You can visit the southern edge of this screen at the beginning of the game by going north from the Greenwood Village farm, but until you find the Axe and the Hammer, the only thing you can do is summon the Slime Bunny in the river (check the light blue spot next to the reeds).
If you're on the north side of the river, don't miss the Egg.
Muddy Bush: Three waves.
Wave #1: Three Muddy Bushes and two Red Wasps
Wave #2: Three Muddy Bushes and two Leaf Turtles
Wave #3: Two Green Jellies, two Leaf Turtles, and one Rock Turtle