My parents grew a rosemary bush from a 'cutting' they were offered while on holiday (because my sister, then a toddler, is called Rosemary) and the advice they were given was the 'glass of water' technique (plant it when you can see roots). The plant eventually outgrew the space and got 'leggy' so they took another cutting, and when that was doing well, scrapped the old bush. My sisters and I all got cuttings, and all doing well...... It is probably the easiest herb in the world to take cuttings from. The small plants you have bought can be put in big pots outside to grow up, and by spring will be fit for taking cuttings off anyway.

I wouldn't bother with rosemary from seed unless it is the only way to get a variety you especially want. Try nicking a twiggy bit or so from the next garden where you see it<g>

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Flowers come in too many colours to see the world in black-and-white.