A Guide to Hibernacula Design and Construction By Vince Fiorito with assistance from the editors of the Blazing Star ( NANPS.org ) Hibernaculum (hibernacula in the plural) is a zoological term referring to a place of abode in which a creature seeks refuge for the purpose of hibernation or overwintering. Hibernacula attract insects, spiders, amphibians, mammals, reptiles and even birds. I build hibernacula because I believe balanced ecosystems include both plants and animals. Also, I find the increased wildlife activity associated with hibernacula to be an entertaining highlight of my native plant gardens. Hibernacula can occur naturally or be built by humans. Examples of natural hibernacula include bear dens, bat caves, beaver lodges and loose soil under fallen trees. Manmade hibernacula include more elaborate, intentional structures (see below) or simply the loose soil under compost piles and crevices behind retaining walls. Most hibernacula offer hibernators a welcome above-freezing microclimate during the winter. These conditions exist in underground chambers below the frost line or near moving water or they result when decomposing organic material generates heat. If you decide to construct a hibernaculum, plan wisely. Hibernacula are long-term structures that evolve over time. Choose an out-of-the-way location where the hibernaculum won’t become an obstacle. Leverage the site’s existing structures (rocks, stumps…), grade (flat, sloped…), vegetation (trees, shrubs…) and edges (fences, walls…) to advantage. Hibernacula will attract different species based on size, depth, humidity and composition. Snakes and reptiles prefer well drained crevices between rocks below the frost line. Chipmunks dig into soil to create tunnels and chambers to hoard food. Salamanders and toads bury themselves deep under leaf litter and loose soil. A small hibernaculum of a few cubic meters or less may only support insects and a few small mammals. The larger the hibernaculum, the more life it can support. A large enough hibernaculum of the right design in the right location might even attract a bear. (Contact me if you think you have the right location!) Before starting, always prepare the hibernaculum’s site with a thorough cleaning to remove undesired synthetic and contaminated materials (glass, plastic, treated wood, paint…). The Brush Pile Hibernaculum The brush pile hibernaculum recreates habitat associated with a fallen tree and requires clean organic material. Thanks to this winter’s ice storms in eastern North America, you may already have an ample supply. Instead of burning or hauling away dead trees and branches resulting from the storms, consider using this “windfall” for a brush pile hibernaculum. The first step in building a brush pile hibernaculum is to loosen up the site’s surface, making it easier for the future inhabitants to dig tunnels. Next, create a stable foundation with the biggest logs. If the site is sloped, partially bury some of the largest logs to aid stability. Arrange the logs so that they have crevices, gaps and open spaces between them, large enough for hibernaculum’s target species. Fill the gaps between the foundation logs with woodchips, leaves, grass cuttings and/or other organic material which will decompose, create heat and warm the brush pile hibernaculum’s interior. Then progressively add smaller logs, branches and sticks filling in the exposed open spaces. Once all the wood has been placed, add plant cuttings, leaves, wood chips and grass cuttings to create roof-like insulating structures over the open interior spaces. Decomposing organic material in the brush pile hibernaculum attracts insects (including lacewings, assassin bugs and bumblebees), grubs, slugs and spiders, which in turn attract fireflies, toads, wrens, warblers and woodpeckers to eat them. Rotting wood supports mosses, mushrooms and other species of fungi. Deep inside, the warm microclimate provides winter nesting sites for mice, squirrels, rabbits and groundhogs. These small mammals then become prey for hawks, weasels, foxes, coyotes and other predators. I’ve observed that most people don’t immediately appreciate the beauty of a pile of dead branches which is why I tend to construct brush pile hibernacula in discreet locations, hidden behind an outbuilding or shrubs. I highly recommend taking advantage of a brush pile’s trellis-like structure to grow vines. If you are in a hurry to hide your brush pile, consider planting Virginia creeper (Parthenocissus quinquefolia) and river bank grape (Vitis riparia) around the edges. Any vigorously growing native vine should create the desired camouflaging effect. Showier vines include Virgins bower (Clematis virginiana), trumpet creeper (Campsis radicans) and American bittersweet (Celastrus radicans). Enhance the appeal of the brush pile with berry bushes and fruit-bearing shrubs to attract birds. Choose plants that grow tall while providing food and cover. Examples of suitable vegetation include blackberry (Rubus canadensis), raspberry (Rubus strigosus), elderberry (Sambucus canadensis) and nannyberry (Viburnum lentago). Complete the brush pile with companion plantings of tallgrass prairie species. You will want fast growing, tall forbs like perennial sunflowers (Helianthus spp.), cup plants (Silphium perfoliatum), asters (Symphyotrichum spp.) and goldenrods (Solidago spp.). Cup plants provide an additional benefit: the leaves form a cup around the stalk to create a tiny reservoir that holds rainwater or dew for birds and insects to drink. The brush pile hibernaculum will compress over time as the organic material decomposes into rich compost. You can maintain its original size by continually adding twigs, branches and leaves. The end result should be a focal point of wildlife activity in your garden that resembles a thicket rather than a pile of dead wood. The Rock Pile Hibernaculum Rock pile hibernacula create habitat for cold-blooded animals which are more accurately described as ectotherms. Ectothermic snakes, reptiles and amphibians are among Ontario’s most threatened species. The primary stress for these endangered animals in our area is habitat loss due to agriculture, industry and urbanization. Rock pile hibernacula can help these species survive human disturbances and assist in their recovery. Many ectotherms will seek out warm surfaces like dark, sun-exposed rocks when they are active to raise their body temperature. In the fall, they seek frost-free overwintering sites which are safe from predators. Rock pile hibernacula remain above freezing throughout the winter because they have crevices which reach underground chambers below the frost line. Simply digging a hole below the frost line and then filling it with rocks may be ineffective. During a spring thaw or heavy rain, this structure may fill with water and could drown the inhabitants. Ideally, you want to construct your hibernaculum on a south-facing slope and dig laterally into the side of the hill. The foundation layers and innermost recesses of the rock pile hibernaculum should be accessible, well drained and well below the frost line. Create access to and drainage away from the interior with weeping tile and coarse gravel. Since this part of the structure will be hidden, you can also use broken clay tiles and pots, brick, smashed concrete and other clean recycled materials. Poured concrete can create stable roof-like structures over open interior spaces and act as a load-bearing base that blocks surface level sand, gravel, soil and debris from washing into the underlying open spaces and restricting access to the interior caverns. Dark rocks can passively generate heat with solar energy. Flat rocks on the surface can act like stepping stones, allowing easy access for future maintenance. Once the rocks are placed, wash gravel and sand into the surface level openings. Fill the gaps between surface rocks with topsoil, creating an insulating layer and foundation for companion plants. Remember that the purpose of some of the rocks is to create warming surfaces for cold-blooded animals. Since you are not trying to create shade or shelter above the surface, typical rock garden plants work well for a rock hibernaculum. Look for shorter, smaller grasses and forbs that thrive in narrow crevices with minimal soil in full sun. Recommended rock garden favourites include early saxifrage (Saxifraga virginiensis), nodding wild onion (Allium cernuum), blue-eyed grass (Sisyrinchium montanum), long-leaved summer bluet (Houstonia longifolia), prairie smoke (Geum triflorum), yarrow (Achillea millefolium), prickly pear cactus (Opuntia humifusa), Carolina puccoon (Lithospermum caroliniense), Virginia spiderwort (Tradescantia virginiana), cylindric blazing star (Liatris cylindracea), pearly everlasting (Anaphalis margaritacea), large-flowered beardtongue (Penstemon grandiflorus), tall thimbleweed (Anemone virginiana), sundial lupine (Lupinus perennis) and silverweed (Potentilla anserina). The brush and rock pile hibernacula described above are just two possible designs that create above-freezing microclimates. Aquatic versions of both types exist along shorelines and streams providing overwintering habitat for mudpuppies, frogs, turtles and fish. Hybrid versions combine rocks and organic material. A hibernaculum can be a showy centerpiece in your garden or a subtle feature. Either way, it creates habitat for wildlife, an increasingly necessary service native plant gardeners can provide in our developed world.
Posted on: Tue, 23 Dec 2014 15:29:12 +0000
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