Q.Is it too late to trim back for more blooms
The portulaca have been blooming beautifully but now there are many spent blooms. Can it be cut back for new growth? Today is September 7. We live in Pennsylvania. Usually no frost until near November.

Certainly trim off the dead flowers, deadhead.
This should encourage more blooms until the weather cools.
Trimming the plants back to far will likely not hurt but the temperatures and sunlight conditions will not be enough to bring on a big flower display.
https://www.gardeningknowhow.com/ornamental/flowers/portulaca/portulaca-plants.htm