Puzzle 5 is completed when your villagers learn mastery of six special plants located in various parts of the village. Drag a villager to each of them and watch for the message that each has been mastered. The plants are (beginning in the northwest part of the village and moving clockwise): the odd-looking plant immediately to the left of the very dry grass, the blue flower between the thorns and the rocks, the small lavender flower at the northern edge of the graveyard, the black flowers near the waterfall, the bright reddish flowers south of the dam site, and the large orange-red flower at the south edge of the village.

Drag your doctor to each of the plants listed above and watch them until they say something about Mastering the plant. If they just get one of that herb, then pick them up, and take them to the next one, and repeat. Eventually, you'll find the one (or more) that you were missing mastery in.

Look around the village for strange looking plants and put your peeps on them and wait for the mastery message

You don't need a doctor. This puzzle can be solved immediately when you start a new village.