The effect of frost on weeds depends on the length and harshness of the frost. In general terms, dandelions are the most tolerant to frost, while perennial sow-thistle and Canada thistle are the most sensitive. If in doubt, it is best to wait a day or two after the frost and assess the damage to the weeds – if they are still 60% green, they should still be susceptible to Roundup brand agricultural herbicides.