Helianthus is certainly tall, tough and a spreader. There are quite a few different ones, I'd consider seeding in several different species.

You must be thinking of the common asclepias (Asclepias syriaca), there are other ones that are not so coarse looking.

Around here, a good spreader is Conoclinium coelestinum (formerly a Eupatorium). Common name is hardy ageratum.

What about some aggressive woodies like Aronia (chokeberry) - now in the genus Photinia, by the way? These are not terribly dense, but they spread well, kind of like perennials in the way they look.

Have you considering asking the park if you can occasionally weed out some of the knotweed for them?

Here are a few suggestions of large, tough natives, that establish a good strong root system, although I'm not sure how well they'd hold up against Japanese Knotweed -

Helianthus tuberosus, Jerusalem artichoke, or other large perennial Helianthus that spreads by rhizomes. Another large tough perennial with yellow flowers is Silphium perfoliatum (Cupplant).

Large clumping grasses, like Panicum virgatum or Andropogon gerardii (Big bluestem).

Phytolacca americana, Pokeberry. Any of the moisture loving, stoloniferous Dogwoods.

As for Asclepias, A. incarnata is a gorgeous and widespread milkweed, prefers moist soil, can grow 4-5 feet tall, and is very attractive to Monarch butterflies and many pollinators. It does die back a bit in the early fall though.

Some pics -

A patch of Helianthus tuberosus -
{{gwi:1052182}}

Asclepias incarnata with a Fritillary & Monarch -
{{gwi:1052184}}

How about some Coreopsis tripteris? It has good fall color too

Here is a link that might be useful: goldfinch's like also