S/Reed and Ginger s2

Weeds in our Area (Part Sixty Six)

By Bob and Ena McIntyre, Botanical Society of S.A. – Garden Route Branch

A recent trip into the countryside triggered this week’s choice - Acacia dealbata (Silver wattle). Silver wattle is listed as a category one invader in the Western Cape, in the rest of the country it is a category two. Exactly when and how Silver wattle was introduced into South Africa is not known. It can easily be confused with A. mearnsii (Black Wattle) and may have been mistakenly imported and planted as Black Wattle. Its commercial value does not equal that of Black Wattle and it is therefore not planted for commercial purposes. The eastern regions of the country are the worst affected with isolated infestations along the Garden Route and the Little Karoo extending to the Cape. Silver wattle has a major impact on water courses and crowds out indigenous flora.

Identification: A. dealbata like all other Australian Acacias is unarmed (no thorns as is the case with all indigenous Acacias occurring in Africa). The leaves of A. dealbata are bipinnate, silver grey to light green, finely hairy with short leaflets (2 – 5 mm). The glands on the central leafstalk are positioned at the junction of each pair of pinnae. The flowers are pale to bright yellow, globular in large fragrant sprays during July and August. Fruits are greyish or purplish-brown pods that are occasionally constricted (indented).

Control: Silver Wattle produces large numbers of long lived seeds and coppices readily when cut, necessitating sustained control and follow-up. This species can however be treated in the same way as black wattle. For easy reference we repeat the description of the treatment. Trees may be killed standing by using a frill and herbicide application. Young plants may be strip-barked; the bark is stripped all round the stem from about 50cm above ground down to the root ball. In this case no herbicide is necessary provided you ensure that no bark remains on the root ball – a little more work but certainly an eco-friendly method. The species may also be killed with a basal stem treatment (painting the stem with the herbicide mixture from about 50cm to ground level) with a suitable herbicide mixed with diesel. Like Black wattle, the Silver wattle’s vast numbers of seeds can remain dormant for up 50 years; therefore any clearing project must include a well-planned and thorough follow-up programme. Germination is triggered by fire, which means that the seed bank can be greatly reduced if the area cleared can be safely burnt with the local authorities’ permission. Post-burning action must include herbicide application and/or manual follow-up to eradicate the thousands of seedlings that will germinate.

Substitutes: Ptaeroxylon obliquum (Sneezewood), Pterocelastrus tricuspidatus (Kershout, Cherrywood), Rhamnus prinoides (Blinkblaar), Rothmannia capensis (Mock gardenia)

BotSoc Forthcoming Events

C R E W (Custodians of Rare and Endangered Wildflowers) workshop at the Moriarty EE Centre, 49 Caledon Street, George on 22 July 2006 – Please contact McIntyre’s below.

Bitou Twig: Outing to St Francis Bay – 15 July 2006 to the Irma Booysen Reserve with Prof Richard Cowling – Please contact Jean Sparg or Maaike Murphy below. Don’t miss this great opportunity.

BotSoc Contacts in your area: George/Wilderness: McIntyre’s 044 877 1360

Knysna: Chris Gow 044 387 1978, Ariane Rohloff 044 382 7993

Mossel Bay: Lucretia van der Walt 044 691 3230. Stilbaai: Janet Naude 028 754 1106

Plettenberg Bay (Bitou Twig): Jean Sparg 044 533 2625, Maaike Murphy 044 531 6857