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SUMMER ANNUAL WEEDS

A profile of two of the key summer annual grasses pest managers need to know how to tackle for successful lawn management.

Summer annual grasses are some of the most persistent and prolific weeds of turf in Australia. Among them, summergrass (Digitaria spp. pictured above) and crowsfoot grass (Eleusine indica) are the most economically significant across the majority of the country with a vast portion of summer weed control programs being targeted at these species. The good news is that with a well-timed application of a pre-emergent herbicide, pest managers can provide season-long protection in a single application.

Two of the key summer annual weeds are summergrass and crowsfoot.

Summergrass (Digitaria spp.)

Description:

  • Spreading, mat-forming monocot with light, yellow-green leaves
  • Membranous ligule, folded vernation purpling at the sheaths and stem bases
  • Inflorescence consists of multiple small, finger-like spikes radiating from a central point.

Propagation:

  • A prolific seeder fruiting throughout the warmer months and germinating between 12-15 ̊C
  • Propagated through seed and vegetatively through stolons.

Crowsfoot Grass (Eleusine indica)

Description:

  • Tough, dark green monocot with flattened stems and strap-like leaves
  • Smooth leaf blades with folded vernation and white sheaths
  • Short membranous ligule divided at the centre
  • Tolerates low mowing heights
  • Inflorescence consists of between two and ten spikelets on a long stem.

Propagation:

  • Seeds profusely from late spring through to autumn and germinates between 16-18 ̊C.
Crowsfoot Grass (Eleusine indica)

Pre-emergent control

Pre-emergent herbicides provide pest managers with a unique opportunity to approach weed control in a proactive manner. Reactive applications of post-emergent herbicides are often an inefficient approach to weed management, as it is simply supressing the symptom rather than eliminating the problem altogether. A well-timed application of a premium pre-emergent herbicide can offer season-long protection from germinating grassy weeds such as crowsfoot and summergrass.

Correct timing is crucial when it comes to pre-emergent herbicides. As most pre-emergents have no activity on existing weed populations, the initial application must be positioned prior to germination of the target species. However, applying the herbicide too early can also be problematic as the herbicide level in the soil can drop below an effective level prior to the season’s end. Traditionally, application timing is accurately predicted based on soil temperature.

However, measuring soil temperature may be too onerous for most pest managers. As such, with most pre-emergents providing long-lasting residual control, going in early in the season and planning to apply a further one or two applications through the season, will not only provide complete protection but allow regular service calls on your customer. A number of pre-emergent herbicides are available in both liquid and granule format, but as with all herbicides make sure it targets the weed in question and that it is suitable for use on the turf grass present.

If you miss the start of the season and weeds have started to emerge, you may need to apply a post-emergent herbicide. However, there are some pre-emergent herbicides that also possess post-emergent performance properties, so it may be possible to get away with a single application even if the weeds have germinated and started to emerge.