Effect of serrated tussock-killing herbicides on trees

M. H. Campbell

56 Green Lane, Orange; formerly NSW Agriculture, Orange, NSW, 2800

When aerially spraying serrated tussock in hill country, it is not possible to avoid applying herbicides to trees.  Flupropanate herbicides have little or no detrimental effect on trees whereas glyphosate herbicides can kill trees when applied at the high rates necessary to kill serrated tussock.

Taskforce herbicide

When applied at rates recommended to kill mature serrated tussock (1 to 2 L/ha of product with 75% active ingredient), Taskforce has little or no effect on mature eucalypts, wattles, kurrajongs and radiata pine.

In experiments in New Zealand, mature radiata pine trees tolerated rates of Taskforce as high as 9 L/ha. Under these extreme conditions the trees were damaged but more than 90% survived.

In experiments near Braidwood in NSW, two-year old radiata pine trees had 100% survival when Taskforce was applied at 2 and 3 L/ha to the tree foliage or to the soil under the trees. At the higher rate some leaves were killed on the main stem just under the apical bud one year after application but this did not affect growth rates. 

In glasshouse experiments, seeds of radiata pine and five eucalypt species germinated, emerged from the soil and grew unharmed under pre-emergence rates of 2 and 4 L/ha Taskforce (Campbell and Nicol 1998). Under these conditions the survival of wattle (Acacia dealbata) and she oak (Casuarina cunninghamiana) seedlings was, respectively, 78% and 85%, 58 days after sowing.  

When aerially applied, Taskforce is initially deposited on the tree leaves but it is washed to the soil by rain where it is taken up by serrated tussock roots. In most cases it will kill the serrated tussock under scattered to moderate densities of eucalypts. Naturally, distribution of the herbicide can be hindered by moderate to dense stands of trees but, with a favourable cross breeze at spraying, acceptable kills of serrated tussock can occur.

Glyphosate herbicides

When applied at rates sufficient to kill serrated tussock (4 to 6 L/ha of product with 49% active ingredient), glyphosate will severely damage or kill mature eucalypt trees. Most eucalypts will tolerate up to 2.5 L/ha but some species, for example stringy bark and iron bark, will be damaged at these relatively low rates.

Young trees are more susceptible to glyphosate. For example, the percentage survival of three and five month-old eucalypts seedlings (mean of nine species) varied from 8% to 44% when sprayed with 0.48 L/ha glyphosate (Campbell and Nicol 1998). Seedlings of radiata pine of the same ages had 100% survival at 0.48 L/ha but only 44% survival at 0.96 L/ha.

Conclusions

For selective removal of serrated tussock from trees in hill country, Taskforce, at 1 to 2 L/ha, will remove the weed without seriously damaging trees of any age. In addition, some native grasses (kangaroo grass, red grass and poa tussock) and broadleaved plants will tolerate Taskforce at these rates. However, wallaby grasses and weeping grass will be severely damaged. 

Campbell, M. H. and Nicol, H. I. (1998). Tolerance of tree seedlings to pre- and post- emergence herbicides. Proc. 9^th Aust. Agron. Conf. Wagga Wagga, 276-78.