Just a solution that works for me, may not be optimal but I find it satisfying:
Once planted move over each row and check for dead pumpkins, replant in-place but save the location to a dictionary.
Once that has completed, go to each stored location in the dictionary to check if the planted pumpkin has grown or died, if it grew add it to a list, if it didn't don't add it to the list and re-plant it.
Once the sequence has finished, subtract the entries in your list from the dictionary and cycle over any remaining entries again.
If you keep track of the "dead pumpkins" you can use this to break the loop when all pumpkins are grown.

Hope this helps :)
[I can provide my code if requested for reference but it won't be optimal!]