, warm climate with a long growing season.

7. Don’t mistake earlier seed starting for earlier crops

“Even though a lot of gardeners still like to start very early,” say Jakob and Anna, suburban home gardeners with Northern Homestead in Alberta, Canada, “Starting seeds indoors 3 to 4 months before the last frost day in most cases does not result in earlier and healthier tomatoes.”

8. Don’t start seeds in cell trays if you can use 3-inch pots

Tomatoes grow fast. Once they sprout, they produce a couple of sets of leaves. And they put out roots. Lots of them. And there are enough leaves for the size of the pot and the size of the plant to send a signal to the flower buds that it’s time to get blooming. They get leggy, so you need to re-pot them before it's time to set them in the garden. That piles a shocker to their little systems which sets back growth. You can avoid that delay by using 3-inch pots or larger when you start tomato seeds. 

9. Don’t consider indoor flowering a good thing

As Ms. Tomato fills its seed cell with roots and pushes out more leaves and branches, it thinks, “I’ve got to get flowering. If I don’t, then I won’t make any fruit and I won’t have any seeds, so I won’t reproduce.” Soon the plant has buds all over the place and it’s still a couple of weeks before you need to set it in the garden.

You may be excited. But the plant? It’s desperate. And all of its energy is going into flowers.  It will be in for a shock when you put it in the garden. And shocked plants take a while to recover. 

So while starting your plants and getting them to the bloom stage seems like a smart plan for an early crop and bigger tomatoes, it’s not. Plant seeds accordingly.

10. Don’t start determinates too early

A determinate tomato plant produces fruit for a couple of weeks and then production fades out. Meanwhile, indeterminates – also called “vining tomatoes” – continue to grow and produce all season. In fact, with the proper care indeterminates can be held over season to season and treated as perennials.

Meanwhile, dwarf tomato varieties are used to having their roots limited to a smaller area. They’re often grown in pots. 

So if you absolutely must start tomato seeds early, choose a dwarf variety (and move it to its final container before setting it outside) or an indeterminate. 

A final word about when to start tomato seeds

There are plenty of other important tips to follow when growing tomato seedlings, including choosing a sunny spot … keeping the soil moist … preventing drafts.  

But remember the key point about when to start your tomato seeds: read the seed packet. Follow the vendor’s timetable.

Your tomato seedling is wired with a single goal: to put out seeds. To put out seeds, it needs to produce fruit. So when to start tomato seeds indoors isn’t so much about getting the seedlings going ahead of time. Rather, the main question you need to answer is, “What can I do to give those plants the best chance for producing fruit?”

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