Great St. John's Wort (Hypericum ascyron) is the giant among this genus, with individual flowers well over two inches across, blooming atop sturdy stalks about four feet tall.
Here's the itty-bitty member of the St. Johns' Wort clan, the aptly named Dwarf St. John's Wort (Hypericum mutilum), with flowers barely a quarter-inch across.
Canada St. John's Wort (Hypericum canadense) often rivals Dwarf St. John's Wort for tininess, although I have occasionally found it growing larger than the one pictured here. Note the slender leaves, much narrower than those of Dwarf St. John's Wort.
Spotted St. John's Wort (Hypericum punctatum) doesn't always display its spottiness quite so clearly as this one does, but I can often tell this species at a glance by the tight cluster of buds and blooms atop the knee-high stems.
Pale St. John's Wort (Hypericum ellipticum) is smaller than this photo would indicate, with flowers rarely more than half an inch across. I often see this species growing in masses at the river's edge, often extending well out into the shallow water. I find the common name to be quite the misnomer, for a plant with such bright-yellow blooms, vivid-orange flower buds, and scarlet seed pods.
Just to complete my account of the St. John's Wort species I'm likely to encounter along this stretch of the Hudson, I'm including the following pink-flowered ones, even though I did not find them yesterday.
Marsh St. John's Wort (Hypericum virginicum) is a truly beautiful flower that thrives in bogs and fens, as well as on damp banks along this stretch of the Hudson River.
If I didn't see Marsh St. John's Wort yesterday, it's likely because it usually blooms a bit later in July than the yellow-flowered species I mentioned above. And even if it were in flower by now, I wasn't around at the right time of day to catch it in bloom. I josh that I could set my watch by when these pretty pink blooms show their face, every afternoon about three. But even if only in bud or seed, this plant is quite colorful, its blue-green leaves edged with purple, and both flower buds and seed pods a beautiful scarlet.
This second pink-flowered Hypericum species, called Fraser's Marsh St. John's Wort (H. fraseri), probably does grow along this stretch of the Hudson. But I bet that if I have seen it in the past, I doubtless confused it with H. virginicum.
I happened to find the particular one pictured here last year on the shore of an island in Lake Desolation, over in the western region of Saratoga County, and I'm not sure what caused me to question its taxonomy. It just looked a little bit different: stubbier flowers, less prominent sepals, just a bit smaller than usual. My Newcomb's Wildflower Guide was no help, since it listed only H. virginicum as the pink-flowered St. John's Wort. But a search of the New York Flora Association's Plant Atlas confirmed my suspicions that I had found a second pink-flowered Marsh St. John's Wort. Now I will keep my eyes open, hoping to find it along "my" stretch of the Hudson, home to so many other species of this quintessential summer-flower genus.