Weeds in our Area (Part One Hundred and Nine)
By Bob and Ena McIntyre – Garden Route
BIOLOGICAL CONTROL CONTINUED
More Water Weeds – Pistia stratiotes (Water Lettuce) and
Pontederia cordata (Pickerel weed)
This week we are looking at two aquatic weeds - one steadily marching down the coast from the
east, but luckily under control with an effective a bio-control agent - Pistia stratiotes (Water
Lettuce) and another which has been spotted in the area for which there is no bio-control agent
available - Pontederia cordata (Pickerel weed).
Pistia stratiotes (water lettuce) is a listed category one invader of
uncertain origin. The plant was first recorded in North Africa around
AD77 and is now found in tropical and sub-tropical areas around the
world. The water lettuce floats on the surface with the long fibrous roots
submersed – looking remarkably like a floating lettuce. It is a perennial
with thick, soft leaves that form a rosette. The leaves can be up to 14 cm
long and have no stem. The leaves are pale green, with parallel veins,
wavy margins and are covered in short hairs. The hairs form basket-like
structures which trap air bubbles, increasing the plant's buoyancy. The
male and female organs are on separate plants, and are hidden in the
middle of the plant amongst the leaves. Small green berries form after
successful fertilization. The water lettuce can also reproduce asexually.
Mother and daughter plants are connected by a short stolon and the plants form dense mats.
First recorded in South Africa in northern KZN Pistia stratiotes has since been found in
Mpumalanga and in the Eastern Cape. Hopefully the move in our direction can be averted.
Impacts: The forming of dense mats blocks waterways, creates breeding areas for mosquitoes and
bilharzia carrying snails and displaces indigenous aquatic vegetation.
Control: A leaf and stem borer was released in 1985 which has proved successful as a control
agent for large areas of infestation. Small infestations are easily removed by hand. Although care
must be taken as the main means of reproduction is vegetative. A herbicide is registered for aerial
application.
Pontederia cordata (Pickerel weed), a listed category three plant has been
spotted on dams in the Hoekwil area. Pontederia sp. is endemic to the
Americas and is distributed from Canada to Argentina, where it is found in
shallow water or on mud. Pontederia plants have large waxy arrowshaped leaves, succulent stems and a thick pad of fibrous roots. The roots
give rise to rhizomes that allow rapid vegetative colonization. Pickerel
weed is perennial, and produces large spikes of blue flowers (50 - 150mm
long) during summer. While P. cordata is in the same family as Eichhornia
crassipes (water hyacinth) and both are invasive in many tropical and
temperate parts of the globe, they can be useful as efficient biological
filters of polluted water in constructed wetlands under controlled
conditions.
Impacts: Displaces indigenous vegetation, invades river banks and blocks drainage lines.
Control: There are no herbicides registered at this time. The only effective control is mechanical or
physical.
References: Alien Weeds and Invasive Plants by Lesley Henderson, Copyright © 2001 Agricultural
Research Council. Global Invasive Species Programme © 2004, Problem Plants of South Africa by
Clive Bromilow 2001. http://www.wikipedia.org and http://www.arc.agric.za.