Watching a lets play with nice graphics and mods and i noticed bushes/plants popins. What causes this and what eliminates it

Plants and objects pop in in the distance because even the best of existing graphics cards cannot render everything.

On pc you can modify Skyrim.ini and add the line:

[General]uGridsToLoad=7

or 9, or 11, or more. An interesting thread there: http://www.gamesas.com/topic/1274926-ugridstoload-skyrimini-comparisons-and-explanation-default-57911/