Well as a secretary of many years standing and something I was taught when taking my English 'O' level - I agree with Blandfordp.
Think of the original word - does it naturally end in an s or not.
If you are talking about a flower in the singular then it is 's - e.g. the flower's colour.
If you're are talking about flowers in the plural, that already ends in an s, so the apostrophe goes after -e.g. the flowers' colour. You leave off the s. (A common mistake is keeping the s - you often see St James's Street which is incorrect).
The other use of the apostrophe is when it's indicating that something's missing (see my use in this sentence - it is indicating that the 'i' is missing in each case).
Hope this helps! You're probably well confused by now LOL!
Cheers
Moon
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