I have been trying with notable lack of success, to remove Californian Thistle. It is growing in my vegetable garden, brought in with horse manure - and subsequently it has been flourishing in three different patches throughout our property, now for three years. Our fourth year lies ahead and there they are... showing their prickly rosettes again and picking up speed in their growth.
I am lame and wear an Orthotic Brace and had prepared my garden to be No Dig because I am unable to dig properly any longer you see. I have attempted to remove them by hand, digging down as far as I can but it is difficult as the soil underneath is as hard as charity and full of stones. The no dig part above this is in beautiful condition and rich and friable and full of worms... and of course, Californian thistle.
I have used a spray which did knock them, but not for long unfortunately. I have also tried common salt. I have cut their flowering heads off so there are no seeds to blow around - only to have seeds blow in from next door [sigh!] They are also amongst my old roses and are also growing very well there too. I am feeling desperate but do not want to use poisons.
Of all the weedy thistles in New Zealand, the Californian ones are probably the hardest to cope with. Their long tap root system will make complete removal difficult as they will be able to grow back from small pieces of roots left in the ground.
However, they can be exhausted. Repeated cutting of top parts (before flowering) will use up the reserves stored in their roots and eventually they will give up and die. The only question is who is exhausted first, the gardener or the thistle?
Green manure crops can help smother out thistles. One of the best plants for this purpose is Buckwheat (Fagopyrum esculentum). Prepare the area in question for a seedbed, broadcast the buckwheat seed fairly densely (20-50 per square metre) in late spring (after late frosts). Let the crop grow to when the plants just start to flower, then work the whole lot into the soil. Wait three to four weeks, keep the soil moist during this time. Prepare the soil again and broadcast another crop of buckwheat. Continue as outlined above. Grow a total of three successive crops. A labour intensive and time consuming way, but well worth it.
From a Reader:
I have had success in organically controlling californian thistles by hand pulling them. This method removes a large portion of the plant's underground reserves. The easiest time to weed thistles is after the stem has developed but before the flowers have opened. Admittedly this is a tedious process and it must be kept up for several years but the results are obvious even in the second season.