To release the items stuck in the tar, after killing all the growths dig a trench starting from a lower point (there is usually a creek nearby) with a gradual ascent up to what you think will be the bottom of the pit. This will free up most or all the items stuck in the tar. Sometimes items are stuck even when there does not appear to be any tar, simply dig a small 3-4 foot trench beside the item to free it. In most instances 2 growths will continue to spawn every 2 game days, so be cautious when approaching tar pits you think you have cleared and drained.