Gardeners living in areas where the ground freezes often feel vegetables can only be planted during the frost-free periods. However, some cold hardy vegetables can be planted up to six weeks prior to the average last killing spring frost. Many of these vegetables also can be planted late in the fall and into the winter months even when the ground is frozen. These vegetables will often be ready for harvest a month or two before your neighbors even think about planting their gardens.
Source: CSU Extension