Southern took complete control in the second spell when it scored 27 unanswered points to beat Alhambra-Union at Bathgate Park.
Alhambra-Union had been competitive in the first spell and was still in the game at halftime.
But it ran out of steam and ideas as Southern lifted its game in the second spell.
The decisive moment came after 10 minutes when Southern scored two quick tries to stretch the lead to 25-3.
Flanker Sam Crompton scored the first after a prolonged build-up by backs and forwards.
It was followed two minutes later when fullback Tony Fenner counter-attacked and first five-eighth Julius Glasson was up in support to score the try.
Southern repeated the dose in the final two minutes of the game when winger Hamish Buick scored and No 8 Andrew Stead used his brute strength to crash over.
Stead was the pick of the Southern forwards with his ability to take the ball to the game line and his strong tackling.
He was backed by Crompton who made ground with breaks down the blindside and hooker Ean Griffiths with his skills in the tight and with the pick and go.
Glasson was the pick of the backs with his ability to read the game and take the ball to the line and his tactical kicking to the corners.
No 8 Regan Turoa was the best of the Alhambra-Union forwards and first five-eighth Peter Breen the best of the backs.
- Southern 35 (Hamish Buick, Luke Herden, Julius Glasson, Andrew Stead, Sam Crompton tries; Tony Fenner 2 con, 2 pen), Alhambra-Union 3 (Peter Breen pen). Halftime: 8-3. Referee: Liam Scanlon.