By Steve Reiners, Professor and Chair, Horticulture Section, School of Integrative Plant Science, Cornell University, Cornell AgriTech. 
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May 25, 2021

Every gardener has experienced it.  Transplant shock.  You take a plant that has been growing in a protected environment, perhaps in a greenhouse or under lights in your basement.  The temperature has been optimum as has the soil moisture.  The plants never wilt and there is likely some plant food in the growing media. There is no wind. And the light, although bright enough to grow the plants, is a fraction of what sunlight provides.

Now take that happy, protected plant and put it in your garden.  The sun beats down on it and the thin waxy cuticle on the leaf can’t protect it.  It’s hot during the day and cool at night.  The plant has a hard time keeping up with its water needs and may wilt a bit.  The stem bends from the wind and can even break. 

With all this stress, your precious pepper or tomato plant goes into survival mode.  To save moisture, it closes the stomata on the leaves, the small pores through which water transpires. Instead of growing bigger, it diverts resources to develop a thicker waxy cuticle to protect the leaves. It goes nearly dormant as it struggles to begin growing in its new environment.

The water that transpires through the leaves helps to cool them. With the stomata closed to limit water loss, the leaves can overheat and photosynthesis is severely reduced. The leaves may yellow, brown, and even turn white, as chlorophyll is reduced by the harsh conditions. 

In time, the plants usually recover.  You will see new growth and a greening up of the leaves. But there is a cost. The check in growth delays maturity. But there are steps you take to minimize transplant shock.

Facing up to the cruel world

We normally recommend that you ”harden off” your transplants before planting them outside.  This means acclimating the plants gradually from growing in a protected environment to the harsh realities of the outside world.  Just like you wouldn’t spend all day on a sunny Caribbean beach in March after spending the winter in Ithaca, it’s the same for plants. Sudden exposure to bright sun, high or low temperatures, and drying winds creates stress. 

Hardening can become a very long process.  Many recommend moving your plants outside one to two weeks before you plan on transplanting.  First to a shady spot for a couple of hours the first few days.  Then slowly increasing the sun they receive. 

They also recommend allowing the plants to “almost wilt” before watering, and withholding all fertilizer.  Once you have done all that, the plants are better able to recover once planted in the soil permanently. 

All of this is good in theory.  But I’m a bit of a contrarian when it comes to hardening off.  I don’t know about you, but I have better things to do than spend every day moving plants outside and bringing them back in.  And in my experience, I don’t think all this fuss is necessary.  I seldom harden my plants for my field-scale tomato and pepper studies.  They go right from the greenhouse to the field. In one study, my unhardened plants experienced unseasonably hot temperatures over 90F right after transplanting.

After a few days, the plants looked bad.  They stopped growing and some of the leaves whitened and browned.  But guess what?  They grew out of it and about a week later, were growing just fine.  I had a few plants that were hardened and didn’t experience the yellowing and browned leaves.  But they did not resume growth any faster than the others.  They grew out of their hardened state the same time as the unhardened ones.  And both produced the first ripe fruit at about the same time.

Here’s the crux of the matter.  Hardening slows the growth of the plant which allows it to adjust to the change in conditions.  A perfectly hardened plant will take some time to adjust before it starts growing again.  A nonhardened plant will also take time to adjust to the outside conditions.  It’s change is more abrupt, and it may show more damage.  But both will start growing again at about the same time.

I usually take plants straight from my greenhouse and plant them outside.  And I’ll bet many of you do too if you are buying your plants from a greenhouse.  Most commercial growers are not hardening their plants.  They just don’t have the time or labor available.

The key for using the nonhardened plant is to create ideal conditions in your garden immediately:

• Keep the plants well-watered. 
• Protect them from strong winds. 
• Plant on a cloudy day or in the evening if possible so plants can recover out of strong sun. 
• Provide a little nutrient solution to the roots, especially one that is high in phosphorus.

Another tactic is to use transplants with bigger root balls.  A pepper plant in a 3 inch pot will recover faster than one grown in a small cell pack.  Try to minimize root damage when you plant.  Make sure the root ball is soaked when you plant and never allow the soil around the root ball to dry out. 

One last thing to keep in mind: If you are growing transplants yourself indoors under lights -- perhaps in a basement or bright room without any direct sunlight -- you might want to acclimate those a bit.  Perhaps a few days of outdoor sunlight and breezes.  As these plants will be much more likely to suffer severe and possibly permanent damage.  But the idea of a two week hardening period is, in my opinion, just a waste of your time. 

In the garden this week

Keep up with your greens harvest.  Early-planted spinach will flower and bolt (go to seed) if not harvested.  Lettuce will do the same but usually not for another month.  If reusing tomato stakes or cages, wash them before reusing as they can be a source of bacterial diseases.  Clean off all dirt and spray, or better yet, soak in a 1 part household bleach to 9 parts water solution.  And speaking of stakes and cages, don’t wait until your tomato plants are large to set these up.  I set up mine soon after I transplant my  tomatoes and then I’m set.  No trying to wrestle with my plants to make them fit in the support or damaging roots with the stakes.  That’s not good for them or for me.