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The term “hardening off” means to toughen up your tender little indoor seedlings before you completely move them to their outside garden spot.
Plants that are raised inside can undergo a significant shock when moved to a new environment. It can be enough of a shock to actually kill some delicate plants, or to at least set back their growth so that the plant doesn’t do well for the rest of the season.
1 – Use a Fan
Having plants do their early growing and development indoors means they only have a quiet atmosphere of still air to deal with. While that may not seem like a problem, it means that the structure of the stem has not had to develop the strength to resist the wind.
Think of how strong a muscle is if you never have to use it. Kind of the same thing.
So even your indoor plants need some movement to grow a sturdy stem, which is necessary to survive outside. If your plants happen to be growing near a window that gets regular breezes, you can use that.
Otherwise, add a fan to your seedling space, ideally one that oscillates so that the movement isn’t always coming from the same direction.
Get this set up as soon as your seedlings are sprouted, so they grow up with your man-made breezes. This isn’t something you need to start just when you are ready to plant outside.
2 – Move Them Gradually
Whether or not you take advantage of the fan technique, you will want to introduce your young plants to the outside gradually for the best results.
Get your seedlings organized and set up in a tray for portability, and move the lot of them outside for a period of time each day.
Choose your location carefully so that your little plants are getting enough sun without overdoing it. Even if you have been growing under lights, having the actual sun beaming down on them can be traumatic.
In the evening, bring them back inside for a safe and warm night. Keep repeating until your plants are acclimatized and ready to plant.
While this is fundamentally a very simple process, it can be awkward and time-consuming if you have a large number of plants to deal with.
3 – Use a Cold Frame
Serious gardeners who need to harden plants regularly will use a cold frame. It works like a small, simple greenhouse to give your new plants a more protected space while they adjust to life outside.
After you’ve hardened your plants in the cold frame, they get planted out in the garden.
A basic cold frame is simply a 4-sided box with an open bottom. The top is clear or translucent, often created from an old window or even a piece of plexiglass. It can be hinged, or just lifted off. Whatever your construction skills can accommodate.
Your frame should be 20 inches to 2 feet in height so that you have room for your plants to grow, and also to allow for the right amount of air flow beneath the lid.
Set it out in a sunny area with your new seedlings inside, with the lid off or open wide. As the day cools off in the evening, you want to close the lid to protect your little plants from the chilly night air.
The warmth of the soil in the “bottom” of the box will keep the inside slightly heated for hours.
Not only does a cold frame mean you can leave your plants out overnight, the high sides also keep out harsh winds that could topple your seedlings or snap their stems.
Light rain is excellent when you are hardening plants, as long as it is not too heavy. When expecting a downpour, close the lid.
Even if they need the water, they won’t be able to stand up to the strong force of the rain for very long. It’s better to use a watering can later than take the risk of having your plants pummeled.
One last note on using a cold frame to harden plants, you should not underestimate how hot it can get in there if you leave the lid closed during the day. Think about what happens inside your car when the windows are up in the summer.
A cold frame must have the lid open at least enough to vent out the heat during the day. If you are new to this, it would be smart to check on your plants, or even leave a thermometer inside the box so you can keep an eye on things.
4 – Make a Cloche Cover
So far, these are all techniques to apply before your seedlings are planted out. You can also do your hardening after you’ve done your transplanting instead. It’s really just a variation on the cold frame, except that you are adding a temporary enclosure around each plant.
You can make a quick DIY cloche by cutting the bottom off of a big plastic soda bottle. Press the cut edge into the soil around the plant, and leave the lid off so that it can vent out the heat. Put the lid back on when it gets cooler in the evening.
There are also light-weight fabrics you can use to tent over your plants while they adjust, giving you a bit more flexibility in how you place it and how much space you can allow for the plants underneath.
The pop bottles are quick but limited by their shape and size.
How Long to Harden?
Once you pick your technique, how long do you need to keep it up? There is no set rules for this, and you’ll have to make some educated guesses as you watch your plants develop.
Generally, a few weeks should be enough. After that, if they are not tough enough to withstand the outdoors, they may never be.
It might seem like a lot of work but your new plants will thrive and do so much better this season if you invest a little more effort once their time in the house is over.