FYI From Nutrition AU:
"Most vitamins will keep well in frozen vegetables. Carotene (a compound that is converted to vitamin A in the body) may actually be better preserved in frozen produce because packaging keeps the vegetables away from light (which destroys carotene).
Some losses of vitamin C and folate (also known as folic acid) occur during commercial freezing. About 25% of the vitamin C, and perhaps a greater percentage of the folate, will be lost during the blanching process that precedes commercial freezing. A smaller quantity (perhaps 10%) of the thiamin (formerly called vitamin B1) will be lost during blanching. Little further loss occurs during the time the food is kept frozen, provided that it has been stored properly (-18 degrees C for no more than six months).
However, the vitamin losses associated with blanching and the thawing/cooking process are similar to those that occur during normal cooking of fresh vegetables. This means that, provided they have been stored and then cooked properly, frozen vegetables provide similar levels of nutrition to fresh vegetables. It is also worth noting that for cooking both frozen and fresh vegetables, microwave cooking and steaming are both superior (in terms of retaining nutritional value) than boiling in a large volume of water."
So to sum it up, frozen is fine (and sometimes better) up to 6mo. Steam or Microwave them don't boil.