They're orginally from South and Central America and were planted all over the city.
I recommend going to almost any viewpoint to take a look out over the city at the gorgeous colours. Parktown is always good for viewing as is Northcliff Hill (though I hear you need to be alert up there).
And when they fall off the trees, the flowers cover the roads wiht a carpet of purple (and make a fantastic popping noise as you drive over them!)
