When is it safe to plant trees in Riverton this year?
Fremont County Wyoming
If trees are in pots they can be planted any time the ground is thawed. However, if they were bought from a nursery in an area with a milder climate, the trees may have started to leaf out and you would be correct in your caution to plant them without protection this time of year.
A fun website to explore the frost free dates, hardiness zones, etc. is: http://www.plantmaps.com/interactive-wyoming-last-frost-date-map.php According to this, the average last frost date for Riverton is May 21-31. Mother Nature is really unpredictable so those dates certainly aren't set in stone.
Let me know if you have additional questions!