PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
PHYSICAL &
MECHANICAL
METHODS
Knowledge of how
plants respond to
damage can be applied
to develop more
effective physical
control methods for
weeds.
Blade ploughing
Mimosa pigra cuts off
plants about 10 cm
below ground level
and is more effective
than cutting them at
ground level or 15cm
above ground level
which results in most
plants resprouting
Plastic irrigation
equipment does
not respond well
to being flamed
LIGHT & SEED
GERMINATION
Seed germination of
some weed seeds, eg
sowthistle, is favoured
by light, however, some
germination occurs in
the dark as well.
PROBABLY THE OLDEST METHODS OF WEED CONTROL.
Barriers. Garden beds can be edged to prevent weeds entering from lawns. Weed
Gunnel is a permeable and degradable weed barrier which can be placed around trees
and shrubs, also used for fence lines and various sizes of pots.
Hand weeding, chipping, hoeing, before flowering or seed set, controls scattered
weeds and small patches of annual weeds in garden beds and landcare areas. Some
woody weeds are easy to pull out. Can be labor intensive and costly for large areas,
and not suitable for median strips, parking areas where pebbles might be disturbed.
Various tools have been developed to remove broadleaved weeds in lawns, tractormounted
mechanical weeders work around fruit trees and vines.
Mowing, slashing, grazing weeds before seed set prevents viable weed seeds from being
added to the soil seedbank and is useful for controlling annual and biennial weeds.
– Some weeds tolerate close mowing, eg winter grass in turf. May encourage
growth and flowering of prostrate plants such as white clover and wireweed.
– Repeated mowing, slashing and grazing will restrict some perennial weeds by
weakening food reserves, by defoliation and preventing flowering and seed set.
– Used to reduce fire risks associated with grass and other weeds.
– Cheaper than cultivation and it preserves the ground cover reducing erosion and
improving access in wet weather.
– Mowing may be used in conjunction with herbicide applications.
– Selective grazing by stock can cause unpalatable ungrazed species to become
dominant and troublesome. Pastures and grazing management includes grazing regimes,
prudent fertilizers, heavy grazing forces stock to eat the less palatable weeds. Sheep
graze closer to the ground than cattle so do more damage to weeds. Avoid overgrazing,
use appropriate stocking rates, rotational grazing to avoid overgrazing, bare ground and
subsequent weed invasion. Goats graze on thistles, bracken and gorse.
– Protect trees from lawnmower, whipper-snipper (and herbicide) damage.
Equipment trailed behind harvesters, which destroys any weed seeds in harvester
chaff and re-spreads the chaff over the field, is currently being researched.
Rolling weeds and cover crops with special machinery, flattens them; they then break
down slowly into mulch.
Scalping is used in forestry for plantation establishment, and involves the removal
of the weeds and topsoil with a tractor or bulldozer on flat sites, but there are costs
and tree nutrition problems, resulting from removal of topsoil.
Some implements bury seeds (rotovators), while others lift them up to the surface
(spring tines) where they can be removed mechanically as they germinate.
Aquatic weed harvesters cut, load and dump weeds out of waterways.
Flooding is timed in rice fields so that weed seed germination is suppressed and
growing weeds drowned meanwhile the more water tolerant rice is unharmed.
HEAT.
Burning weeds and crops using flame throwers (low pressure gas burning torches),
are occasionally used by trained personnel, but may be prohibited by local legislation
or at certain times of the year. Flames or superheated steam (searing) boils
moisture in weeds which die back to the crown. They may be hand-held or tractormounted.
Most effective on weeds with unprotected growth points and/or thin
leaves, eg chickweed, and weeds with a low capacity for root suckering, thin bark,
etc. Degree of control of woody weeds depends on the species.
– Does not kill perennial weeds the effect on weeds being similar to that of
mowing or slashing except that burning is more complete.
– Many natural ecosystems are adapted to regular fires and species diversity
may decline unless they are burnt. Burning may stimulate germination of some
soil-stored weed seeds such as some legume seeds. The intensity of fire determines
which seeds are stimulated. Too frequent burning can lead to fire-resistant weeds,
loss of surface organic matter, poor soil stability, loss of the desirable species in
pasture, erosion and depletes reserves of phosphorus. Can kill seed present on the
soil surface of natural bushland, eg boneseed.
– Ideal for suppressing weeds where chemical use is not appropriate or where
machines cannot access, eg paths, lawns and gardens against fences, and around trees.
Selectively controls weed seedlings in established cotton and maize crops.
– Stubble burning, followed by a post-emergent herbicide, can reduce weed seeds.
– Does not disturb the soil and the technique is accepted by organic groups.
Infrared weedkillers of various types are being developed overseas; some of which
look like mowers, can be wheeled down greenhouse/nursery rows to clean up crop
debris, kill weeds, weed seeds and spores. Hand held types are also available.
Pasteurization. Aerated steam (60 o C for 30 minutes) is used to treat potting and
propagation media in nurseries to kill most plant disease organisms, leaving some
beneficial microflora. Some weed seeds are killed, but higher temperatures are
needed to kill many species of weed seeds.
Solarization prior to planting, properly implemented can cause soil temperatures to
increase to such an extent that some young weeds, many seeds and some plant disease
organisms are destroyed (page 330). Solarization is not possible in mixed or perennial
plantings. Often not very effective against weeds with deep roots and rhizomes. Water
beds before solarization to improve control. Moisture under the plastic helps conduct
heat and stimulates weed seeds to germinate prior to killing them.
438 Weeds - Integrated Weed Management