Since tomato plants are often started from seed indoors, there comes a time when they must make the journey to the outdoors. This process is known as ‘hardening off,’ and it is critical to do it properly.
In this article, I’ll cover the basics of hardening off tomato plants, including some key tips and tricks to avoid damaging your plants. I’ll go over when to start hardening off, how to avoid sun scald, pest prevention, and more.
What is Hardening Off?
So what is hardening off exactly? It is actually a process that we go through with many plant types that are started from seed indoors, including tomatoes, peppers, eggplants and others.
Hardening off is the process of gradually adjusting an indoor plant to the outdoors before transplanting permanently outside. This slow acclimation to direct sunlight, wind, rain, and temperature fluctuations is very important to avoid damaging your tender tomato plants.
Why bother hardening off?
It can be tempting to simply put your tomatoes outside right into direct sunlight. You should never do this, especially if you started your plants from seed yourself.
The result will usually be sun scald, which is essentially a sunburn on your plant’s leaves. These leaves will harden and eventually die and fall off. This can set back your plant’s progress by weeks!
So yes, hardening off is pretty important for tomatoes. Thankfully, the process is very simple. It just requires a bit of patience and a little extra time for moving the plants around for a few weeks.
When to Start Hardening Off Tomatoes
With a basic understanding of what hardening off is, next is figuring out when you should begin the process with your tomatoes. The timing will vary based on your location, but it is largely temperature dependent.
If temperature weren’t an issue, you wouldn’t have to start your tomato seeds indoors. Starting early gives the plants a month or so to get a head start on the season, leaving more room for the plants to grow to maturity and be productive.
Outdoor temperatures will dictate when you begin hardening off tomato plants. If you planted your seeds at the right time (4-6 weeks before the last frost), you’ll be able to begin hardening the plants when they are about 1 month old.
Around this time, outdoor temperatures should begin to creep up into the high 60s and low 70s (°F), which is perfect for tomatoes. Once you have warmer outdoor temps, you can begin to bring your young tomato plants outdoors during the day.
In general, I begin hardening off my plants about 2-3 weeks before temperatures are consistently above 55°F (13°C) at night. This is enough time to adjust the plants to handle all of the outdoor elements.
If you see that the temperatures will dip below 55°F during any night, be sure to bring your tomatoes indoors for the night. In the morning, once the temperatures are back up, you can continue hardening off.
After temperature, sun exposure is the next most important factor to consider during hardening off tomatoes. Direct sun exposes your plants to radiation that indoor grow lights and filtered sunlight don’t.
For this reason, I always recommend choosing a cloudy day for your first few days of hardening off. Filtered sunlight is much more gentle, and will decrease the chances of getting sun scald on your tomato leaves.
If you live in a location without many cloudy days, place your plants in a shaded location. While in the shade, the tomatoes will still get plenty of light, but will also be adjusting to the natural winds and changes in temperature outside.
In addition to temperature and sunlight, there are a few other things to keep in mind during hardening off.
Animals like to eat young tomato foliage. For this reason, always keep your tomatoes up off of the ground on a bench or table while they adjust. This will prevent rodents and rabbits from reaching your vulnerable plants while they harden off.
Wind can be a problem for early tomato plants. Young tomatoes are not very sturdy, so I recommend avoiding especially windy days for the first week of hardening off. You can also stake your tomatoes with a small stake to keep them upright while they are young.
How to Harden Off Tomato Plants
If it isn’t already obvious enough, hardening off involves patience and close attention to the weather. Tender tomato seedlings shouldn’t just be thrown out into the sun for 12 hours without a proper hardening period!
Hardening Off Tomatoes (Steps):
- Choose a cloudy, warm day to start.
On the first 2-3 days of hardening off, choose cloudy days. Alternatively, keep the plants in a shady spot (under a tree, etc.) to avoid direct sunlight. After this break-in period, you can begin gradually exposing the plants to more sunlight.
- Avoid cold temperatures.
Typically, I start hardening off my tomato plants around 2-3 weeks before the outdoor temperatures are consistently above 55°F. During the day, temperatures should ideally be around 70-80°F while the plants adjust.
- Increase sun exposure daily.
After the first few days of shading the plants, move them into the sun for about 20-30 minutes. Then, move the plants back to the shade. If the leaves begin to wilt, just move them into shade until they perk back up. The next day, increase sun exposure by another 20 minutes or so, watching for signs of stress.
- After 2-3 weeks, move outside permanently.
After increasing sun exposure daily for 2-3 weeks, your plants should be ready to stay in a permanent location. As long as nighttime temperatures will be above 55°F, you should be safe to move the tomatoes outside for the season.
Tips for Hardening Off Tomato Plants
With the basics covered, I’d like to share a few extra tips that can help during the hardening off phase. In certain circumstances, these have helped to save an otherwise doomed tomato plant!
Use Shade Cloth
If you plan to move your plants to a full-sun location, you should also be hardening off in a full-sun location. However, if you live in a different place than where your garden is located, it may be impossible to truly harden the plants off before bringing them to their final planting spot.
I used to live in an apartment, but my in-ground beds were located at a community garden plot across town. Therefore, my plants hardened off in partial sun, but then they needed to be moved into full-sun. This sudden change can lead to sun scald.
So, as a solution, I used row-cover fabric to keep the plants partially shaded after being transplanted. This extends the hardening off period, even after the plants have been put in the ground. After a week or two, the shade cloth can be removed to expose the tomatoes to full-sun.
Tip: Shade cloth is also great for protecting your tomatoes from animals (rabbits & mice) and from unexpectedly cold weather overnight.
As I mentioned earlier, staking can help protect your tomatoes form wind. I highly recommend staking the plants as soon as they go outside to avoid broken stems or branches. I have seen it happen too many times!
If your plants are still very small, you can use a small stick or food skewers to keep them propped up. These will suffice until the plants are in the ground, where they can be supported with a larger stake.
Tip: I like to use garden velcro – it is easy to use and gentle on the plant stems.
While your tomatoes are outdoors, they will likely use significantly more water than while indoors. This is because wind causes more water to transpire through the leaves and evaporate from the soil. The higher heat and direct sunlight can add to this as well.
Keep a close eye for wilted-looking leaves and feel the weight of each pot to check if the plants are thirsty. Try to keep them evenly watered and to never let them dry out.
Once your tomato plants are fully hardened off, they will be well on their way to growing tall and producing lots of fruits. Never rush the process of hardening off tomatoes – it is always best for the plants to transition slowly, so your patience will pay off!