add a string prefix to each value in a string column using Pandas

I would like to append a string to the start of each value in a said column of a pandas dataframe (elegantly).
I already figured out how to kind-of do this and I am currently using:

df.ix[(df['col'] != False), 'col'] = 'str'+df[(df['col'] != False), 'col']

This seems one hell of an inelegant thing to do – do you know any other way (which maybe also adds the character to rows where that column is 0 or NaN)?

In case this is yet unclear, I would like to turn:

    col 
1     a
2     0

into:

       col 
1     stra
2     str0