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GARDENING: Wondrous wetlands

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The mass rearing of biocontrol insects by schools is helping to turn the tide against aquatic invasive weeds.

Celebrate World Wetlands Day by creating a mini wetland – with no invasive weeds – on your property to provide a glorious habitat for biodiversity

February 2 is World Wetlands Day. The day marks the anniversary of the adoption of the Convention on Wetlands in Ramsar, Iran, in 1971. This year’s theme is “Wetlands and Biodiversity”, to highlight the important role wetlands play as habitats for plants and animals.

Scientists say wetlands are disappearing three times faster than forests. Pollution, agriculture, drainage for land development, climate change and invasive alien plants (IAPs) pose serious threats to wetlands.

Many of South Africa’s worst aquatic invaders were introduced as ornamental plants for gardens and aquariums. Having jumped the proverbial garden fence, they spread into rivers, dams and wetlands.

Wetland invader. The yellow flag iris (Iris pseudacous) forms dense stands along riverbanks and ponds, reducing water flow. Picture: CBC

Dense vegetation or thick mats on the water’s surface block inlets and outlets and restrict the flow of water. Leaves floating on the water’s surface also block sunlight from reaching the depths, affecting all parts of the food chain and impacting negatively on biodiversity.

Removing aquatic invaders The removal of IAPs is an important step in the rehabilitation of wetlands. IAPs are either physically removed or chemically controlled using herbicides. A third method, biological control, uses the plant’s natural enemies like insects, mites, fungi or pathogens, as targeted biological control agents.

“The organism is brought over from the home range of an invasive species to control the invasive species naturally,” explains Kim Weaver, community engagement officer at the Centre for Biological Control at Rhodes University. She explains that the testing phase can take up to five years. Once the agent is deemed host specific, an application is made for its release from the quarantine facility.

Five of the worst aquatic invaders currently have biocontrol agents deployed against them.These include the water hyacinth (Pontederia crassipes) previously known as Eichhornia crassipes, Kariba weed (Salvinia molesta), water lettuce (Pistia stratiotes), parrot’s feather (Myriophyllum aquaticum) and the Brazilian waterweed (Egeria densa).

Aquatic Invader. Water lettuce (Pistia stratiotes), forms dense surface mats and creates a breeding ground for mosquitoes and bilharzia-carrying snails. Picture: Lukas Otto

Success story Insect-rearing ponds at local high schools are at the forefront of biocontrol success stories across the country. Currently, billions of tiny insects are rapidly destroying a water hyacinth mat covering Hartbeespoort Dam, north-west of Joburg.

Going forward, a range of biocontrol insects are in quarantine and undergoing host specificity testing for yellow flag iris (Iris pseudacorus), delta arrowhead (Sagittaria platyphylla) and the Mexican or yellow water lily (Nymphaea mexicana).

Invasives in garden ponds All eight invaders listed here, as well as pickerelweed (Pontederia cordata), can be lurking in your garden. These invasives are either category 1a or 1b on the National Invasive Species List, which requires them removed and destroyed.

Louisiana iris thrive in bogs and water gardens; split in late summer to early autumn. Picture Etienne Cremer

“Biological control insects are available free to the public from our mass-rearing facility,” said Weaver. “If aquatic weeds are found in small ponds, we recommend that they be removed immediately and disposed of correctly, so they don’t end up in a new water body.”

Plant alternative species Remove invasive species on your property and replant with these alternative species:

Aquatic: blue water lily, floating hearts and waterblommetjie (Aponogeton distachyos)

Marshy areas: Louisiana irises, marsh lily (Crinum campanulatum), white arums and Cyperus spp.

Margins: river star (Gomphostigma virgatum) and river bells (Phygelius aequalis)

Moist soil: horsetail restio (Elegia capensis) and Cape thatching reed (Elegia tectorum)

For more information on biological control agents for invasive aquatic plants, email cbcinfo@ru.ac.za.

Send clear identifiable images of your aquatic invasive species before applying for insects.

Create a mini wetland

Grow water plants in shallow containers set in gravel. Picture: Eco Balance

Do you want to attract more wildlife to your garden? A garden pond or mini wetland is the answer. Water attracts birds, frogs and insects to the garden, filling it with colour and sound.

◆ A natural depression, a place where water pools after rain, creates the perfect place for a mini wetland. You can also create a wetland alongside an existing garden pond.

◆ If you don’t have a natural depression, choose an area where you can direct rain water from a paved area or gutter downpipes to create a wetland.

◆ Lay out a basic shape using a rope or your hosepipe and dig out to a depth of 40 to 50cm, with gentle sloping sides.

◆ Line the hollow with thick PVC plastic. Use a pencil to puncture several holes along the sides of the liner to allow for slow drainage in very wet periods. Backfill with a mix of soil and compost. Fill with harvested rainwater.

◆ Place rocks and pebbles along the margins, to provide shelter and perches for wildlife and to hold the liner in place. Plant up to soften the edges.

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