Once it is established do you provide supplemental water?

No. This is a very drought tolerant plant and should need no irrigation after establishment. Especially in areas that "normally" receive decent summer rainfall. I live in a recurring summer drought area - we receive less than a 10th of our annual rainfall during the summer months - and even under our normally very dry summer conditions lavender needs no supplemental watering.

Different types of lavenders bloom at different times - the Spanish or L. stoechas is the earliest (begins in April in my area), followed by English or common lavender (L. angustifolia - starting in mid to late June) and wrapped up by the hybrids or xintermedia cultivars like 'Provence' or 'Grosso' in July (all timing is appropriate for my area only). 'Blue Cushion' is a cultivar of English lavender. The bloom period for any type is extensive, lasting most of the summer and in some areas, well into fall and can be increased by shearing or deadheading.

Thanks for the detailed information gardengal48.