The Hardening Off Schedule That Guarantees Strong Tomato Plants

When you take the time to harden off tomato seedlings before you plant them out, you ensure the healthiest growth and epic yields when harvest time arrives. There are multiple ways to do this, but setting a schedule for yourself and your seedlings every year makes it easy to keep up with. 

Hardening off seedlings is especially important when you live in a region that has cold winters. Getting the timing right on planting your seedlings benefits the garden in the long run. The same goes for growers in areas with hot summers. You need to get those tomatoes in at the right time to get a good spring harvest before flower and fruit set cease for the hot season.  

One note before we get into it: it’s good to know your last frost date and your season length before you decide when to plant out your tomatoes. This helps you determine which varieties are suitable for your climate. For those with longer seasons, longer maturation rates should be fine. In shorter seasons, look for cherry tomatoes or those with fewer days to maturity.

Heirloom Tomato Seed Bundles

Heirloom Tomato Seed Bundles

Cherry Falls Bush Cherry Tomato

Cherry Falls Bush Cherry Tomato Seeds

Fortamino Rootstock Tomato

Fortamino Rootstock Tomato Seeds

Tomato Temperature Tolerance

Above 85°F, fruit production slows.

In general, tomatoes need soil temperatures of at least 60°F (16°C) to germinate. Once they’re planted out, they need ambient temperatures between 55 and 75°F (13 and 24°C). This is the best range for adequate fruit set. 

Once temperatures rise above 85°F (29°C), flowering and fruiting slow. Around 90°F (32°C), it almost stops. In regions with a mild fall and spring, but a hot summer that typically stays above 90°F (32°C), expect a break in the season that commences when mild autumn temps arrive. 

Seedling Protection

Keep tender seedlings protected early on.

As you harden off your seedlings, you’re getting them used to the elements. If you live somewhere (like me) with intermittent frosts that occur after your frost date, it’s nice to have some cover handy in case of snap freezes. 

For instance, I just planted my tomatoes and tomatillos, and we’re expecting a few nights of 30 to 40°F (-1 to 4°C). I’ll move the seedlings I can under cover during those nights to keep them safe and plant them out after the potential for frost has passed.

There are multiple ways to do this. If you have a raised bed with a cold frame cover, you can simply place your tomato seedlings within on cold days. Or you can plant them under the cold frame cover and remove it when conditions are suitable. You could also gently cover the seedlings with a frost cloth. 

One easy way to deal with snap frosts is to move your seedlings indoors on those days. While you’re not leaving them outdoors for an entire day at the start, it’s nice to have something to keep your seedlings safe in case of wild weather. 

Tomato Hardening Off Schedule

Now, let’s discuss the best schedule to follow to harden off tomato seedlings. The principle behind hardening off is very simple, and it just requires your attention for success. 

Days 1 Through 7 

Keep them in a spot close by so you don’t forget them.

In this first week, you’re giving your seedlings a little dose of what it’s like to be outdoors. Start by finding a good spot that gives them a stable place to sit. I like to keep my hardening off spot close to my back door so I can easily access them if something unexpected happens with the weather. 

When you harden off tomato seedlings, the ideal location for doing so has indirect sunlight. If you’ve been working with a really good grow light indoors, you can get away with full sun. As the days go on, you can increase their access to sunlight. 

Here’s where the schedule begins. On day one, place your seedlings outdoors for two hours in the mildest part of the day, and then bring them in until the next day. Increase by two hours each day on either side of that mild period. This gradually adjusts their tolerance to the wind, sunlight, and outdoor temperature fluctuations. 

As you increase the time they’re outdoors, increase the amount of light they’re taking in if you haven’t already placed them in full sunlight. By the end of this first week, you’ve managed to have them outdoors for an entire day. By this point, the tomato seedlings should be well-adjusted to the elements. 

Days 7 Through 14 

Check their growth to determine the next steps.

In the second week of hardening off, you have two options. You can increase the timing, allowing the tomatoes to experience nighttime temperatures little by little, or you can simply leave them out in their hardening off spot. If your seedlings have multiple sets of true leaves, you can fertilize them with a gentle, diluted, liquid organic fertilizer. 

Don’t forget to water them, and prune away any damaged areas as a result of contact with intense weather or curious mammals and pests. Definitely scout for these on a regular basis. One thing I like to do in this second week is to place the starter pot in the location where I’ll plant the tomatoes. 

This gives young tomatoes an indication of what conditions will be like when they’re planted in their container or garden bed. Once you’ve made it through this second week, you can plant your tomatoes in this spot. 

Planting Out 

Plant the seedling deeply to expand the root system.

If you’re working with indeterminate tomatoes, remember to provide support in the form of a trellis or tomato cage either at the time of planting or shortly thereafter. While determinate tomatoes tend to be bushy, it’s nice to have some support handy for them as they get loaded up with tomatoes around the same time. 

You can plant your tomato at the level it was growing in its starter pot. You can also plant it deeper or almost on its side to improve lower root development and give it a strong base. Note that planting the tomato deeper will defer energy to root development rather than flowering and fruiting. However, gardeners in areas with high winds will benefit from this practice. 

Once you’ve planted your tomatoes in their final growing spot, you can fertilize them with tomato-specific fertilizers. Use an organic one, and dilute it with water. I like to apply this fertilizer every week or so when fruit production is at an all-time high. Then, enjoy the tomatoes that grow!

Key Takeaways

Using a schedule to harden off tomato seedlings is a great way to systematize the most sensitive phase of tomato growth. This ensures the plants make it through to the production phase and that you get a chance to enjoy the fruits of your labor. 

As you’re getting everything together, remember the following:

• Keep the adjustments gradual, and consistently expose the seedlings to more sunlight, wind, and the elements every day. 
• Do this over two weeks, keeping them in a good, stable position, preferably close to your home so you can respond if something happens. 
• After the first week, keep a close eye on them as they continue to adjust. At this point, you can place your seedling container in the location where the tomato will eventually be planted. 
• Keep a cover handy if you live somewhere that snap freezes occur. This ensures the plants are protected from frost. 
• Plant out tomatoes with supports, and don’t forget to water and fertilize them as they’re hardening off.