Dear Jeevers, yes there is a noticable difference in the flowering of trees around Perth, isn't there? I think this is usually put down to genetic variation, coming from seed grown trees. You can see it in the colour range of Eucalyptus ficifolia, the red frowering gum that put on such a show in mid to late summer- some trees are full of flowers and some have washed out colouring. The jacarandas vary too, I've seen some that flower either very late or early and some that hardly flower at all.