They were discovered Friday during construction work on a new industrial park.
The fossils of the prehistoric flora, mainly conifers, are the only ones ever found from the early Jurassic Period in the Western United States.
The site sits next to the Dinosaur Discovery Site at Johnson Farm where eight years ago thousands of dinosaur tracks were discovered, which are now protected as a city-owned museum.
Now developers will work with scientists to preserve the site.
"This plant site is extremely important to help us examine further the vegetation recovery of plant life during the mass extinction at the end of the Triassic Period," said Utah State Paleontologist Jim Kirkland.
Kirkland said the area where the plant fossils and tracks are found was once on a vast lake's northern shore that attracted dinosaurs.
Andrew Milner, paleontologist for St. George, said one of the plant species, Saintgeorgeia jensenii is named after the city and another has been named after him, Milnerites planus.
Trapped in the sedimentary stone, brown juniper-like leaves can be recognized sprouting from stems. Fossilized pine cones were also found.
"They [developers] have been very cooperative," Kirkland said. "Because it is on private property, they could have told us to get lost and sold everything on eBay."
Don DeBlieux, a paleontologist with Utah Geological Survey, said several scientists descended on the site Sunday and have been splitting open stones since.
"It's neat," DeBlieux said. "We're finding a lot of things we haven't seen before.
Scientific museums around the country are interested in the find and have requested some of the fossils.
Kirkland said samples will be shipped to the Smithsonian Institution, the Museum of Natural History in New York City, the University of California Berkeley, the University of Utah and Brigham Young University among others.
