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The day after I posted Tomatoes – Starting Your Own is Easy, I received several emails from gardeners fairly new to seed starting asking me how to harden off seedlings.

When plants are sown into the garden under weather conditions in which they thrive, they start acclimating themselves to the outside world the minute they poke out of the soil.

In order to get the longest production possible from heat lovers like tomatoes and peppers, we have to give them a head start inside under artificial conditions. We don’t even think of putting them into the “real world” until temperatures are much warmer.

These pampered seedlings must have a transition period to become accustomed to the world of wind, sun and rain. A chance if you will — to become more “thick skinned”.  And that is exactly what happens in the hardening off process.  The plant’s cuticle (skin)  becomes thicker.

The time it takes to harden off will depend on the plant, the temperature and the fluctuations in temperature. Generally, the transition can be made in about 7 days.

The key word to remember is “gradual exposure to the elements“.

If you don’t harden off your seedlings you stand a good chance of loosing all your invested time and effort.

You can do a couple of things from day one of their emergence to help the hardening off process.

• Air movement (a fan) where your seedlings are located inside really makes a difference.
• If you have been brushing your seedlings with your hand once or twice a day, this too will have helped make them thicker.

If you haven’t done these two things, give your seedlings a day or two of this before proceeding to the great outdoors.

Two important things to keep in mind before you begin hardening off outside.

• Your plants will still need protection from cold, strong wind, and driving rain.

• When you water your seedlings, water them well and then let them almost dry out before you water again.

Guidelines for Hardening Off

The only way to learn the hardening off process is to jump in and do it. It’s not an exact science. But there are some simple guide lines you can use.

• Place your plants in a sheltered, shady spot outdoors out of direct sun light.  (Maybe under a tree or on your porch.)   Keep an eye on them for any signs of stress like wilting or drooping.  If things appear to be ok, leave them out for about 3 or 4 hours. Day 2 through 5,  add 1 or 2 hours to the time you leave them outside.  Still keeping an eye on them just in case.

• About the 4th or 5th day, place them where they will get direct sun for a few hours.  You might want to expose them to morning sunlight first, rather than hot afternoon sun.  Return them to the shade in the afternoon. Each day increase the exposure to sun.

• By the 7th day they should be able to handle sun all day.

They should be able to handle nighttime temperatures without problem after 10 days. Of course, you’ll have protect against any freezing temperatures. And they’ll still need your protection again high winds and sudden downpours until they are large enough to fend for themselves.

(My tomatoes seedlings – 5 weeks old and 3 inches tall – went through their first hard downpour today.  They passed the test with flying colors!)

Encountering Mishaps

If during this process you encounter some mishaps – like some of the leaves of your plants turning white and drying up (sunscald) — there is still hope.  Just take them in and give them some time to recover and start over.  It won’t hurt to loose a few leaves.  Look for new growth.  That’s what’s important.

I’ve seen lettuce in a flat totally wilt from the stress of too much rain or sun and then have some of it make a comeback when moved to cooler more desirable conditions.  So don’t throw in the towel until you know for sure that all is lost.

There are too many variables for this to be an exact science. Just use your best judgment.  You’ll have to experience it to learn.

A Cold Frame

Using a cold frame or some type of covering (it doesn’t have to be fancy) can make the process a bit easier, quicker and you won’t have to “hold their hand” so much.

I put a rock or brick under the cold frame to make sure my plants get the outside air.

Follow the guidelines above.  But on day 3, after the seedlings have been out in the air for the allotted time, place them in a cold frame. For the remainder of the day, make sure the cold frame is open enough that air can get in and circulate. If temperatures are not forecast to dip low leave them in the cold frame all night.  Make sure it’s closed.

(Here you have to make a judgment call.  My tomato seedlings were out of the house and into the cold frame after they reached the ripe old age of 2 days old at the end of February.  Temperatures at night ranged from about 39 to 50.  If you want to be more cautious, go with 50 to 60 degrees.)

Two day old tomato seedlings.

Another Judgment Call

Some seemingly pretty knowledgeable gardeners say tomato seedlings subjected to low temperature (like mine in the example above) will never grow and produce to their potential in the garden. The only thing I can tell you is that over the years I have subjected tomato seedlings to some pretty horrible conditions.  (I’m not suggesting that you do that.) In spite of it, I have always had bumper crops of tomatoes for the past 33 years. So – it’s your call.

Transplanting to the Garden

Once your plants are hardened off, you can transplant to the garden.

Helps for Young Seedlings in the Garden

• You might still watch for low temperatures and severe weather and be prepared to give them some protection with a cold frame.  (Even a gallon jug with the bottom cut out and the top off makes a great mini-coldframe.) Once they’ve grown some this won’t be necessary.

• If that doesn’t work for you, pull your straw mulch up very close to the plants.  Maybe even lightly sprinkle some over the plants.  Works for me all the time when I don’t want to take a cold frame back to the garden.

I removed the top and cut the bottom out of this plastic gallon jug and fit it snugly over the seedling until the weather warms.

Final Words

Now go out there and harden off those plants!  It’s time! 

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