Conditional formatting

Hi

I have a workbook and in one column I want to apply conditional formatting if any cell is >=3 then the cell goes green and if the cell is =0 then it is red. I can do this fine but I need to apply it to the whole column and as it stands at the moment all the blank cells in the column are red too. I want blank cells and the cells yet to be populated to remain unformatted until a number is entered.

Thanks

Viv Haig