Various perennial weeds that can thrive in short regularly mown
lawns can be troublesome. They usually originate from seeds carried
by wind or birds. Once lawn weeds have germinated, mowing then rapidly
spreads them. Thus, early treatment of the weeds is required to
eliminate them from the lawn.
Some weeds, e.g. cape-weed (Arctotheca calendula) and
daosoes (Bellis perennis), may look attractive in utility
lawns, but are undesirable in high-quality lawns. In some regions
they are declared as noxious weeds. Many weeds survive regular cutting,
and so must be controlled with herbicides or by hand weeding.
The most troublesome lawn weeds are winter grass (Poa annua),
nut grass (Cyperus rotundus), slender speedwell (Veronica
filiformis), wax weed (Hydrocotyl americana) and baby’s
tears (Soleirolia soleirolii). These persistent weeds are
not controlled by herbicides registered for lawns. Moss may also
be a problem.
Other lawn weeds that are less of a problem as they are controlled
by herbicides include yarrow (Achillea millefolium), Mouse-ear
chickweed (Cerastium glomeratum), bindii or Onehunga weed
(Soliva pterosperma), broad-leaved plantain (Plantago
major), self-heal (Prunella vulgaris), creeping buttercup
(Ranunculus repens), Sheep’s sorrel (Rumex acetosella),
lesser yellow trefoil or suckling clover (Trifolium dubium),
and common white clover (Trifolium repens).
Good lawn care is an effective preventive measure. The presence
of numerous weeds in a lawn usually indicates that the grass is
not growing sufficient vigorously to prevent weeds from establishing.
Low soil fertility and drought are the most common causes for poor
growth of turfgrasses. Soil compaction and mowing the grass too
short may also lead to invasion by moss.
Removing weeds by hand is effective for a few scattered rosetted
weeds such as English daisy (Bellis perennis), dandelion
(Taraxacum officinale), and plantains (Plantago
spp.) Use a daisy grubber or hand fork to lift the weeds, and then
firm back the displaced turf.
Moss is prevalent in lawns where soils are compacted, have poor
drainage, low fertility, insufficient light, closely cropped cutting
regimes, and too acidic or basic soils. Moss can be removed temporarily
using a special herbicide containing benzalkonium chloride, copper
sulfate or dichlorophen (Table 6), followed by scarification. Iron
(ferrous) sulfate in lawn sands can also be used to eradicate moss.
However, unless the soil is in a condition conducive for healthy
turf, the moss will return, necessitating removing the underlying
cause of moss intrusion. Improving soil aeration, drainage and fertility,
and top-dressing on light soils to assist in water retention, could
improve lawn health. Adopting an annual maintenance programme (Table
4) will alleviate moss invasion.
The herbicides registered for use in turf and the weeds they control
are listed in Table 1. They are selective for specific weeds, and
usually do not harm the turf grasses when applied at recommended
rates to lawns more than six months old. Higher rates may kill even
a mature lawn. Younger turf may be susceptible to herbicides, even
when they are applied at rates recommended on the label.
The herbicides translocate within affected plants from the leaves
to the roots. Within a few days of application, the weeds usually
begin to distort and shrivel.
Proprietary lawn herbicides often combine two or more active ingredients
in order to control the widest possible range of weeds in one application.
Active ingredients that are commonly mixed include 2,4-D, which
is selective against broad-leaved rosetted weeds. Mecoprop kills
small-leaved and creeping weeds such as clovers (Trifolium
spp.) and yarrow (Achillea millefolium). Dicamba may be
combined with 2,4-D or MCPA to widen the spectrum of control. Bromoxynil
is usually combined with MCPA or dicamba to control Onehunga weed.
Apply the selected herbicide to a lawn in spring, about two to
three weeks after growth commences, when plant growth is vigorous.
Then, weeds are more susceptible to the effects of the herbicide,
and the grasses will rapidly colonise the spaces vacated by the
dead weed plants.
Allow at least three days after mowing before applying the herbicide,
to allow enough time for the weeds to develop new leaf surfaces
to absorb the herbicidal compounds. Further, wait three days after
herbicide application before mowing the lawn to allow time for the
herbicide to translocate to the roots.
Lawn weeds differ in their susceptibility to herbicides. Dandelions
and plantains can be killed after one to two applications. Clovers
may require two to three applications at four to six-week intervals.
Creeping speedwell (Veronica persica) may survive several
Check label for instructions on disposal of lawn clippings. For
some herbicides such as chlorpyralid, clippings should not be added
to compost heaps that are used as mulch around sensitive plants,
or disposed of at any garden waste recycling centre. In such a case,
clippings should be mulched back into the lawn, or burnt.
Ants can deposit small heaps of fine soil on the lawn surface
as they remove soil particles to extend their underground nests.
Brush the soil mound away when it dries. The ants can be killed
with insecticides carbaryl, permethrin powder, or pyrethrins washed
into the nest site with water.
Porina (Wiseana species)
Caterpillar feeding can be masked by nitrogen fertiliser application.
Grass grub (Costelytra zealandica)
Controlled by drenching insecticide into the soil. Sometimes heavy
rolling or wheel pressure at the right time can crush grub larvae
when they are feeding close to the surface. A biological control
agent (Invade) is available commercially for grass grub.
Lawns containing couch, kikuyu and paspalum grasses are particular
affected by the dark-brown caterpillars of the armyworm. They grow
to 40 mm long feeding on leaves, stems and seed heads of the turf
grasses. The can be controlled with carbaryl or trichlorfon.
Urine patches caused by dogs and cats can be dealt with culturally
or with animal repellents. Dry soils and new lawns that attract
cats can be wetted regularly to provide an unfavourable area for
them. A thin layer of sawdust raked into the turf surface may prevent
patches of lawn being ‘burnt’ by the high concentration
of urine, as the microbes that colonise the wood particles utilise
the surplus nitrogen, thereby attenuating urine’s toxicity.
Repellents based on pepper dust or naphthalene need to be applied
regularly, particularly after rain, and provide only temporary protection.
Thanks to Bill Walmsley, New Zealand Sports Turf Institute, for
his article which was abridged for the section ‘Perennial
Ryegrass for Lawns and Turf’, and Sam Wakelin for the drawings
in Figure 1, Figure 2 and 3. Thanks also to Robert Lamberts for
the photographs of weeds.