Since the beginning of the pandemic, there has been a huge increase in the number of people starting their own vegetable, herb, and flower plants from seed. For those of you new to the venture, I would like to pass on some advice for how to safely transition your tender seedlings outside. This practice is called hardening off.
When I was 15, my brother gave me an herb growing kit for my birthday. It included a tray with a domed greenhouse-style lid, planting pockets filled with soil and a variety of herb seeds. I was very excited to begin the project, as I had never grown plants from seed before.
In late March I sowed the seeds and eagerly watched for germination. I was very pleased to see a good success rate for the 6 different herbs in the kit. Since I didn’t have grow lights, I set the tray of seedlings in front of our large south-facing living room window.
By early May, the weather was warm enough to start putting the plants outside. Not knowing any better, I set the tray of seedlings out on the south-facing front porch, protected from the wind, and headed off to school. When I got home after 4:00, I was devastated to find that all the seedlings had died! I have made the beginner’s mistake of not gradually hardening off those tender plants for a successful transition outside.
A simple explanation of what happened is to relate this experience to putting a naked baby out on a blanket in the sun. A baby’s tender skin has never been exposed to the ultraviolet rays of the sun, so will immediately burn. The tender tissue of my herb seedlings burned too.
Seedlings grown indoors have not been exposed to ultraviolet sun since window glass blocks most of those rays. To transition your seedlings successfully outdoors, start by setting them out in early morning sun for no more than 30 minutes the first day. Each day increase the time they are out by another 30 minutes until you see they have toughened up. You always start with morning sun as it is less intense. Also be sure the seedlings aren’t put out when it’s too cold or windy. They won’t be able to tolerate those conditions initially either.
You may wonder why plants you buy from a greenhouse are already acclimatized to the sun? Ultraviolet rays come through the clear plastic that greenhouses are covered with. Greenhouse workers wear sunscreen and hats on sunny days to prevent sunburn!
Be aware that if you do buy plants early and keep them inside your home in a sunny window, you will need to harden them off again before they are taken outside.
Plants purchased at garden centres with a heated indoor sales areas do have to be protected from the cold and will need to be gradually transitioned outside too. You should keep plants under shelter on days when the temperature is 10 Celsius or less. Be sure to pay attention to wind chill temperatures too. Most tender plants can not tolerate cold wind!
Some gardeners will buy plants early and place them in a garage or shed on cold days and nights, putting them out on warm sunny days. This is a lot of shuffling back and forth, but many people are willing to do this to get a jump start on spring! If you do plan to buy early and we get a cold snap requiring them to be under cover, just be sure to set plants on shelves or boxes up off the cold floor. Young plant roots rot very easily if they are too damp and cold.
Also, be sure you feed plants you are holding on to until weather is warm enough for planting. I recommend using an all-purpose water soluble or liquid concentrate fertilizer. Mix it at ¼ of the strength recommended on the package and apply once a week to keep plants healthy.