The Seed of Greed is a chestnut. It is found in the Wistful Wild, inside one of the two Orange Bulborbs in the area. To get it, you'll need to defeat the Dwarf Red Bulborbs in the area and the Swooping Snitchbug near your landing site. After they're gone, move on and take on the Yellow Wollywog, Gatling Groink, and the Cloaking Burrow-nit. Then build the bridge so your Purples can pass and defeat the Fiery Blowhog. Before you defeat the Bulborbs, knock the Withering Blowhog out of the sky and then attack the Orange Bulborbs.