Do you cut back thistles?
Pruning and caring for thistle
Being a very easy plant to care for, thistle requires practically no care at all. ... Cut wilting flowers off as they die and cut the plant back very short in fall. During the summer season, it's important to water in case strong of extended drought.
When should you cut back thistles?
For the first year you should cut off any flowerheads as soon as they appear, to let the plant build up strength. Harvest them in years two and three, and ditch the plant thereafter.