Seeds Series: Preparing Seedlings

  • Difficulty Rating: Beginner


Joe L'ampl in fiskars field

By now, you have planted your seeds, watched them germinate and put on leafy growth. You have carefully divided them out of crowded starter pots and moved them into larger containers where they have continued to grow larger root systems and stronger leaves and stems. So what’s next?

Hardening them off, of course!

Hardening off refers to preparing tender young plants for the harsher conditions of the outside world. Whether you begin seeds indoors in a brightly lit window or start them off in a greenhouse on a heating pad, young plants that have been coddled along will need some toughening up before they will have a fighting chance in the wilds of nature’s fickle spring weather.

A surprise hailstorm on a late spring day isn’t unheard of. Those pounding, frozen pellets of water can easily puncture and demolish young plants. Too, a heavy rain can do irreparable damage to delicate sprouts. Something as simple as moving a pampered plantlet from the warm, protected indoors to the cold, harsh outdoors, can cause irreparable damage – unless you transition your plants’ environments slowly. So, rather than risk losing your tender young babes and end up several weeks behind on your production schedule, take a few simple precautions to help them build a thicker “skin”.

When temperatures begin to warm during the day, gather up your tender plants, and place them in a protected, sunny outside spot for a few hours during the day. Then, before nightfall, move the plants back inside.  Repeat this for several days, extending the number of hours your plants are left outside as the days go by. As you do this, you should begin to notice that the stems and leaves become thicker and sturdier as the days go by. After about a week of this indoor-outdoor-indoor rotation, you may be able to leave your plants outside overnight. Just be sure overnight temperatures won’t get anywhere near freezing and nothing harsh will be falling from the sky or blowing about. And, be sure to keep your young plants in a protected spot.


Planted seeds


Season extenders are one of the best tools a gardener can employ for hardening off tender young plants. Passive greenhouses, hoop houses, and cold frames have the capacity to capture and hold warmth. This means they are ideal new homes for your sensitive seedlings.  If you have access to one, try using it as your “protected location” throughout your hardening off period. Just be sure to vent it slightly during times – even cold days – when these structures may maximize sunlight rapidly and fry your plants. Venting will also keep air flowing, which helps reduce disease problems. And, venting will let in a bit of the outdoors to start training your youngsters to hold up under new conditions. Shutting these structures before nightfall will help hold heat and keep your plants from shocking out in the cold. And, in cooler, short-season climates, season extenders prove priceless for keeping your crops protected until warm weather really arrives for good. Plus, as the name suggests, they are great tools for growing much later in the year than nature itself might allow.


Next up in this series: Transplanting Your Starts to Their Final Destination

Last in this series: Potting Up: What, Why, When & How