When and where to plant oxalis?

You can get many types and colours of Asters, pink, mauve, purple and white. They also vary in height from dwarf to 4' high, so the spacing depends on which ones you buy. They flower in late summer through into the autumn.

Oops! I misunderstood - I thought you were referring to the previous question on Asters! Sorry, Oscar.
Right. Oxalis. A completely different genus. There are quite a lot of different types, growing from corms, rhizomes or roots. Some can be invasive - the ones that grow from rhizomes. Most of them are small and don't grow more than a few inches in height. They come in shades of pink but there is a yellow one - O.lobata. Most flower in early summer and keep going into autumn. I recommend O. tetraphylla 'Iron Cross' which is grown from corms space about 4" apart. Its leaves have a distinctive pattern and the flowers are salmon pink. It pops up quite late in the spring but is still going strong now! Some of the little alpine clumpy ones can suffer from rust, and they should be removed and burnt. Hope this helps.