Queries :: Add Days From One Field To Date Field

I am trying to track vegetables that I will be planting in my garden. I will plant some vegetables. I have a StartDate and HarvestDate and Estimated HarvestDate Fields. I also have a field named GrowDays. This field states how long it takes for each vegetable to mature i.e. 75 days or 58 days or 42 days etc. etc. What I want to do in a query is to automatically add the number of days from the GrowDays field to the StartDate and come up with the Estimated Harvest Date. If I plant a Tomato on April 13th and it takes 75 days to mature then I want the EstimatedHarvestDate field to reflect 6/27/2013 which is the StartDate + 75days. I am trying to figure out how to do this when the GrowDays field changes numbers. I can use this "Estimated HarvestDate: DateAdd("d",+75,[GrowDays])" except this only works if the GrowDays Field is 75. I need to be able to accomplish this no matter what the GrowDay field says.