PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Weeds in containers
Weed types
Annual and perennial broadleaved and
grass weeds, eg creeping hairy bittercress
(Cardamine hirsutus), common groundsel
(Senecio vulgaris), oxalis (Oxalis corniculata),
winter grass (Poa annua), liverwort
(Marchantia polymorpha), chickweed
(Stellaria media), willow herb (Epilobium sp.).
Some weeds are difficult to control in
containers.
Impact
Weeds in containers may compete for light,
water and nutrients, detract from appearance and
value of plants, be an unknown purchase in pots
which spread to gardens, landscapes, bush areas.
They physically hinder workers and are a source of
insects such as aphids, thrips, whiteflies and mites
and be symptomless reservoirs of virus diseases.
Weed biology
‘Overwintering’. Soil acts a primary storage bank
for weed seeds and vegetative propagules. Nursery
property fence lines and adjacent properties can act as
large sources of weed seeds that are blown in by wind.
Spread. Infested crop seed, propagation material,
containers. Seed is spread by wind, water, animals,
and people. Weed infestations close to a nursery and
in the immediate area around containers. Many
manufacturers avoid using uncomposted material and
soil in potting mixes but they can become contaminated
during transport, and by untreated recycled water.
Conditions favouring. Weedy surrounds, poor
weed management.
Management (IWM)
Good weed management is important for any nursery
business and is one of the most difficult.
1. Plan the weed control program well in advance. Have
a calendar so you will know what to do when. Include
site assessment, source of planting material (plant
species selections), potting up, maintenance and
preparation for sale. Cleanup between batchs.
2. Crop, region. The container species you are growing
will determine your weed control options. What
information is already available?
3. Identify present and predicted weed problems. If
necessary seek advice (page xiv). Identify problem
weeds, eg bittercress, know source of weed seed,
weed life cycles, etc.
4.Monitor weeds in the nursery and/or impact and
record your findings (page 429). Early detection of weed
problems is essential and regular scouting can begin as a
first assessment on foot, walking the perimeter of the
nursery and then across sections. Create a working
system to allow workers to report weed infestations.
5. Threshold. How much weed infestation is
acceptable? Have any thresholds been established? If
so, what are they, eg economic, aesthetic? Do your
customers have nil tolerance?
6. Action/control. Some weed control options do not
apply to container plants. Last minute hand weeding
can be used to remove weeds that escape herbicide or
other treatments prior to sale. Pre-emergent herbicides
will remain economical for the present as a necessary
part of many IWM programs. However, their use
should be minimized.
7. Evaluation. How effective was the weed control
program? Evaluate a selection of plants 6 weeks
after they have been sold and planted out
whether in a garden, bushland, or in a commercial
planting. Recommend improvements if required.
Control methods
Weed control is one of the largest cost issues for
growers and is the No.1. issue for organic growers.
Cultural methods prevention
Use weed-free potting mixes. Soil-less
mixes should be weed-free. Organic materials, if
used, may contain large amounts of seed, and must
be adequately composted.
Pot toppers (fabric discs) are suited for larger
container stock, eg 2-litre size upwards. Weeds
readily grow in any gaps, eg around edges, and
along any cuts made to assist fitting.
– Non-damaging to plants, stable and resistant to
wind, protected situations, control weeds for at
least 1 year. Price competitive and easy to fit or
apply. Commercially attractive at point of sale.
– Disks must be UV resistant and permeable to
fertilizers. Wool material deteriorates too rapidly.
– Some pot toppers are coated with copper. Others
contain herbicides and slow-release fertilizers and
control weeds longer and better than standard
herbicide applications.
Loose fill mulches are suited to perennials grown
in small pots. Mulches must be permeable to water
and air but exclude light. Seed may germinate on top.
– Mulches are best applied immediately after
potting.
– Surface must be weed-free. A weed mat could be
used in containers with a weed-free mulch on top.
– Recycled paper pellets applied at 25mm may not
be cost effective but may have merit in certain places,
enclosed structures where herbicide use is restricted,
environmentally sensitive areas, eg water, some plants
difficult to weed, eg thorny plants, plants grown in
large container where between container loss is great.
– Mulches treated with pre-emergent herbicides
are used overseas. They could be added as top layers
during pot-filling in assembly-line plantings in
commercial production or in landscapes of
commercial and non-commercial plantings.
– Pre-emergent treated bark nuggets results in
increased and extended herbicide efficacy.
Sanitation prevention
Crop-free fallow. Growers, who have a break in
their production cycle, can empty these areas and
control weeds. Remove debris well before another
cropping cycle, could put down new screenings.
In enclosed areas, eg in greenhouses and propagating
sheds, wash and disinfect containers for re-use.
Propagation areas should have concrete floors
which can be regularly hosed down and disinfected.
Well-draining blue metal or gravel aggregate in
areas where container plants are grown or held for
sale prevents water puddling and contact with soil.
Sources of weed seed include:
– Control weeds in surrounding areas to prevent seeding
by mowing, etc and maintain at least 6 metre weedfree
area around greenhouses to eliminate weeds
near vents which can be screened to limit introduction
of wind-blown seed.
Sandbeds must be kept weed-free at all times.
– Remove weeds before they set seed, place in bag,
dispose of outside greenhouse or production area.
– Dispose of unwanted plants promptly before they
become a weed seed bank for clean stock.
Handle weedy pots before weed-free pots.
– Do not use the same buckets to dispose of handpulled
weeds and for transporting cuttings.
Store mixes in covered bins to keep out wind blown seed.
Do not re-use potting mixes.
– Check overhead baskets, a source of weed seeds.
– Remove weeds in containers before they flower and
produce seed. One prostate petty spurge weed
may produce up to 50,000 seeds per plant.
Biological control.
Rarely used in nurseries or container stock.
464 Weeds - Examples of weed situations