Whether you’re a hobby grower or a large commercial grower here is a tried and true system for overwintering container grown perennials and herbs.
There are many ways to overwinter perennials. This is one of the systems used successfully by many hobby and commercial growers throughout the country. Cold Frames are never out of style and are easy and inexpensive to erect.
How to Construct a Cold Frame for Overwintering Plants
Your cold frame should be about 5 feet wide. To create the frame use wood boards that measure 10 inches x 12 feet, and cut or add to desired length. Lay down one layer of heavy weight landscape fabric.
Landscape fabric is porous. Moisture must be able to enter from the ground so your pots don’t dry out. Remember, this structure will be sealed, and you won’t be able to and wouldn’t want to water during the winter.
Inside the frame starting at the beginning screw “U” Bolts every 4 feet until the end. Repeat this on the other side. Insert 8 foot long rebar into these “U” Bolts to form a hoop. Do not use PVC pipe as a hoop, because if you live in an area that gets snow PVC will not be strong enough to hold up to a heavy snow load.
Related: Winter Garden Design Ideas
How to Protect Plants from Rodent Damage
Next and most important you must put rodent bait in the beds. There is some controversy to baiting. Some are concerned that a Fox, Owl or Bird of Prey will catch the poisoned rodent and get sick or die.
This should not be a concern, because once a rodent enters a covered bed they don’t leave. They’re protected from the elements, and have all the food they could ever want.
If you don’t want to bait, use mechanical traps, but you will have to check regularly. If you choose not to use any protection than it’s best not to overwinter this way. Voles are voracious feeders and will eat every last plant.
How to Make Inexpensive Rodent Bait Traps
Make traps out of 1 foot long, 1½ inch PVC pipe. Pipe is very easy to cut using a hand saw. Attach an elbow on each end.
You will need to remove this elbow to put in your bait so don’t permanently seal it. Use block bait only. Pellets can fall out too easily.
When and How to Place Cover Over Cold Frame
Before you cover, plants must have been through three good freezes. In zone 6 this is usually around the middle to end of December. Plants must always be in the dormant stage, and have all dead foliage removed before covering.
The covering must be white greenhouse film only. Use 2 layers of 4ml thickness (total 8ml,) 12 foot wide. Some greenhouse film is sold as 6ml and 24feet wide. This can be doubled to get two layers of 12 foot wide plastic, plus the doubled 6ml will give you even more protection. White greenhouse film can be reused for years.
Now you will have to batten down the plastic to the boards. After you lay the plastic over the rebar hoops screw 2×4’s over the plastic and into the boards to keep the plastic in place.
If you don’t want to do this, you can use cinder blocks, but make sure everything is secure, because if the wind gets underneath, your plastic will get ripped off, and you will have to chase it in the dead of winter down the highway, and there won’t be a soul around to help you.
If you live in a colder zone you can lay a heavy weight white fiber cover (floating row cover,) over-top of the plants before you cover with the plastic. Use the row covers when overwintering small 3 inch pots for added protection. On both ends place cinder blocks to hold down the plastic.
In zone 6 cold frames are generally uncovered around March 20th. It all depends on the weather. The plants are starting to show some growth at this point. Collect all your bait traps first, and safely dispose of the unused bait. There will be very little, if any bait left in the traps.
This system is an inexpensive alternative to large Quonset huts, and has worked successfully for many large scale commercial growers.