In the Willamette Valley, we do not have winters with prolonged deep freezes. So if you want to build your own hibernaculum here in Oregon, most of the plans online will not be appropriate. Our frost line rarely exceeds seven inches and building a hibernaculum six feet deep is a waste of time.
I am in the USDA plant hardiness zone 8b with winter temps down to 10 – 15 degrees Fahrenheit. Reptiles for the most part only need to be twelve or so inches underground to be safe. So here is my hibernaculum plan:
Picking a Hibernaculum Site
Hibernaculum sites should be sloped toward the east and/or south with entrances facing in the same direction. In the spring the morning sun will warm the ground most efficiently with this orientation. It needs to be above the flood plain as well or reptiles will drown. Might site is next to a barn which will protect the inhabitants from the frigid north winds.
Constructing a Hibernaculum
While the depth can be rather shallow, I made my hibernaculum thirty inches deep with the two back corners thirty six inches deep for extra drainage. I dug it using a backhoe and made it a trapezoid shape. The back is thirty feet long and the front is twelve feet long. Both sides are fifteen feet long. I took all the dirt I dug out of the hole and put it to a side to cover the rock I brought in forming an insulation layer. Originally the site was fairly flat so using all the dirt plus the rock allowed me to build a sloped area.
I filled the two back corners with a yard of ¾” minus gravel for drainage. Then I brought in eighteen tons of rubble rock between six and eighteen inches long. On top of the rubble I placed several very large boulders to form entrances. You should not use any sort of pipes to make tunnels. This will only allow weasels and other predators access to the snakes when they are cold and extremely vulnerable. Reptiles do not need artificial entrances – they aren’t found in nature! They will squeeze down through the rocks for safety.
Next, I covered the upper two thirds of the site with landscape fabric to keep soil from filtering down into the rubble and filling in the spaces. You could just as easily use old carpet. On top of the fabric I placed all the soil I dug out of the hole. The remaining third of the site was left open and not covered with soil – this is where the entrances are.
On top of the rocks I place a large log for added cover and to create a more bio diverse site. Many of our local reptiles and amphibians eat insects and slugs and a log is a great nursery for these delicacies. For decorative reasons I added a dry river bed down the center of the site using river rock and a couple small boulders. No one says these have to be ugly, however they should not be planted and kept manicured like you would a urban landscape.
Planting a Hibernaculum
Providing cover on and around the hibernaculum is essential if you want to attract snakes. Species such as rubber boas will not stray far from the hibernaculum and require cover nearby while racers will travel out from the site in the spring; sometimes going as far as a mile away. They need the cover when emerging in the spring and returning in the fall.
The north and west side of the site can be planted with dense shrubs and groundcover which will protect the reptiles from predators. It will also provide a shady spot during the summer to help them regulate their body temperature during the hottest part of the day.
The east and south side need to have low vegetation to take advantage of the spring sun. When the temperatures start to rise and the sun hits the rocks it will warm the site and trigger the emergence of the inhabitants. I am planting this area with native wild flowers and grasses that die back in the winter and cover the area in the summer. This has the added benefit of attracting pollinators to my property.
Cost of Building a Hibernaculum
In the end, this project took two people with a backhoe two days to build. The backhoe I borrowed from a neighbor and I already had the boulders on site. The landscape fabric was left over from an earlier project but you can find free carpet from installers that are ripping out old carpets or from trade show service companies that rent carpets to exhibit booths. Eventually their carpet gets old and cannot be rented any longer.
The only material I had to pay for was the rock which cost me $382 dollars delivered and the diesel to run the backhoe. The wildflower seed are being collected by a friend and the shrubs I am digging from another part of the property. Now all there is left to do is wait and see how the hibernaculum is received by the local reptiles and amphibians.