I've got a Sage plant in my back garden that's quite a nice size but its about to flower ( covered in buds), once flowered does it need replacing or will it be ok to use again?
will be fine, mine is 3 years old now and is still used let it flower and save the seeds and plant them i have 2 new baby ones planted ready for next year, then its r.i.p old sage plant in the green bin!
Yep, they are good for at least 3 years or more.
If you do propagate from it, my experience is that over-wintering new plants outside is far better than in a greenhouse or cold frame, despite what the experts have to say.
These were sown last year, and kept in big pots in my cold frame until March. I thought I was going to lose them at one stage:
(http://i40.tinypic.com/148nski.jpg)
This, on the other hand (I have 12 of these), sown at the same time, braved it outside huddled behind a bit of seaweed mulch to keep the worst wind off.....
(http://i41.tinypic.com/qywdc3.jpg)
I lost several over winter... but the survivors are in full bud... :)