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How to Harden Off Seedlings


harden off seedlings
Colleen Vanderlinden

You've spent weeks coddling the tiny plants you grew indoors from seed, and now, it's time to move them out into the garden. But you can't just take a seedling that's lived in sheltered, monitored conditions for its entire existence and just thrust it into the harsh, cruel world that is your garden. Think about it: bright sunlight, wind, rain. Insects! A bit more babying is in order to help your seedlings transition well to life in your garden.

We call this process of acclimating our plants to our garden "hardening off." It's not a difficult process, but it does require some planning and attention. Here's how to do it.

When to Harden Off Seedlings

The process of hardening off seedlings takes from one to two weeks, depending on how patient you are. Two weeks is better, because this allows your plants to very gradually adjust to conditions in your garden. However, if you only have one week, you can still do a good job of hardening off your seedlings.

To figure out when to harden off, you'll need to know two things:

  1. When is the best time to plant your seedlings?
  2. You'll want to read your seed packet and follow the instructions for when to plant. Some cool season crops can be planted at or even before the last frost date, or as soon as the soil can be worked. Others, such as peppers and tomatoes, should be planted after the last frost date, and after the soil has warmed. Once you know this information, count back two weeks (or one, if you find you're cutting it close) and that's the date upon which you'll start hardening off the seedlings.

  3. What is the weather forecast for the next ten days?
  4. But, the date is less than useful if your garden is supposed to be snowed on, or flooded with rain, or much colder than normal during the time in which you're supposed to harden off. So take a look at your forecast. If normal conditions for this time of year seem to be in the forecast during your scheduled hardening off time, then you're good. If not, put hardening off off until the weather normalizes.

How To Harden Off Seedlings

The process of hardening off entails slowly acclimating your plants to conditions in your garden. This means increasing both the time and amount of exposure they get, so that by the time the hardening off period is over, they will be used to living in the garden full time, in all types of typical conditions.

Increasing the Amount of Time

You'll want to start out putting your plants outside for an hour or two, then bring them back inside. Each day, you'll leave them out a couple hours longer, until, by the end of the two weeks, they're spending all day and all night outside.

Increasing Exposure

When you first put your plants out, you'll want to place them in a sheltered condition, out of the strong sunlight and out of the wind. Much in the same way you'll be increasing the amount of time they spend outside, you'll increase the amount of exposure they get as well. Each day, move them farther out of their sheltered area, ending up where they'll be growing in your garden. The first few days should be spent in a sheltered location, just increasing the time. But after the third day or so, start moving them out of their protected spot.

Keep your plants watered during this time; you'll find that they dry out faster than they did indoors thanks to increased evaporation from wind and warm sunlight.

Common Questions About Hardening Off Seedlings

What if we get a freak cold snap? Or if it starts snowing? Or hail, or...?

If any of these things happen, or are in the forecast, bring your plants inside and keep them there until the weather normalizes. Then start the hardening off process again.

Why can't I just plant them outside when I'm ready? Why go through all this?

You could just plant them out, and hope for the best. You wouldn't be the first gardener to do so. But here's why you should give your plants at least a week of hardening off time: stressed plants are more susceptible to pests and diseases. And no plant is more stressed than one that has been removed from its warm, bright, sheltered location and plopped into the cold soil, buffeted by wind and rain and exposed to bright sunlight. Who needs the extra hassle of coddling a sick or pest-infested plant? Take some time to harden off, and your plants will get off to a better start.

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