Tomato plants should be planted outside after all danger of frost has passed, but that doesn't mean you can just plant them in the ground on the last frost date and walk away. Learn about hardening off tomato plants, and when and how to transplant tomato seedlings.
How and when to transplant tomato plants outside
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Whether you raised your tomato plants from seeds indoors, or you bought transplants at a garden center or nursery, you can't just plant them outdoors on the first nice day.
Not if you want your plants to live, that is.
Tomato plants (and other vegetable plants) are available at the stores and garden centers earlier than they should be planted outdoors. There are actually several things to consider before you can transplant them in your garden.
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Plant after your last frost date
First, you'll need to know your average last frost date. This varies according to where you live, and it is an average of the last spring frost dates in your area from years past.
But this isn't a guaranteed last frost date! You can still have an overnight frost two weeks after your average last frost date. Here's how to figure out when to plant your tomatoes outside.
To find your local frost dates, use the calculator at Dave's Garden and simply put your zip code into the box. The calculator will spit out your last spring frost date as well as your first fall frost date.
Tomato plants won't survive an overnight frost, so if you plant them too early they will simply die from the cold weather.
You'll need to wait to plant until after your last frost, and keep an eye on the extended forecast so you'll know if there's a late frost coming.
If you planted your plants too early, you can cover your tomato plants with upside-down buckets or cardboard boxes or even bedsheets on frosty nights and they will probably survive, depending on how cold it actually gets. Be sure to remove the covering in the morning.
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Hardening off tomato plants
Before planting, tomato plants should be hardened off.
"Hardening off plants" refers to the process of getting tomato plants used to being outside. It also applies to any other vegetable plants that you might have started indoors from seeds.
Moving to the harsh outdoors can kill your young vegetable plants in a single day. Cold weather, hot weather, bright sun, rain, wind and any number of other "normal" weather conditions are hard on a young plant that was raised in a protected environment.
So gardeners gradually get their plants used to being outside before they actually plant them.
Start by moving your potted tomato seedlings outside to a shady, protected area during the day, and bring them back inside at night.
After a few days in the shade, gradually move your plants into the sun, a little at a time. A spot where they get morning sun and afternoon shade is ideal. Just move them a bit every other day or so, so they are getting more sun and less shade.
This process should take 10-14 days; don't rush it.
Eventually you can leave the potted plants outdoors overnight as well.
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The difference between hybrid and heirloom tomatoes
When to transplant tomato seedlings
When my tomato plants have been hardened off and my last frost date has passed, I check the weather forecast to choose the best day to transplant my tomato seedlings.
Look at the overnight low temperatures and make sure the temperature won't dip below 35°F at the minimum.
Actually, tomato plants won't begin to thrive until the temperature is above 50°F. They can survive if it's above 35°, but they won't take off and grow for awhile yet.
Check the high temperatures too. I avoid planting my tomatoes right before a stretch of unseasonably warm spring weather. You know how crazy spring can be, right?
My ideal transplanting weather is several overcast, cool days in a row. This protects my plants from hot, direct sun and possible too-high temperatures.
While gentle spring showers for a few days is great, avoid a period of heavy rain and possible hail.
I transplant my seedling tomatoes in the evening, so that they can adapt to their new circumstances overnight. Especially if the forecast is for sunshine (but not too hot!) and you can't plant on an overcast day, this keeps the young plants out of direct sunlight for several hours and has worked really well for me.
In summary, transplant tomato plants when the overnight temperatures are over 35°F, although tomato plants are happiest when it's at least 50°F. Ideally, plant in the evening before a period of several overcast days.
How to transplant tomatoes
After your last frost date has passed, there is no frost predicted in the forecast, and you've hardened off your tomato plants, it's time to transplant them.
Whether you're planting in a raised garden bed or directly in the ground, the process is the same.
Tomato plants should be planted 18-24 inches apart. I know, they look so tiny when you first plant them, but they will spread out quickly. Tomato plants need airflow to be healthy, so don't plant them too close together.
I fill the planting hole with water and let it soak in before I add the plant. When the water has soaked in, I add a small handful of blood meal and bone meal to the hole to help the tomato plant get off to a good start.
Remove your tomato plant from its pot. Using a shovel or hand trowel, dig a hole deep enough so that the tomato plant's stem is buried deeper than it was in the pot, so that about 3-4 inches of the plant is above the ground.
Carefully remove any leaves from your tomato plant that might be underground when you are planting deeply.
If your plant is really tall, you can plant it sideways instead of digging a really deep hole. In other words, dig a trench rather than a deep hole, and lay the plant on its side with the top of the plant emerging from the trench.
Don't worry that the top of the plant isn't pointing straight up, after a few days the plant will have straightened itself up on its own.
Fill the hole or the trench with soil and tamp it down lightly with your hands to firm the soil around the plant and its roots.
Gently water your plant. I form a "moat" around the plant to hold water while it soaks in.
I water my newly-transplanted tomato plants daily for the first week. Don't let your plants dry out completely during this critical just-planted time, but don't over-water.
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Transplant shock
Sometimes your plants can look a bit wilted after transplanting. This is called transplant shock. Usually they will bounce back after a day or two, but an extreme case of transplant shock can kill your plants.
You can minimize transplant shock by hardening off your plants properly and not rushing to plant them too early. Transplanting them in the garden in the late afternoon or early evening on an overcast day is also beneficial.
Keep your plants well-watered after transplanting to avoid stress.
Full sun and high temperatures can also cause transplant shock. If needed, you can try shading your plants during the hottest part of the day.
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Add stakes or trellises
While tomato plants can grow and thrive without a trellis, they will sprawl all over your garden and take up a lot more space. Trellising, especially in a small garden, will save you space and keep the fruit out of the dirt.
Your plants will have better airflow and will be healthier too.
A wooden stake can be driven into the ground next to each plant, with the plant tied to the stake as it grows. It's easiest to install the stake at planting time to avoid disturbing the plants' roots later.
Trellises are another way to grow tomatoes vertically, and I think it's easier to install a trellis and to keep the tomato plants properly supported with a sturdy trellis.
Installing a trellis before you transplant your tomato plants is easier than doing it after planting.
You'll find more information on Tomato trellis ideas here.
How often to fertilize tomatoes
Tomatoes are heavy feeders, so they need to be fertilized regularly during the growing season.
Adding a handful of blood meal and bone meal in the planting hole is a good start. Diluted comfrey tea is a good addition at planting time too.
You can fertilize your tomato plants with diluted fish emulsion fertilizer every two weeks and add more blood meal every six weeks or so. Sprinkle blood meal on the surface of the soil and water it in.
Don't over fertilize though. It's better to apply too little fertilizer than too much.
Stop feeding your tomato plants when the tomato harvest begins.
When is it too late to plant tomatoes
Tomatoes need a long growing season, but all is not lost if your season is shorter, or if you didn't plant early in the spring.
If you weren't able to buy tomato transplants at the store or garden center, you can direct seed tomatoes in your garden or raised beds as long as the soil temperature is above 50°F.
You can measure soil temperature with a soil thermometer like this one.
I have volunteer tomato plants that sprout and grow every year near my compost pile. I transplant them into my raised beds and raise them along with the plants I started indoors from seed. They actually begin to produce at about the same time.
Cherry tomatoes need a shorter growing season than the larger tomato varieties, so they can be planted later than the larger varieties.
To figure out if you still have time to plant tomatoes in your garden, check the seed packet or information tag to see how many days to harvest are required for that variety. This is called "days to maturity."
Find your first average fall frost date at Dave's Garden, and count backwards that number of days to maturity to see if it's too late in your area.
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By using these tips and recommendations, your tomato plants should adapt and thrive in your garden. Enjoy your tomato harvest!
Related posts:
How to save tomato seeds from your garden
What to do with green tomatoes in the fall
Common terms and phrases for new gardeners
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