The Brit beauty turned out to the annual botanical show to do a little species spotting and debuted a stunning new outfit.
Shunning the smart ensembles of fellow visitors Sienna strolled round the exhibition in a pair of tomato-red boots, a loose-fit Breton top, a cosy knit cardi and a pair of classic aviators.
And, it would seem that the fashionista has a new favourite piece of arm candy, as she carried the same over-sized Prada tote she had in tow at last week’s Belvedere Party with Matthew Williamson.
We’re not sure if her get-up was quite gardening appropriate but she certainly looked cool.
By Hayley Spencer