Sofa Spuds are definitely the way to go! Usually one or two Mega snacks at elder harvest. Intermediate harvests yield lvl 6 and 7 snacks. They always drop a seed at elder and they never require more than a lvl 1 pest spell.
since your posting here you a member so. buy like 10-20 magma peas during a seed sale then during double harvests you harvest them doubling your seed count. from there you can within a few events have enough magma peas to keep sixth wizards busy. if you dont wanna buy them you can farm them in Avalon from bosses.
another option is fish on a vine, they drop in Azteca drop rank eighth snacks (25xp) use small plots and very easy to tend.