Hydrangeas that belong to the Endless Summer Collection will typically have two flushes of blooms; that includes: The Original, Bloomstruck, Blushing Bride, Twist N' Shout, Pop Star, Summer Crush. The first flush of blooms occurs from the old wood and, for you, it seems to occur around June based on your comments. The second flush of blooms occurs when the brand new stems that started growing in spring 2024 attain sufficient height in 2024 to trigger flower bud set and immediate blooming. That usually happens in late summer or fall.

If your variety is one of the six listed, it sounds as if the old wood was pruned too early (leafless stems should be pruned by the end of June or even in July if near Canada), the shrub was fed too late last year (early frosts might kill the flower buds if the shrub us still in active growth mode) or perhaps late frosts killed the old wood. The mid-September blooms were from the second flush of blooms. This year, it took until then to get the new stems to grow tall enough to develop those blooms. Shorter varieties like Pop Star and Summer Crush will bloom earlier than the other four which are taller as it will take less time to reach flower bud set height.

If your variety is not listed, your shrub is not an Endless Summer Hydrangea. It may bloom only once a year (it may also have occasional blooms in the fall when the weather tricks the spring 2025 blooms into opening in the fall 2024). It could also be a rebloomer variety not part of the ES Collection but you will need to find the name in the plant label to confirm that.

The newer Pop Star (lacecap blooms) and Summer Crush (mophead blooms) varieties get about 3 feet high only and will produce a second flush earlier than the original Endless Summer Hydrangea as the Original variety can get about 5-6 feet high.

If the old wood is bloom unreliable in your location, ensure your last fertilizer application occurs 3 months before your average date of first frost and consider winter protecting from your city's average date of first frost through your average date of last frost.

From my experience, the new Endless Summer varieties Bloomstruck and Twist N' Shout flower more reliably than the Original. The breeding is newer and the performance is much improved. I would suggest replacing it with a newer Endless Summer variety or one of the newer Let's Dance series such as LD Sky View, LD Arriba, or LD Can Do. The breeding has improved so much in the 30 years since ES was introduced.