GardenBanter.co.uk - Tree peony question

I have a new tree peony, received as a gift last summer, planted last
fall, so this is its first full season in the ground. It was unlabeled
and I don't know what variety it is, but it's clearly the kind that's
been grafted onto hardier rootstock. In its nursery container it looked
like the graft union was sticking out several inches above the soil;
there were some new buds at the graft, and a few more buds at the soil
level. I didn't like the looks of that, and feared that the lower set
of buds were from the rootstock. I planted it quite deep so that the
graft and the upper set of buds would be at soil level or an inch or so
below.

This spring the plant grew nicely and had three modest-sized blossoms.
After the blossoms were gone, it sent up an extremely vigorous new stem,
with leaves similar but not identical to the existing topgrowth. They
are a bit larger, lighter-colored, and less deeply dissected. But they
still don't look anything like herbaceous peony leaves (to me, anyway).

So, all you tree peony owners out there, what do you think? Do tree
peonies normally put out an explosion of topgrowth in mid-spring just
after flowering, and is it normally a little bit "coarser" than the
older leaves? Or do I have a sucker coming from the rootstock which
should be removed?

(As you can probably tell, this is my first tree peony...)

I can put photos online if it would help...

Thanks in advance
-Alex