Are you wondering how to harden off tomato plants? Is it really necessary to do? How long does it take to harden off the plants? I’ve got answers to all of your hardening off questions below, but the short response is yes, you do need to harden off indoor-grown seedlings before you move them outdoors. It isn’t hard to do and takes about a week. Keep reading to learn how to harden off tomato plants using my simple seven day schedule.
Why do you need to know how to harden off tomato plants?
I was just a teenager when I learned the importance of hardening off seedlings like tomato plants. As a new gardener, I was starting seeds indoors for the first time. I planted a few trays of vegetable, flower, and herb seeds and was growing them beside a window in the family dining room. I felt like a proud parent and, one sunny day in early May, I thought I’d do my seedlings a favor and take them outdoors for a few hours of direct sunlight. When I went to bring them back inside I discovered that all my seedlings had flopped over and many were bleached by the sun. Needless to say, none survived. Why? The reason is simple: I hadn’t hardened them off.
Hardening off indoor-grown seedlings is a step you can’t skip. It acclimatizes the young plants for the transition from indoor to outdoor growing conditions and essentially toughens them up. Seedlings started indoors under a grow light or in a sunny window have a pretty pampered life. They have plenty of light, regular moisture, a steady supply of food, and no weather to deal with. Once they’re moved outside they have to learn to not just survive, but thrive in bright sun, strong winds, and fluctuating temperatures. That lesson doesn’t happen overnight, and this is why gardeners need to learn how to harden off tomato plants.
How long does it take to harden off tomato plants?
The hardening off process takes about a week. Again, the goal is to slowly expose the tender seedlings to outdoor growing conditions. Hardening off thickens the cuticle and waxy layers on the leaves which protect the plants from UV light and reduce water loss in hot or windy weather. Failure to harden off tomato plants, as well as other indoor-grown seedlings like peppers, zinnias, and cabbages, leaves plants unprotected. This can result in the leaves being scalded by the bright sun or the plants wilting from moisture loss.
If, after the week of hardening off, the day and night temperatures are still cool and unsettled, you should put off your transplanting plans for another couple of days. It would be great to say that after seven days the young seedlings are set to go into the garden, but Mother Nature sometimes doesn’t play fair. You may need to adjust the amount of time it takes to properly harden off plants. You don’t want to go to all the trouble of growing tomatoes from seeds, hardening off the plants, and moving them to the garden to lose them to a late frost. Adjust your hardening off strategy to the weather.
Do you need to harden off tomato plants from a nursery?
Tomato plants bought from a nursery are generally hardened off and ready to be moved into the garden. If you buy them early in the season and they’re still growing in a heated greenhouse, it’s a good idea to ask the staff if the plants have been hardened off. In that case I would give the seedlings a couple of days outside on my sunny back deck to adjust before I moved them into my raised beds. Better safe than sorry!
When to harden off tomato plants
As spring temperatures begin to settle and the planting date nears, it’s time to start to think about hardening off tomato plants. Tomatoes are a warm season vegetable and won’t tolerant cool temperatures or frost. Don’t transplant the seedlings into garden beds or containers until the risk of frost has passed and day temperatures are above 60 F (15 C) and night temperatures are above 50 F (10 C). Don’t try and rush tomato seedlings into the garden! Cool season vegetables like cabbage and broccoli often adapt better to cool and inconsistent temperatures. Heat-loving crops like tomatoes and peppers are more susceptible to cold damage so proper hardening off and proper timing is essential.
I typically start the hardening off process around our last average frost date. I’m in zone 5B and my last average frost date is May 20th. That said, it’s not a guarantee there won’t be frost after that date passes. This is why I start the process around the last average frost date. By the time the seedlings are hardened off a week later, the weather should be fine for transplanting. Not sure what the last average frost date is in your region? Find out your last frost date by zip code.
Where to harden off tomato plants?
When talking how to harden off tomato plants we also need to discuss how to choose the best spot for this process. A site with shade is essential. I’ve hardened off seedlings in the shade of my house, alongside a garden shed, and even under patio furniture. I’ve also created shade by making a mini hoop tunnel and floating a length of shade cloth on wire hoops.
Keep in mind that the sun moves in the sky during the day and a spot fully shaded in mid-morning may be in full sun by lunch. You need a site with full shade for the first couple days of the hardening off process. You may find it more convenient to harden off tomato plants under a piece shade cloth floated on top of wire hoops. As noted above, I often use these quick DIY tunnels for this task. It only takes a few minutes to make one, but they make hardening off very easy. Make sure you select a piece of row cover that is long and wide enough to completely cover the tunnel, not just the top.
How to harden off tomato plants
I start my tomato seeds in cell packs and re-pot them into four-inch diameter pots as they grow. To maximize the space beneath my grow lights, I place the pots in 1020 trays. Having the pots of seedlings contained in trays also makes it easier to move them around when you’re hardening them off. Loose pots can blow over on windy days, potentially damaging the seedlings. If you don’t use trays, consider putting the pots in a box or tub to secure them. Another consideration is moisture. Water seedlings before you start hardening them off. Potting mix can dry out even in a shady spot on a cloudy day, particularly if it’s windy, so make sure your tomato plants are well irrigated.
To make hardening off easy, I’ve created a seven day schedule. Gradual exposure to light, wind, and weather is key and you’ll see that I recommend you bring your tomato plants back indoors the first few nights. This is important, especially if the night temperatures are cool. Tender plants, like tomatoes, are prone to cold injury. As noted above, don’t set tomatoes out until the night temperatures are above 50 F (10 C). If the temperature dips after planting, you can use row cover to insulate and protect plants.
How to harden off tomato plants: A seven day schedule
For the first day, choose a day where the temperatures are forecast to be over 60 F (15 C). Move your trays, pots, or cell packs of tomato seedlings outdoors. Check the soil moisture level to make sure the growing medium is moist. You don’t want the potting mix drying out and stressing the plants. Place them in a spot shaded from the sun. Leave them outdoors for a couple of hours and then bring them back indoors. If you’re not home during the day, you can leave them out in shade all day, but make sure it’s a spot that stays shaded.
Once again, move the plants outdoors (assuming the temperature is above 60 F), and place them in a spot with shade. Don’t worry about wind, unless it’s an extremely gusty day. A light breeze helps the plants acclimatize to being outdoors so that’s a good thing. Bring the plants back indoors after a half day in the shade.
Bring the tomato plants outdoors in the morning, moving them to a site where they will get an hour of morning sun. After the hour of sun is up, you can pop them beneath a shade cloth covered mini hoop tunnel or place them back in a shaded location. Bring the seedlings indoors in late afternoon or early evening before the temperature drops below 50 F (10 C).
It’s time to start introducing more sun to your tomato plants! Take the plants outside and give them 2 to 3 hours of morning sun. Provide shade from the intense afternoon sun. And check the soil to see if they need watering. Again, water-stressed seedlings are more prone to damage from weather. If the night time temperature is above 50 F (10 C), leave the plants outside in a sheltered site. I would add a layer of row cover overtop the seedlings for extra protection.
The spring shuffle continues! Move the plants outdoors giving them 4 to 5 hours of sun. You can leave them outdoors at night if the nighttime temperatures are above 50 F (10 C), but again consider covering them with a lightweight row cover just in case the temperature drops.
Continue to increase the amount of sunlight the plants receive each day. If the outdoor conditions turn cloudy or rainy at this stage in the hardening off process, you’ll likely need to add an extra day or two of acclimatizing time. Hardening off on overcast days can be a challenge. If it is sunny, give the plants a full day of sun, checking on them mid-day to make sure all is well and they don’t look wilted or show signs of stress. Water if necessary. Leave them outdoors overnight if the temperature is mild.
Day 7 is moving day for your tomato plants. If you were wondering how to harden off tomato plants when you started this article, you’re now a pro! As long as the weather is still mild and day and night temperatures don’t plunge, you can begin transplanting seedlings into vegetable garden beds or containers. I always keep row covers handy and usually set up a mini hoop tunnel covered in a piece of lightweight row cover overtop the bed. I leave this in place for the first week or two to further help my tomato plants settle in.
Before transplanting my tomato seedlings I work in some compost or aged manure and a slow release organic vegetable fertilizer. Also, be sure to plant tomatoes in garden beds or pots that are located in full sun.
Interested in learning more tomato growing tips? Be sure to check out these articles:
- Tomato growing secrets for a BIG harvest
- 22 Companion plants for tomatoes
- How often do you water tomato plants?
- When to harvest tomatoes for the best flavor
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