Occurrence of selected perennial weeds has been monitored at control sites across the Czech Republic since 1989, and trends of their occurrence have been evaluated. The monitoring has been aimed mainly at Cirsium arvense (L.) SCOP, Sonchus arvensis L., Artemisia vulgaris L., Stachys palustris L., Bolboschoenus planiculmis, Bolboschoenus laticarpus, and Elytrigia repens (L.) NEV. on arable land and Taraxacum officinale L.A. on pastures and meadows. High increase in the frequency of occurrence of C. arvense and E. repens was found out in the mid-nineties while the infestation frequency with these two weeds decreased later on. The occurrence of infestation with the other perennial weeds Cirsium arvense, Elytrigia repens, Sonchus arvensis, Bolboschoenus planiculmis, Bolboschoenus laticarpus, Stachys palustris, Artemisia vulgaris and Taraxacum officinale L.A. slightly but continually increased.