What is "Hardening Off" Your Plants and What Does it Mean?

If this were any other year, we’d still be enjoying Spring’s annual show of Daffodils, Hyacinths, Tulips and Bluebells.  Instead, Mother Nature has fooled us into thinking it’s late April and time to go plant shopping.  Nurseries are scrambling to fill our overwhelming desire and insatiable need for annuals, perennials, herbs and vegetables.

The weather is so conducive and our urge to plant is so overwhelmingly strong, we’re willing to shop until we drop and plant until our knees and backs cry “UNCLE!”  Release that shovel, grab a cold drink and relax. 

Let me teach you about “hardening off” your springtime annuals and perennials and why it’s one of the most important steps to giving your plants the right start this spring.

So, what exactly does “hardening off” your plants mean? If you have ever spent all morning picking out your favorite plants at the local nursery, planted them lovingly, only to have them wither and look pitiful for a few weeks, your plants weren’t hardened off. 

Did you wonder why those huge, robust and tropical Elephant ears that were so stunning in the garden center became shredded on that windy day?  Have you ever have some Hosta that looked like they were sunburned, even though you only left them in the sun for a day or two? 

Hardening off is all about acclimating your plants to their new home.  They need to be slowly introduced to their new digs.

Generally, the plant material you buy from your favorite greenhouse has spent all winter and early spring under very controlled conditions.  The greenhouse temperatures are normally consistent.  Water and fertilizer are given at regular intervals and the plants have been “babied” their entire lives.  Gentle greenhouse fans barely rustle the plants’ leaves.  

In your garden, spring breezes that make for superb kite flying can shred plants recently planted in the ground.   Because they haven’t been exposed to sun, wind and rain, the cuticle of the plant is soft and tender, allowing any change at all to throw the plant into a tizzy.  They need to be hardened off- you can do it successfully a few ways.

The first way to harden off is by withholding water.  You can stress the plant enough that it actually responds by “toughening up.”  By waiting to water until the plant exhibits a wilted appearance, you build a stronger plant.  Doing this for a week to ten days will toughen up the plant enough to place it in the ground.  I think of my plants as my “little babies” and I just don’t have the heart to do this.  I prefer hardening off my plants another way.

I place my plants under a covered patio if I know that they have been grown in a greenhouse.  I leave them under the protective cover of the patio roof for 3 or 4 days and water normally.  Then, I’ll move the plants outside, on a sunny day, for a few hours each morning.  I bring them back in about 11 o’clock.  I’ll do this for 3 or 4 days, lengthening the time I leave them outside each day.  Then, around the 9th or 10th day, I leave them outdoors permanently. 

I find this method less stressful than watching my plants wither. 

Of course, if temperatures get below 45 or 50 degrees, I’ll cover them with a frost blanket or sheet for the night.  Make sure to pull the covering off in the morning.  Additionally, when you bring your houseplants outdoors this spring, treat them the same was as you do your new greenhouse plants.  I’ve seen many cases of eager gardeners placing their houseplants outside on a beautiful day, only to see them sunburned and withered at the end of just one day.  The heartbreak IS preventable.

A third way to harden off your plants is to place them in a cold frame.  I got one for Christmas this year and have really enjoyed starting plants from seed in it.  Unfortunately, my cold frame isn’t large enough to accommodate the plants I am hardening off.  I find the method of “babying” them easy and stress free.

Plants that have been exposed to the elements for a week or more in the nursery don’t need to be hardened off.  Just be sure that, once they’re planted, you remember to cover them if we happen to get low temperatures at night.  This has been a wacky spring, Mother Nature just might have a frosty night up her sleeve!

By treating your plants right in the beginning, they will return the favor and reward you with vigor and stupendous growth all season long.  So, get out there and shop- just make sure that you harden them off becomes a regular part of your planting process!