Weed of the Month: Thistles

Now is the best time to treat thistles (except Californian). Once thistle seed germinates it forms a seedling or ‘rosette’.

Searching for and treating rosettes now will save you a lot of time, money and worry as you will be destroying the plants before they mature, flower and produce more seeds next summer and autumn.

In Tasmania we have a variety of thistles including Spear, Slender, Nodding, Californian, Variegated, Cotton, Saffron and Milk thistles.

Thistles impact on pasture grasses and crops in agricultural areas and also in disturbed areas on roadsides and conservation areas.

They also invade the home garden. Seeds germinate mainly after the autumn rains but a few shoot at other times when adequate moisture is available.

During winter an extensive root system, consisting of several branched fleshy storage roots, develops. Rosettes grow rapidly through spring.

The best general control of all Thistles is done by encouraging competition from useful plants. Control methods include:

• mechanical grubbing using a mattock and making sure that you remove/sever the tap root at a minimum of 5cm below ground level
• chemical (refer to DPIW Service Sheets)
• cultural with the best method being to encourage competition by desirable plant species

Detailed information on thistle control is available from the Department of Primary Industries and Water website. General information on weeds can be found at the DPIW Weed Index
DPIW Service Sheets are available for:

• Spear thistle
• Slender thistle
• Nodding thistle
• Saffron thistle
• Californian thistle

If in doubt about which thistle you have and/or the best control method(s) contact DPIW Weed Management Branch on 1300 368 550.