On the ones in your yard, can you take a picture of the back of the flower so that we can see the bracts (the green things holding the flower)?

I hope this is okay
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Definitely a Helianthus. The leaves and stem don't look hairy at all - are they smooth?

They are smooth perennial and over 7 feet tall.

The key I have doesn't include your area (but it is close, it is for the mid-atlantic states and Georgia). Here is the key (using this one, I would've said H. smithii based on what I can see in your pictures):

Key D - perennial sunflowers with leafy stems and yellow disk flowers
1 Stems below the capitulescence glabrous or nearly so, sometimes glaucous.
2 Leaves whorled at principal nodes, either alternate or opposite at other nodes ........................................................................... H. verticillatus
2 Leaves either alternate or opposite (or both).
3 Leaves grayish-green or bluish green in color, sessile, and abaxially glabrous and glaucous.
4 Rays 5-10; leaves glabrous or glabrate adaxially, smooth or only slightly rough to the touch; phyllaries 2-3 mm wide .... H. laevigatus
4 Rays 10-14; leaves strumose adaxially, rough to the touch; phyllaries 3.5-4.5 mm wide .........................................................H. eggertii
3 Leaves light to dark green, sometimes whitish abaxially, but not grayish or bluish green in color; leaves sessile or petiolate, glabrous or
pubescent.
5 Leaves linear-lanceolate, with only a single main vein ............................................................................................................... H. smithii
5 Leaves linear-lanceolate to lanceolate, lance-ovate, or ovate, triplinerved at base.
6 Rays few, usually 5 or 8; heads small, the involucres 9 mm broad or less.
7 Leaves abaxially whitish in color and glabrous and glaucous, lacking subsessile glandular trichomes ("resin dots") .....................
................................................................................................................................................................................. H. glaucophyllus
7 Leaves abaxially greenish in color, usually tomentulose (sometimes glabrate), with abundant subsessile glandular trichomes .....
................................................................................................................................................................................ H. microcephalus
6 Rays usually 10 or more in larger heads; heads larger, the involucres usually > 9 mm broad.

8 Leaves sessile, rounded to cordate at base, and trinerved, with the 2 lateral veins diverging from the midrib at the very base of
the leaf .......................................................................................................................................................................... H. divaricatus
8 Leaves sessile to petiolate, but narrowing gradually to base and triplinerved, the 2 lateral veins diverging from the midrib above
the base of the blade.
9 Anther appendages yellow.
10 Leave blade lanceolate to lance-ovate, sessile to petiolate but the petiole usually conspicuously graduated and imbricate, usually loose and spreading .......................................................... H. grosseserratus
10 Leaf blade ovate to elliptic, with a distinct petiole usually > 2 cm long and 1/2 as long as blade or longer; phyllaries
conspicuously graduated and imbricate, usually appressed, not exceeding disk ................. H. occidentalis ssp. occidentalis
9 Anther appendages dark or reddish-brown.
11 Plants producing abundant tubers; leaves subsessile, the petioles ............................................................................................................................................................................ H. schweinitzii
11 Plants rhizomatous, but not producing tubers; leaves petiolate, the petioles 1-5 cm long; [collectively widespread in our
area].
12 Phyllaries equal to or slightly exceeding disk, apex acute; leaves moderately serrate to entire, with a petiole 1-3 cm
long, and abaxially with usually abundant subsessile glandular trichomes ("resin dots") ............................. H. strumosus
12 Longer phyllaries usually exceeding disk by half their length or more, apex acuminate; larger leaves moderately to
conspicuously serrate, with a petiole 2-5 cm long, and abaxially with usually relatively few subsessile glandular
trichomes ....................................................................................................................................................... H. decapetalus

How did you come by this plant, by the way? Was it purchased or growing wild?

The one from the park reminds me of Helianthus grosseserratus (or H. giganteus which is said to be quite similar) based on the habit and the leaf shape. Leaves should be serrate if so.

I wonder if the planted specimen is a hybrid, such as Helianthus x multiflorus.

Thanks to both of you. I bought it in 2003 at Bowmans wildflower preserve native plant sale.

Here is a link that might be useful: Bowman Hill wildflower preserve

I wonder if they would have kept records? Well, it's a very nice looking plant and definitely a Helianthus.