Gardeners Please ID this tree

Please have a look at these two photos and let me know if you recognize this. It has small, dark green, leaves with pronounced serrations. Much to my surprise, last weekend it came out in thousands of long cream-coloured blossoms. I expect they have opened further since these photos were taken – if so, I’ll add an updated photo next weekend if I’m still in the dark.

I have three reasons for wanting to identify it:

One, I’m curious;

Two, it needs a significant trimming and I’d like to look up the best time of year to do it;

Three, it is growing on my septic field, and I feel I should have an idea of what sort of root structure it might have and how much of a problem that might ultimately pose.

Any opinion or advice greatly appreciated!