This story was illustrated on the cover of a sci-fi magazine in the 60s. The illustration shows 2 astronauts in NASA Gemini-style space suits. They are struggling as their feet grow roots and anchor them to the ground and their arms sprout tree limbs.
Are you thinking of this cover
by artist Robert Adragna (www.robertadragna.com) from Fantastic Stories of Imagination, December 1962? There are three astronauts instead of two, and their suits don't look all that much like Gemini space suits, but they are definitely "struggling as their feet grow roots and anchor them to the ground and their arms sprout tree limbs."
The picture illustrates the short story "Cocoon" by Keith Laumer, which you can read at the Internet Archive (click here for download options). The illustration is symbolic; there is no new planet in the story, no space suits, no astronauts, no tree roots or branches growing from people.
The same painting was later used for the cover of Transformations: The Story of the Science Fiction Magazines from 1950 to 1970 by Mike Ashley.