Bushland Weeds Manual - Environmental Weeds Action Network

14
Chapter 3 Grass Weeds
• Transition occurs when the growing points stop
producing vegetative material and start developing
flowers and the tissue between nodes, the
internode, elongates. Because of internode
elongation, slashing or mowing at this stage can
remove many axillary buds, thereby reducing leaf
production in the regrowth.
• Flowering begins when the seed head is just
emerging from the leaf sheath (boot stage) and
continues through to seeding. Some grass-selective
herbicides are only effective if applied prior to the
boot stage. Slashing or mowing during flowering
may facilitate the spread of seed.
The period of active growth depends largely on
whether the plant is a cool season (C3) or warm
season (C4) grass. Cool season and warm season
grasses differ in the way they use carbon dioxide. Cool
season grasses (Wild Oat, Blowfly Grass, Perennial
Veldgrass) photosynthesise more effectively, and thus
are most actively growing, during the cooler periods of
winter and spring (15-25° C). Warm season grasses, or
summer grasses, photosynthesise best with full light
saturation and consequently their period of most
active growth is late spring and summer (25-40° C).
The summer growing Couch (Cynodon dactylon),
Kikuyu and Lovegrass are all C4 grasses.
For successful herbicide uptake grasses should be
sprayed in the season appropriate to their
photosynthetic pathway and when actively growing.
Fire and the spread of grass weeds
Many grasses, especially caespitose species, have long,
narrow, vertical leaves that are efficient in strong light
allowing sunlight to penetrate deep inside the clump
(Gibbs Russell et al. 1991). This structure allows the
production of a large biomass in a small space. Stems
die each year and new ones develop. Over time a large
biomass, much of it dead material, creates a significant
fire hazard. As a result, grass weed invasions can lead
to changes in fire frequency and intensity.
These changes are self-perpetuating as the disturbance
caused by fire tends to promote germination and
establishment of seedling grasses (Cheplick 1998,
Williams and Baruch 2000). At Shenton Bushland,
Perth, Perennial Veldgrass was mapped in the year
following an intense fire that burnt half of the site. A
comparison between a pre-fire bushland condition map
and the post-fire Veldgrass map, highlighted a
significant post-fire expansion in the population.
Previously healthy bushland now recorded 75-100 %
cover of Veldgrass (Brown and Marshall
unpublished data).
Fire contributes greatly to grass invasion in lownutrient
soils, where post-fire increases in nutrients,
light and space availability are more effectively
exploited by invasive grasses than by the native flora.
It is important to reduce the possibility of fire in
bushland areas at risk of grass invasion. Where
bushfire does occur target resprouting plants and
seedlings for control before they become
established.
Management and control
Prevention and early intervention
Degradation of our native bushland remnants through
clearing, fire, disease and mismanagement may be of
particular relevance to the establishment and spread
of grass weeds. Often an observed decline in the
canopy cover within Banksia woodlands around Perth
correlates with an increase in the occurrence of
Perennial Veldgrass. Reduction in canopy cover leads
to an increase in light availability, a condition favoured
by many grass weeds (Williams and Baruch 2000,
D’Antonio et al. 2001).
Weed grass invasion can be limited by:
• Maintaining the bushland canopy. Many grasses
prefer open sunny sites and do not establish or
compete successfully in the shade.
• Reducing the potential for bushfires. As
mentioned, fire can significantly increase the
establishment and spread of grass weeds.
• Identifying unfamiliar grasses. It is important to
determine if grasses are introduced or native.
Early identification of grass weeds allows you to
assess the likelihood of invasion and prevents
mistaken eradication of native grasses!
• Establishing weed barriers.
Weed barriers
A common grass weed problem in urban remnants is
the spread of Kikuyu, Couch or Buffalo Grass
(Stenotaphrum secundatum) from adjacent parks and
road verges. A successful ‘weed break’ can be created
by embedding a physical barrier such as weed mesh or
conveyor belt vertically into the ground and using
concrete kerbing above it. This prevents rhizomes
creeping into the bushland and provides a mowing
edge for council mowers (Box 3.4). Some grasses have
rhizomes 50 to 60 centimetres below ground and any
root barrier must allow for this.
The bush itself often provides an excellent weed break
(Box 3.2). However, the sandy soils and open
vegetation of Banksia woodland or the friable soils of
York Gum–Jam woodlands are more susceptible.
Establishing a dense cover of locally-occurring low,
medium and tall shrubs along the edges of degraded
sites and paddocks could act as a sieve, preventing
entry of wind and water carried seed. Knowing the
prevailing wind direction, position in the landscape
and relationship to waterflow can help to determine if
the adjacent paddock, weedy rail reserve or over-run,
semi-rural backyard, is the weed source. When planting
or direct seeding, use native seed collected from the
area of bushland you are protecting and only plant
where run-off or prevailing winds are definitely
carrying weed seed in.