Eucalyptus Species, blue gums,bloekomme
(especially E. grandis – saligna gum; and E. camaldulensis – red river gum)
Eucalyptus grandis (saligna gum)
E. camaldulensis choking Melkrivier
Description: Tall, (25-50 m) evergreen trees; trunks shaft-like (E. grandis) or branched (E. camaldulensis) with high canopy. Bark generally smooth, mottled grey-white (or reddish for E. camaldulensis), but that of E. grandis peeling in long thin strips to expose powdery pale undersurface. Long narrow, green leaves, strongly aromatic when crushed; creamy white flowers; fruits are brown capsules. May exude a sticky gum from breaks in the bark.
Origin: Introduced from Australia and cultivated for timber, shelter and firewood, as windbreaks, as sources of honey; and to reduce marshland. Of over 700 species of Eucalyptus, (native to Australia and southern Asia), some 200 have been cultivated in southern Africa and 9 have been declared as in invasive weeds in SA. Only E. grandis and to a lesser extent E. camaldulensis, are important in the Waterberg.
Occurrence: Eucalyptus species have been planted extensively on farmland borders across the Waterberg (very few as timber plantations). Dense stands occur in proximity to moisture, with the result that they are especially prevalent along watercourses and adjacent to roads.
Why it is a problem: Many species of Eucalyptus have been of great benefit to the South African economy, and so care should be taken only to eliminate those species that have been identified as invasive and particularly those that occur along watercourses.
Eucalyptus is an extremely thirsty plant: it is estimated that a single mid-height tree consumes ~70 litres of water per day. Groups of the trees therefore contribute materially to lowering of the water table, to the reduction or elimination of stream flow and to the obstruction of watercourses.
They can out-compete indigenous species and it seems that they are allelopathic – i.e. that they secrete chemicals that suppress the growth of an understorey.
Dropped seeds readily germinate beneath the shelter of their parent trees and when the latter are felled, access to sunlight promotes very rapid growth of a high density of saplings.
The tree contains highly flammable aromatic oils which, together with a typically thick ground layer of leaf litter, presents a high fire risk and can aggravate the spread of uncontrolled veld fires.
The two species mentioned here are declared invaders (Category 2) in most parts of the country. New legislation, still to be promulgated, will propose that all listed species are prohibited within riperian areas (e.g. watercourses), wetlands, declared montane catchment areas, declared high fire risk zones and in protected areas.
Elimination / Control Methods: The species are amenable to physical and herbicidal control, but their readiness to re-grow and the propensity for seedlings to germinate once exposed to sunlight necessitates repeated attention and herbicide application.
The tree can be killed by ring-barking, which is simple, cheap and effective; however, this results in an unsightly and potentially dangerous tall dead tree, surrounded by a mass of young, fast-growing saplings. With somewhat more effort, it can be chopped down for firewood or timber, assuming that access to the site permits the removal of the wood; but once again, the removal of the adult tree allows for the growth of saplings. In both cases, therefore, both the stump of the adult tree and the saplings require repeated application of an appropriate herbicide over several years.
Numerous herbicides have been registered for application to Eucalyptus. Most are based on one of the following active ingredients:
Glyphosate (isopropylamine), 360g a.e./l SL (for cut stump application): e.g. Springbok (L6719); Roundup (L407); Erase (L6206); Glyphosate (various); Mamba (L4817); Kilo WSG (L7431).
Metsulfuron methyl 600g/kg WG (for foliar or cut stump application): e.g. Brush-Off (L4535); Nicanor 50 WP (L6583).
Picloram 240 g/l SL (for frill or cut stump application): e.g. Access (L4920); Browser (L7357)
Imazapyr 100 g/l SL (foliar application): e.g. Hatchet (L7409); Chopper (L3444)
Triclopyr (butoxy ethyl ester) 480 g/l EC (foliar application): e.g. Triclon (L6661); Garlon 480 EC (L4916); Ranger 240 EC (L6179).
Note, however, that recognition of specific Eucalyptus species is difficult. The products mentioned may be effective on the species listed on the product labels, but be ineffective on others. It is not possible to recommend a single product that will effectively control all Eucalyptus species, including hybrids developed for commercial purposes.
NB: Follow carefully the instructions provided on herbicide label. Many herbicides can be toxic to other plants and or game and livestock if used inappropriately. (Mis-use of herbicides is also a criminal offence in terms of Act No. 36 of 1947).
ARC-LNR Weeds & Invasive Plants website: www.agis.agric.za/wip
ARC-LNR SAPIA News 12 (July 2009): ARC – Plant Protection Research Institute, Pretoria. www.arc.agric.za
Bromilow, Clive (2010): Problem Plants and Alien Weeds of South Africa. Briza. Pretoria
Henderson, Lesley (2001): Alien Weeds & Invasive Plants. Agricultural Research Council (ARC), Pretoria.
Van Zyl, Kathy, compiler (2005): Control of Unwanted Plants. Xact Information, Pretoria.
Van Zyl, Kathy, compiler (2012): Problem Plant Control Compendium, AVCASA, Midrand
Special thanks to Dr Gerhard Verdoorn of Griffon Poison Information Centre, to Mr Ferdie Jordaan of Arysta Lifescience and to Ms Lesley Henderson of ARC for their invaluable advice and guidance. Their enthusiastic support for this voluntary project is greatly appreciated.