Frogs are under threat throughout the world and loss of habitat is one of the factors contributing to their demise. A frog pond is easy to construct, adds interest to your home garden and will provide a haven for the frog species in your area. The pond must be free of predatory fish and polluting chemicals. It must also have gently sloping sides for the frogs to emerge and vegetation shelter around the pond.
Please don't introduce frogs that don't occur naturally in your area. They may introduce frog diseases or upset the local balance in other ways.
The following information is based on Frog Facts Number 2, produced by the Frog and Tadpole Study Group, a special interest group of the Australian Herpetological Society Incorporated (December 1992, revised June 2001).
A pond with flowering water plants can be a very attractive focal point in a garden. These plants are generally resistant to pests and diseases and require little maintenance. Observing the lifecycle of frogs throughout the seasons provides added interest to your gardening. Frogs also help to control insect pests.
- An ideal place is part sunny, part shady, but not directly under trees. Give a wide berth to trees that have poisonous leaves (oleander, Bleeding Heart and pines for example).
- To get the most enjoyment from your pond, locate it so that it's visible from the house but in the back garden, a bit away from your own house and your neighbour's houses, because frogs can be noisy at times.
- A low garden lamp that is reflected in the water will not only add to your garden's appearance in the evenings but also attract insects for the frogs.
- Your compost heap, another good source of insects, should be close to the pond. Part of the heap can be slightly raised on a wire mesh tray or a pallet. Insects and worms that fall through will provide food for the frogs sheltering below. Other shelters at the edge of the pond should consist of vegetation, a rock pile and planks or fallen logs.
Randwick City Council requires any pond deeper than 30 centimetres to be surrounded by a safety fence.
Shallow ponds can overheat in summer, so consider how it will be shaded or cooled. Plant part of the pond area heavily with sedges and other emergent plants to provide shade and cooler water in the denser areas, or shade with sailcloth, a beach umbrella or a gazebo.
To construct a frog pond:
The hole in the ground should have gently sloping sides so that non-climbing frog species can get out and so that the earth wall does not collapse. It should also have a flat bottom for shallow planter pots to stand on. Alternatively, you can continue the slope down to a depth of about 70 centimetres, and, once the liner is in place, put deeper planter pots in the middle and cover the spaces around them with large round river rocks, thus reducing the water depth back to 30 centimetres.
The underlayer protects the liner from any sharp objects in the ground that may have escaped your notice. You can use a thin layer of damp sand, a thick layer of newspapers, carpet underfelt if it is free of staples, or one or two layers of an old tarpaulin. Also cover any sharp brickwork edges.
3. Pond liner
The pond liner should be a black, flexible sheet and should be heavy duty, from pond product suppliers. Unless you are building a temporary pond (on a patio for example), don't use building grade polyethylene, as it will deteriorate in sunlight. The liner must be big enough to cover the hole and sides, and also the surrounding area by at least 40 centimetres all around. As you are filling the pond, tug and adjust the liner into place. Use soil from the excavation to surround the pond with a shallow mound, to prevent rainwater runoff from entering the pond.
4. Pond edge
To form the pond edge, level and flatten the surrounding earth mound, so that its top is evenly 10 centimetres above the water level. Stretch the outer part of the liner over this and anchor it by placing large flat rock slabs on top. Take care the slabs overhang slightly, to hide and shade the dry part of the liner. Also ensure there are spaces under the slabs, or that they rest partly on top of each other, because frogs cannot negotiate an overhang and would eventually drown or starve in the pond. Then cement the slabs together in enough places to stop them from falling in when standing on them, but without blocking the frog paths underneath.
Add plenty of emergent swamp plants, including those that will look green and fresh in winter, such as Water lilies, Azolla and Duck weed (Spirodela pusilla).
Water lilies in a sunny position provide cover and help discourage marauding birds and algal blooms. Emergents can be grown bare-rooted, held down by a rock resting partly on their roots. They will then take nutrients directly from the water and will improve the water quality.
Alternatively, grow them in trays or pots without open drain holes and cover their soil with sand, securing the pots with bricks or rocks. Certain aquatic plants are fast-growing and should be thinned out by removing the surplus plant. You are also removing surplus nutrients from the pond, aiding the water quality. In time, you should also allow a thin layer of dead leaves to accumulate in the pond. This provides a larger surface area for useful nitrification bacteria and material for browsing by tadpoles. Place some spreading garden plants such as Tassell Rush (Restio tetraphyllus), Spotted Knotweed (Persicaria decipiens) and Knobby Club Rush (Isolepis nodosa) around the pond edge as hiding places for frogs.
The addition of a small submersible pump gives you several advantages:
- The pump allows you to construct a shallow creek bed leading up to the pond. The creek is fed by the pump outlet hose, which is easily hidden in a rock fall. This habitat, although very small, may be suitable for some stream-side frogs.
- As the pump will need a pre-filter, it will also improve the water clarity. Suspended algae, however, are more easily controlled by reducing the sunlight through more water lily cover and by avoiding a build-up of nutrients in the water. Ensure that the filter surface is large enough so that small tadpoles don't get sucked against it.
- You can remove water for gardening purposes from the rock fall without scooping tadpoles out. The pump is also useful if the pond ever has to be drained.
- Flowing water is pleasant and allows easier maintenance of water quality. It is better oxygenated and less likely to stagnate and it reduces overheating of the top water layer.
A frog pond should have fish in it to control mosquito larvae. Tadpoles are ineffective against them, and most frogs eat on land. All pond fish eat tadpoles, but a few species will leave the older tadpoles alone. Never introduce the predatory Plague Minnow - formerly called Mosquito Fish (Gambusia holbrooki) - or goldfish into the pond. To control mosquito larvae, use the White Cloud Mountain Minnow (Tanichthys albonubes) from aquarium shops. It will also breed in your pond. These fish will still eat many newly hatched tadpoles. More will survive if you collect the spawn and keep the young tadpoles in a separate container until they are 15 millimetres long. Place nylon fly screen over your pond so that birds don't get to the fish.
Cats and small children can be discouraged by a surrounding bog area or dense prickly ground cover. Large birds that eat frogs and tadpoles may be warded off by suspended strips of cloth. Tortoises should not be put into a small frog pond.
Tap water often contains sufficient chlorine to kill tadpoles. Pond and aquarium shops sell water conditioner that will safely remove both chemicals. However, in an aged pond containing plants and detritus, there is no need to condition the tap water, provided your regular water changes are small. If your pond is without a circulating pump, replace no more than five per cent of the water at a time, and do this often enough to prevent algae from taking over (weekly if the pond receives much sunlight). You can give a pond with much plant material and good biological filtration a 10 to 20 per cent water change with untreated tap water, provided you spray or mix it in slowly. Periodic partial water changes are useful in removing accumulated nutrients, toxins and acid or alkaline substances. Don't change more than 30 per cent of the water on the same day (even with conditioner), so that tadpoles have time to adjust to the new water.
Frogs and tadpoles have permeable skins and are particularly susceptible to pesticides. Fertilizers and manure also affect tadpoles, especially by their ammonia component. Keep these substances well away from the pond and don't let rainwater runoff or compost heap runoff enter the pond.
This is usually the least of your worries as your local frogs will almost invariably find the pond before long and will spawn in it. With the current outbreaks across Australia of the frog disease chytrid fungus, it is now more important than ever that you don't bring tadpoles or frogs in from the wild (this is also illegal in New South Wales). If you accept tadpoles from other gardeners, they should be from the same suburb. Don't infect new areas with this frog killing disease.
Note: Generally, there is no point in releasing frogs in a garden. They have a homing drive and are unlikely to stay.
Please don't introduce frogs that don't occur naturally in your area. They may hybridise with local varieties of the same species or crowd other species out, or upset the local balance in other ways: remember the cane toad!
Remove the goldfish, drain and dry the pond to remove their spawn and fry. Arrange for sloping sides at surface level and ensure there are plenty of plants and moist sites around at least part of the pond. Fill, and when the water is matured, add White Cloud Mountain Minnows. Wait for your local frogs to find the pond.
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