Asia Jasmine trimming

Hello all, a client of mine has asiatic jasmine that is very thick, about 12 to 14”, and he has asked in the past to cut it lower. I have used my string trimmer to accomplish this with decent results. If I cut it down very much there are no leaves left just vines. Weeds then can infiltrate and grow like crazy. I would like to know the best time if the year to trim it back, how much to trim at a time, and what to use to trim. Any suggestions would be welcome.

Thanks,
klirette