Germination: Key control time for seedlings only
Now acting like perennials - too late for seedling control
Roots survive for 12 months and are replaced annually, the new seasons shoots producing fresh creeping roots.
One seedling can lead to a 4m wide infestation. These roots can reach depths of more than 2.4m.
Exhausting root reserves is therefore the key to control.
Treatment causes die-back but root fragments remain
Remaining buds are forced to shoot
2nd attempt at flowering
Another herbicide treatment the following summer
Total control should be achieved
Late Hardball stage
Male and female plants growing closely together are required to cross pollinate to produce viable seed. Mature seeds are disbursed by wind from female plants, but very few germinate. Viable seed will only grow in bare, undisturbed, open soil without shading. Therefore, spread via seed is minimal in an established pasture. The main method of spread is through new root growth from existing infestations.
Care needs to be taken to differentiate between seedlings versus rosettes that have regrown from existing roots. This can be done by digging plants up to see if there is an established root system.
The ideal time to spray Californian thistles with Tordon Pastureboss is at the flower bud stage. This is commonly referred to as the ‘hard ball’ stage (photo), which is after bud formation but before flowering. Plants at this growth stage are actively sending sugars down to the root system, which helps to distribute the herbicide throughout the plant. Once flowering commences, plants are no longer partitioning sugars to the roots, and the sugars stay within the upper part of the plant. If application was to occur over flowering, this would reduce the effectiveness of the herbicide application. Hence application at the ‘hard ball’ stage is recommended.
Corteva Agriscience trials have shown that a single application of Tordon PastureBoss can provide 92% control of Californian thistle. Some root fragments will remain so retreatment as part of a programmed approach is necessary.
Californian thistle will attempt to flower for a second time in autumn. An alternative control to Tordon PastureBoss should be undertaken to control the flush of stems. This could be a phenoxy herbicide treatment, or a hard mowing or grazing treatment. Alternatives to chemical control are not as effective and can risk prolonging the thistle control program.
Control of Californian thistle
Spot spray Tordon PastureBoss to clean-up final stems.
Total control should be achieved.
Summer - #1 Tordon PastureBoss treatment at late hardball stage.
Spring - new seedling
Autumn - #2 Alternative control treatment at second flowering
Following summer - #3 Tordon PastureBoss treatment as a final clean-up herbicide.
2nd herbicide treatment causes dieback and exhausts most of the root system before winter dormancy”
2nd Herbicide treatment
The quickest and most economical way of returning pasture with moderate to high Californian thistle density back to being productive and profitable is by spraying a Tordon PastureBoss program.
For more detailed information please click on each growth stage of the thistle