Each seed is provided with a well developed pappus or parachute of fine hairs which, in favourable weather conditions is capable of carrying it over distances of up to one kilometre.
The roots not only remain alive from year to year but during the growing season actively spread and increase both the overall size and the density of the infestation. A lateral root spread of 13 metres in a single season and a vertical penetration of six metres have been recorded for this species.
Movement of root fragments by cultivation implements of by livestock is probably the most common method of transfer over short distances.
Distribution and economic significance
Californian thistle can be found in most parts of the State. For very many years it was more common in the south but in recent years the weed has become more evident in the farming areas of the north west. To some extent this results from the conversion of pasture to vegetable crop production.
In the Meander Valley Municipality, although not prolific, there are quite a few infestations on agricultural land adjacent to the Meander River to the West of Deloraine, several infestations in the Chudleigh and Mole Creek areas, and roadside infestations encroaching into the Great Western Tiers area, near Lake MacKenzie.
Although it is more vigorous in the wetter areas where annual rainfall exceeds 750 mm it is capable of growing in the drier parts of the midlands and east coast.
In pasture the thistle competes directly with the sown species and heavy infestation can significantly reduce production. Stock avoid grazing round the rosettes and shoots and this reduces further the productive capacity of infested areas. In dense infestations stock may be totally excluded by the flowering stems.
In crops it not only competes with the sown plants during their growing period but also interferes with their harvest. The weed is a particular problem in vegetables where herbicides tolerated by the crops usually have little or no effect on it.
Californian thistle is very difficult and generally expensive to control in perennial crops such as lucerne and berry fruits. The weed should be eliminated before these crops are planted.
In the south of the State, the foliage of the thistle is very frequently attacked by a rust fungus(Puccinia obtegens) and by aphids. These attacks can severely temper its vigour and limit its vegetative spread. In the northern half of the State, however, where the higher rainfall favours thistle growth, the rust and aphids have considerably less effect and seldom restrict growth. Recently, December 1998, californian thistles growing in the Meander area exhibited a severe infestation of a rust fungus, possibly Puccinia obtegens. This rust infestation has defoliated some plants and in its severest form, completely killed thistles.
Status Under the Noxious Weed Act (1964)
Californian thistle is a Declared Secondary Weed. Plants growing adjacent to public stock yards and sale yards are to be controlled to prevent spread.
CONTROL
Before attempting to control Californian thistle it must be appreciated that a continuous program extending for a minimum of at least two to three years is required. Established stands cannot be eliminated in one season. These heavier infestations which can survive for many years take longer to eradicate because of the enormous energy reserves stored in their root systems.
Cultivation
By itself, cultivation is not a practical method for controlling Californian thistle. However, it can be used as the first step in an integrated control program especially where large, well-established infestations are involved.
The objective of the cultivation is to smash up as much of the root system as possible and force dormant buds on the roots to germinate. Therefore for the initial working, a tool with deep penetrating blades such as a chisel plough should be used. Where possible the ground should be worked to the depth to which the thistle roots have penetrated.
Subsequent cultivations can be done with shallower working implements which sever the growing stems and drag the root fragments to the surface. In heavy or old established infestations a second pass with deep penetrating equipment at right angles to, and three to four months after the first working is advantageous.
The first cultivations should be carried out in the spring before substantial growth has taken place and while the ground is moist enough to make deep working possible. Further cultivations should be carried out during the spring, summer and early autumn, to destroy the top of the thistle before it is able to reach a substantial size or produce flowering shoots.
If used alone, the number of cultivations required will depend on the rate at which the thistle grows, but normally between three and five will be needed in a season. These cultivations should be followed by a forage crop such as an autumn cereal or an annual grass in which selective herbicides can be used should thistle regrowth appear.
As an alternative to additional workings, regrowth may be sprayed with herbicides at the flower bud stage.
Research overseas has shown that plants cultivated when the day length has fallen below 16hrs (Autumn) form leafy non flowering rosettes. Application of some herbicides such as glyphosate at this stage increases herbicide uptake by the root system when compared to flowering plants.
In the second season an additional cultivation cycle can be undertaken if the infestation has very large root reserves.
Following these cultivations the area should be sown down to permanent pasture in the autumn and a herbicide follow-up program used. Care must be taken to ensure a strong and rapid pasture establishment. Departmental officers are able to advise on the best seeding and fertiliser rates to achieve this and on appropriate grazing management.
When Californian thistle infested areas are being cultivated great care must be taken not to increase the area affected by spreading root segments in the course of the work. Care must also be taken not to start new infestations by transferring pieces of root on implements or tractor tyres. Work should be started on the clean or least affected part of the paddock and finish in the most heavily infested area. The implements can then be cleaned there before they are taken onto clean land.
Mowing or Slashing
Since seed plays a relatively insignificant role in the spread of this thistle, mowing or slashing the plants serves little purpose.
Grazing
Grazing is probably one of the least effective methods for the control of this thistle. Sheep and goats are known to graze the young shoots in spring but it is likely that conventional grazing alone will not significantly halt its spread.
Cover Crops
Seedlings of this species are suppressed by shade created by competing crops species and death occurs when light intensity falls below 20% of full daylight. Cover crops may be used in conjunction with other techniques as part of an integrated weed management program.
