What is Hardening of Seedlings?

A new gardening season is upon us and everyone is thinking about seed starting. The process of preparing young seedlings for transplanting outdoors is the turning point for gardeners who are starting seeds for the first time. When you don’t know you have to harden off your plants, you are likely to end up with a high mortality rate. If you try to harden off seedlings that are weak, to begin with, you may end up with similar results. So I’ll back up a little…

Blocks of seedlings in soil blocks in a black tray.

Caring For Seedlings

Once seeds germinate, they need light, moisture, air, and warmth. To prepare them for their permanent home, the goal over the next few weeks is to establish a strong root system and stocky, leafy top-growth. You can accomplish this by gradually moving them from the “perfect” conditions in which they sprouted, to the relatively unprotected environment of the garden while providing good growing conditions all along. This will help them acclimate to their new environment and lessen the chance for transplant shock for young plants.


As soon as the seed leaves emerge, they need as much light as you can give. If you are using grow lights, keep seedlings within inches of the bulbs for twelve hours each day. Flooding the plants with light helps them grow short and stocky rather than thin and rangy.


When watering overhead, use a fine mist or fogger. If the soil dries too much with this type of application, you can water by placing the seed tray, peat pots or soil blocks into a shallow tray of water, allowing the water to “wick” upward in the soil. I’ve found that misting works well for the first week or so, then bottom watering better ensures thorough soaking watering (which is key to good root development). If the soil mix does not contain plant food, provide it in the water.


Good air circulation is important from the start. Remove covers used over trays during germination within a day or so. Keep a small “personal sized” fan on the young seedlings (on it’s lowest setting) . This will help keep plant growth in check, resulting in a stockier seedling that adapts more easily to natural outdoor conditions.


Ideal germination temperatures are often not ideal for producing strong transplants. Keep tender plants like tomatoes and peppers in a place that stays above sixty degrees. Hardy vegetables like kale and lettuce can grow in places with temperatures in the fifties. There is evidence that suggests warm soil and cool air together may be a good combination, but most of us don’t have that degree of control. As the time approaches for hardening off, providing five to ten-degree fluctuations in daytime and night temperatures is helpful.

Hardening off seedlings for planting is crucial for the transition to the garden. Follow these simple to learn how to harden off seedlings this spring


Hardening Off The Seedlings

It’s called hardening off, but it should happen gently. Your seedlings have been sheltered in a greenhouse environment for their entire life thus far. The process to harden off plants prior to transplanting into the vegetable garden isn’t hard, it just takes a bit of time.

Always water the plants well prior to leaving them outdoors, and check on them periodically while they are outside. Pick a calm day with little to no wind, and the temperatures are close to those in the seedling’s current environment. Take them outdoors to a shaded area for an hour or so. The next day, if conditions are similarly mild, take them out again, this time for longer periods of time. The day after that, take them to a spot with direct morning sunlight for an hour and shade the rest of the day. Each day add a bit more “extremity” to their outdoor time.

This process may take a few days or a couple of weeks. It just depends on how different the indoor conditions were compared with those outdoors. The last night prior to planting, they should be left outdoors. At this point your plants should be acclimated.

Following these tips for hardening off plants will ensure that your babies are tough enough to handle real-life conditions in the garden when the time comes. That means you’ll have a bit less stress too. Happy Gardening

Author Mark

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