AI'm having some trouble deciding what I should use for a upcoming cake later this week. I'm wanting to make palm trees but I don't know what I should do to make them, I've played around with the modeling chocolate but it seems a bit unstable and won't stand upright, I'm not sure if it was the recipe I used or what. I've never used gum paste and I'm not sure if fondant would even harden like I'd like. Any suggestions would be great!

AIf they are stand up trees, I might use gumpaste since it hardens so much. In times when I only had a little gumpaste, I would mix it with fondant and that worked just fine for my purposes.

You'd be better off putting the trees on the side of the cake instead of on top. You can still get some leaves to stick out in a 3D way using modeling chocolate but that way, the cake's side can support the trunk and leaves.

If you want the tree to stand free and upright, you are better off using gumpaste but that would be much harder to do. 

I make them from fondant w/tylose. Down here, with modeling chocolate they'd be wilted in half by the time the cake arrived.  I find gumpaste a pain to work with and plain fondant will not get hard enough. (recipe 2t tylose powder kneaded into 1 lb fondant)

Just make the trunks and stick a skewer up about halfway so the tree can be bending a little still. While those are hardening, laying on their sides, form the tree tops out of leaf shapes or whatever you're going to use. I used foam balls cut in half. I think next time I might use larger balls. When attaching to the tree trunks, use a fresh piece of the sugarpaste and tylose glue in between the leaves and the trunks. Stick them upright in  a big foam piece like a cake dummy to dry. These rode just fine to the venue 45 minute away on the cake. And they were big - to help with scale, the cake was a 10-7-5:

Some cake decorators use a large, straight pretzel and then put on gum paste or modeling chocolate leaves.

Here are some examples:

http://www.wilton.com/technique/Palm-Tree

http://cakecentral.com/t/714100/how-do-i-make-a-palm-tree-out-of-gumpaste-fondant

AI used the pirouette cookies from pepperidge farm as a base. Then I cut tiny rows of a 70/30 fondant gumpaste mix to make the trunk. The palms are cut with leaf cutters in varying shades of a 50/50 mix of fondant & gumpaste.

[URL]http://www.verydeliciousdesserts.com/#!Jungle-Baby-Shower/zoom/c1ff7/image5uz[/URL]. I think there are close ups of the trees in my cc gallery