Do Not  Home- Prepare These Foods:

Beets, Turnips, Carrots, Collard Greens, Spinach. 

This is from the AAP Book "Caring For Your Baby and Young Child" page 211

"In some parts of the country these vegetables contain  large amounts of nitrates, a chemical that can cause an unusual type of anemia (low blood count) in young infants.  Baby-food companies are aware of this problem and screen the produce they buy for nitrates; they also avoid buying these vegetables in parts of the country where nitrates have been detected.  Since you cannot test for this chemical yourself, it's safer to use commercially prepared forms of these foods, especially while your child is an infant.  If you choose to prepare these at home anyway, serve them fresh and don't store them.  Storage of these foods may actually increase the amount of nitrates in them."