Hardening Off Seedlings For Planting

Hardening Off Seedlings For Planting

What is “Hardening Off” ?

Now that a new gardening season is upon us and everyone is thinking about starting seeds, we thought it would be a good idea to write about hardening off seedlings.This process of preparing young seedlings to be transplanted outdoors can be 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…

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. This is done by incrementally 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.


As soon as the seed leaves emerge, they need as much light as you can give. If you are using fluorescent lamps, 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. Covers used over trays during germination should be removed within a day or so. Keeping a small “personal sized” fan on the young seedlings (on it’s lowest setting) 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. Tender plants like tomatoes and peppers should be kept in a place that stays above sixty degrees. Hardy vegetables like kale and lettuce may be grown 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

Hardening Off The Seedlings

It’s called hardening off, but it should happen gently. Always water the seedlings well prior to leaving them outdoors, and check on them periodically while they are outside. On a calm day with temperatures close to those in which the seedlings have been growing, take them outdoors to a shaded place for an hour or so. The next day, if conditions are similarly mild, take them out again, this time for several hours. 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, depending on how different the indoor conditions were compared with those outdoors. The last night prior to planting, they should be left outdoors.

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

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