How to Prepare Seedlings & Plants for Transplanting Outdoors (Harden Off)

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The process of transitioning indoor seedlings and greenhouse plants to life outdoors is called hardening off. The seedlings have been pampered and coddled with consistent, comfortable indoor growing conditions. As the risk of frost passes, it’s time for baby boot camp to toughen them up for life in the great outdoors. I’ll show you how I do it.

This is part of a series, Indoor Seed Starting for Beginners | Sow Indoors Grow Outdoors, where I walk you through the process from seed to garden. It’s all the information I wish I had when I was getting started.

A variety of seedlings ready to be hardened off before transplanting outdoors.

How to Prepare Indoor Seedlings for Transplanting Outdoors

This is an excerpt from the ebook, Seed Starting for Beginners: Sow Inside Grow Outside, sharing everything you need to know to start your own garden plants from seeds indoors.

If you would like the entire ebook with all the printable lists in one handy file, you can save the file instantly here.

It’s Time for Seedling Boot Camp

About two weeks before outdoor planting time in spring, as we say goodbye to frosts, it’s time to ‘harden off’ your seedlings and prepare them for life outdoors.

Until now, the seedlings (or young plants) have been accustomed to consistent light, moisture, and temperatures, and all the comforts of home. We don’t want to shock them with the change, so we do it gradually.

This process is recommended for all types of seedlings, including veggies, annuals, perennials, and anything else you started from seed indoors.

This is also how you harden off transplants you have purchased from a plant nursery.

The basic process is, each day the seedlings will be introduced to more light and outdoor elements, until, after two weeks (or so—longer if you have time), and the risk of frost is passed, they will stay outside.

Once hardened off, you don’t have to plant everything all at once. You can keep them in their pots (if they have adequate root room and water) until you’re ready to put them in the garden or add them to outdoor containers.

During this time, keep your eye on the weather, mindful of any temperature swings, or intense sun, wind, or rain and protect them as needed (with covers, mulch, or frost cloths) so your babies aren’t damaged from intense exposure.

I use the hardening-off time (approximately 14 days) to do my garden bed prep (if the ground is dry enough), so everything is ready to go when the seedlings are officially outdoor plants.

How to Harden Off Seedlings
Resources

How to Harden Off Seedlings

1Use Bins to House the Seedlings

Opaque plastic bins make great mini greenhouses! Use them to get seedlings ready for planting and to protect other tender plants.

If you have a lot of seedlings like I do, you are going to want to keep them in easy-to-move containers during the hardening-off process. Depending on the weather, they may be moved inside and outside a few times a day.

I place my seedling pots, trays, and soil blocks in plastic bins which act as mini greenhouses.
I like bins with attached folding lids that let some light in.

2Gradually Increase Exposure to the Elements

Check the weather forecast each day to make sure there’s nothing wonky on the way.

I realize you may not be home all day to be able to do this as described, so just do your best, and, if the weather will be chilly, toss bed sheets over the bins to keep some warmth in while you’re out.

Phase 1: I put the bins out on my covered patio, in a spot that gets indirect afternoon sun, for a few hours a day. This will be the plants’ first exposure to sunlight (as opposed to grow lights) and cooler air temperatures.

As the air cools in evening, I bring the bins back inside.

Remember to keep watering during hardening off.

Set reminders on your phone so you don’t forget to move them in and out each day.

How to harden off seedlings for planting outdoors: use storage bins as mini greenhouses

Phase 2: Gradually increase outdoor exposure, aiming for full-time life outdoors (day and night) at the end of two weeks (14 days).

During the first week, depending on the night-time temperatures, you’ll probably bring the bins inside, or, if it’s not too cool, just close the lids and/over toss a blanket over them to keep the warmth in.

During the second week, weather permitting, they may be able to stay outside 24 hours a day, still sheltered, perhaps.

Again, if any wacky weather is on the way (hard rain, snow, winds, etc.), give them some protection. Winds can snap tender stems, and unusual cold (or heat) can kill them. As mentioned, bed sheets or blankets right over the bins should be sufficient.

Phase 3: When the risk of frost has passed, your seedlings should be adjusted to life outdoors, or close to it. No more living in the house! Now is the time to plan your outdoor planting.

3Plant Seedlings Outdoors

How to harden off seedlings for planting outdoors for a successful veggie and flower garden

It’s time! All that time caring for the seedlings indoors has paid off. The plants will seem tiny in the garden—and they are! —but things will grow fast. The annual flowers, herbs, and veggies will gallop along now that they have natural growing conditions. And the perennials will be nice, mature plants in just a couple of years.

Remember to keep everything watered, and watch for any weather swings. During the first few weeks these little guys will still be adapting and spreading their roots. If there’s stormy weather ahead, have your covers and cloches ready.