Yet another one I think I should know, but don't. What is this plant? He lives in central Ohio.

It appears to be Dame's Rocket (Hesperis matronalis.)
When I was visiting Ohio and Pennsylvania years ago, I saw it blooming everywhere including thick stands of it growing by the roadsides.

Dame's Rocket (Hesperis matronalis)

There's a long list of Hesperis, it could be Hesperis matronalis 'Alba', but it's not guaranteed.

Dame's Rocket (Hesperis matronalis 'Alba')

The form 'Alba' might be the same as Hesperis matronalis var. albiflora ..

http://www.rhsplants.co.uk/pla...

However, that is not listed on the Plant List.

http://www.theplantlist.org/tp...

The list on ATP ..

http://garden.org/plants/brows...

If it's a plant which appears to be endemic then you should check which species are in the area.

Strangely, a search for the genus in Ohio first brought up the straight species Hesperis matronalis with two subspecies, ssp. candida and ssp. voronovii ..

http://luirig.altervista.org/f...

After clicking on one of them, then going back I had to reselect the state Ohio, then it only showed Hesperis matronalis. Any subspecies, as far as I know, can be included in the straight species as they are essentially the same species with slight variations often due to location.

Hesperis matronalis is the only species it could be assuming we have got the genus correct, it looks like it to me.

http://luirig.altervista.org/f...

Thank you, Janet and Danita!!

Hesperis matronalis isn't native to the Americas but is an Eurasian plant that was introduced here. It has spread enthusiastically and is even considered invasive and banned in some states. When I saw it growing in Ohio and Pennsylvania all over the roadsides, there were plants ranging in color from dark lavender-purple to pure white and every shade in between.

I have it in my garden, growing from who knows when or where from. It's loved by some butterflies.

So lucky, I LOVE Dame's Rocket! Used to have it where I used to live and it smelled amazing!