Hardening-Off Seedlings & Cold Frames

What is the difference between a Greenhouse and a Cold Frame?

GreenhouseA greenhouse is defined as a heated structure in which to grow plants, usually made of plastic or glass panes. 

The goal is to provide protection from cold water.  Although some greenhouses use passive solar heat as the heat source or even grow greens in unheated greenhouses in the winter.

A cold frame is typically a small 3’ x 6’ enclosure that people use to extend the growing season either in early spring on late fall. 

Frost Protection Berries Cold FrameFrost prevention is key if you wish to garden outside of your growing season.  Of course, cold frames come in all sizes and shapes.

The main “ingredient” in a cold frame is a transparent/ translucent cover.  This allows the sunlight to get in and warm up the air and dirt in your cold frame.

Hardening-off your seedlings is VITAL in the process of transferring your fresh young seedlings to the great outdoors.  Skip this step and all your hard work will be lost!

Tierra Garden 50-4400 Haxnicks Cold Frame Hardwood Greenhouse Box

In the spring, the cold frame is used as a way to “harden-off” your seedlings.  The seedlings in your house are very delicate and will quickly shrivel up and die if they are not acclimated to the outside environment (direct sun, wind, heavy rain, frost, bugs, and critters).

Often people will use an oscillating fan on the plants to help them acclimate to the wind prior to putting them out in the cold frame.

Late in the summer, experienced gardeners try their hand at growing crops into late Fall.  Crops such as cabbage, kale, and brussels sprouts are examples of crops that can be grown in the cooler weather.

The idea is to get the seedlings to harden off before the frost arrives, allowing them to acclimate to the weather and establish a good root system.

When seedlings are first placed into a cold frame, some kind of sun barrier is used to reduce the amount of sun exposure.  My favorite choice is plastic black window screen.  It’s flexible and will not scratch your glass or plastic cover.

Frost Protection Cold Frame Wooden Start out by placing your plants into the cold frame for only a few hours (2-3 hrs) and then bring them in again.  Work your way up, increasing the amount of hours your seedlings spend outside, until you’re able to leave the seedlings out all day and night.

Allow your plants to acclimate for 7-10 days before you plant your veggies into the garden or raised bed.

Warm weather loving plants do NOT like 50 degree weather.  All of New England discovered that this year.

We all put out tomato seedlings out for hardening-off (temps 45-55 oF) and ended up with leggy yellow seedlings, which was completely due to the cold weather.  Fortunately, whatever seedlings lived eventually perked up with the warmer weather.

Check to see what kind of weather is ideal for your plants.  If they like heat, wait to put them into the cold frame until the temperature is within ~10 degrees of when you want to plant them.

When setting up your cold frame, be sure it faces South to get the best sun exposure.  You also want to make sure that all four sides are flush with the soil or better yet dig down a few inches to set your cold frame into the dirt.

The idea is to make a “hot house” environment.  Any and all holes present will result in heat loss and the cold frame will not protect your plants from frost.

Cold frames are easy to build.  All you need is a sheet of plastic or glass cover and a box. That’s it.  There are many free plans on Google.  Just Google it!

For those of you who just don’t have time to build one or aren’t handy with a hammer… check these out from Amazon.

Tierra Garden 50-4400 Haxnicks Cold Frame Hardwood Greenhouse BoxWooden cold frames are not only practical, but also a pleasant addition to your back yard.  Proudly show off your new seedlings to friends and family.

This unit is made with long lasting Acacia hardwood and stainless steel fasteners.

It has a shatterproof glazing and is twin-walled for maximum insulation, which is ideal when children or pets are around.  The polycarbonate is UV-stabilized for long life.

==> Click here to purchase

Exaco Juwel BioStar 1500 Premium Cold FrameFor the serious gardener who has a lot of seedlings and needs a much larger space.  Try this cold frame.

A friend of mine stated not only does she use the cold frame for her seedlings, she also picks it up and places it right over the garden area where the seedlings will be planted for the summer.

Another option is to use the cold frame year round for greens, such as lettuce and spinach.  This cold frame is great in that it also has a built in screen to keep out the bugs during the warmer weather.

==> Click here to purchase

Univent Automatic Vent Opener Standard - Lifts 15 LbsCold frames need to be vented when it gets warmer or your seedlings will get too hot and wilt!  You can purchase and automated arm, which is very easy to install.  The arm will expand to let the hot air out during the day and retract during the night to keep the warm air inside.

==> Click here to purchase


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