Hey Nimas3, You could try...
Our gardener had insisted that the bushes should be trimmed once a month only. In my view, the hedge has grown too thick over that period.
Khan Abdul is also right. You could also use 'must' - this sounds more authoritative opinion. The word ''should' makes like the gardener sound like an expert and you should follow his advice - However you ultimately can decide whether you listen to him or ignore his advice.
Hope this helps.