You need comfortable, hard-wearing trousers, although some operators will use brushcutter or even chainsaw trousers for hedge-trimming operations.
You might like to note that local authorities and contractors have been asking for a specialist hedge-trimmer trouser.
The result is the HS Multi-Protect. This trouser from Stihl is made of a lightweight, breathable, stretch material with abrasion-resistant material to the knee and thigh areas. Pockets are incorporated for the insertion of extra protective cut- and pinch-resistant Dyneema felt pads.
These fit to the knee and thigh for hedge trimming and to the lower leg for clearing saw work. Knee pads are also available.