Hardening Off: Last Stop Before Planting Out

Author: Rebecca Last, Gardening at Last, Master Gardeners of Ottawa-Carleton; published with permission

To the uninitiated, “hardening off” might sound like the name of a heavy metal band. In fact, it’s the final stage of preparation before you can plant your tender seedlings outside.

I have fair skin and sunburn easily, so I always take precautions the first few times I go outside into that fierce spring sunshine. Just like my winter-white skin, seedlings grown under lights have delicate cuticles that can sunburn easily. Unlike me, seedlings don’t have the option to move into the shade—at least, not without my help. Where my skin turns lobster red when I burn, sunburned plants develop white patches on their delicate cuticles, the equivalent of “skin” on their leaves.

Avoiding Sunscald on Plants

Shortly after, the white patches turn brown as the plant tissue necrotizes and dies. Sunscald is rarely fatal, but it can set back seedling growth considerably, requiring the plants to regrow leaves rather than establishing new roots. Hardening off is the process of acclimatizing seedlings (and houseplants) to their new life outdoors after their pampered start on a sunny windowsill or under lights indoors.

Conventional Hardening Off

The conventional advice used to be that hardening off seedlings required gradually exposing them to outside conditions a little more each day. This meant moving plants outside in the late morning to a shaded and sheltered location and moving them back inside again as the sun began to dip and temperatures cooled off. The process is repeated each day for slightly longer periods of time. This daily plant migration method works if you have nothing better to do with your time and are not prone to forgetting to bring your baby plants back inside. But what a lot of work and bother!

An Easier Way to Hardening Off

Early in my seed-starting career, I discovered a much easier way to harden off seedlings. I begin by checking the long-range weather forecast. I’m looking for minimum temperatures of no lower than -3 to -5°C. Once the weather has warmed to this extent, which in Ottawa usually happens early to mid-May, it’s safe to move my plants outside. I find or create a sheltered location, out of the wind and direct sunlight.

Creating Protective Environments

In the early days, I placed my seedlings underneath a glass-topped picnic table and wrapped the edges of the table with a tarpaulin to create a cozy little mini-greenhouse for my seed babies. There they could safely stay for the seven to ten days it took to harden them off. The only time I needed to bring them all inside was if overnight temperatures dropped below -5°C.

Using a Gazebo

As my gardening addiction grew, I needed a larger space for hardening off my many seedlings. After scorching a few, I discovered that it was also necessary to harden off my houseplants that I move outside for the summer so they don’t die of neglect indoors. Enter the Gazebo! Our inexpensive and now very elderly gazebo has a pale-coloured cloth cover that allows filtered light through. During particularly cold springs, such as we experienced in 2019 and again last year in 2021, I wrap the outside of the gazebo in tarpaulins and shade cloth to provide that cozy, protected environment for my tender seedlings and houseplants that are unaccustomed to the strong sunlight. There they stay for 7–10 days until the Victoria Day weekend, when it’s time to plant them outside in the ground.

So with a little ingenuity and repurposing a garden feature that isn’t yet in season, I am able to easily harden off my seedlings and houseplants without the hassle of the daily plant migration. This season, we plan to buy a hard-top gazebo that will be less permeable to light. Hopefully it will still be effective as an early-season greenhouse, although I might have to remove the protective side panels during the day to let in more light. A new adventure in gardening awaits!

Tip: Harden off purchased transplants too, to minimize transplant shock.