Humongofrog treasure cards can be obtained from three different plants: Deadly Helephant Ears, Fickle Pickle, and Trumpet Vine. I actually happen to have information on all three of these plants! Yay me!
In first place, we have Trumpet Vines! After growing these for 30 days I received a total of 34 Humongofrogs. Second place goes to Deadly Helephant Ears. Thirty days of this plant netted me 20 Humongofrogs. As expected, Fickle Pickles, trudged along in third giving me only 3 Humongofrog treasure cards within a 30 day time frame.
Last week, Dezzy's Myth Wizard's Guide to Dragonspyre Crafted Clothing informed us that Myth wizards will need 25 Humongofrogs to craft their Dragonspyre clothing. If you're in a hurry, I would definitely plant the Trumpet Vines and most likely get 25 Humongofrogs well within a month. Not only do Trumpet Vines provide a nice variety of other Myth treasure cards, they also produce a decent amount of Leather Straps.
However, if you have a little bit of time and/or patience, don't overlook the Deadly Helephant Ears. Not only can you harvest reagents like Aquamarine, Diamonds, and Sunstones, they also give Mega Pet Snacks at elder harvest. You can't beat that!