Hardening Off Seedlings and Other Spring Tasks

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Last week, I mentioned hardening off seedlings before they are taken outside. I want to expand on that subject by telling you about my first experience with sowing herb seeds indoors.

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 being that 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. I didn’t have grow lights but did have huge south-facing windows in the living room where I set the tray.

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 front porch, protected from the wind and headed off to school. When I got home at 4:30, I was devasted 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 to outdoors.

A simple explanation of what happened is to relate this experience to putting an undressed baby out on a blanket in the sun and walking away for several hours. That tender skin has never been exposed to the ultra violet rays of the sun so will immediately sunburn.

Your tender seedlings grown indoors have also not been exposed to ultra violet sun as window glass prevents those rays from entering the house. 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 30 minutes until you see that 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 too 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? A portion of the ultra violet rays comes through the clear plastic the greenhouses are covered with. We wear sunscreen on sunny days when we are working in there!

Tender plants purchased at garden centres do have to be protected from cold temperatures though, as they are coming form a warm environment. If you do buy plants early and put them indoors in a sunny window, you will need to harden them off again before they are taken outside.

Some gardeners will buy plants early and place them in a garage or shed for cold nights, taking them out into the sun during the day. This is a lot of shuffling around, but many people are willing to do this to get a jump start on spring! If you plan buy early, just be sure to set plants on shelves or boxes up off the cold floor. Plant roots rot easily of they are kept too damp and cold.

One other thing I want to mention as it’s very timely, if you had any insect or disease problems with fruit trees or bushes, rose or other deciduous shrubs, now is the time to apply dormant oil. This acts as a preventive insecticide by smothering any insect egg sacks that have overwintered on plants. This oil MUST be applied before buds break, or any green tissue shows.

 You can also spray the same plants with lime sulphur. It’s a fungicide that smothers any disease spores that survived the winter. For early spring application, lime sulphur can be mixed with dormant oil, following the recommendations on the package.

Lime sulphur can also be used throughout the growing season as it doesn’t harm leaves when applied.


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About Me

I began my journey in horticulture in 1982 after graduating from the Humber College Landscape Technician program. At that time, I lead a talented crew of landscapers, taught evening courses in horticulture and had my own landscape design and consulting business. Then I ventured into the garden centre world. I’m lucky enough to be leading the friendly and knowledgeable team at New North Greenhouses. 

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New North Greenhouses

719 Airport Road

Sault Ste. Marie, Ontario

P6A 5K6


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