Last year I pruned my lavender (Lavandula angustifolia) very aggressively, so that the bushes resulted quite small, with very short branches. I pruned it in about October (I leave at about 45° North). This year my lavender produced a lot of short (no more then 10-15 cm long) spikes, for three times during summer (in May, July and September) and its flowers were very small. I think (but I'm not sure!) that this behavior depends on the way I pruned it. I would prefer it to flourish only once during summer, with fewer, more robust spikes and bigger flowers.