Ok so my husband's uncle threw the gauntlet, after he saw what I could do, when I brought a round cake to the family get together.  So now I have to blow their socks off. I am going to make a cake replica of their beautiful garden.  BUT,  I need to figure out a way to make a pergola.  I was thinking about possibly using modeling chocolate or candy melts.  But I can't find the right kind of mold.  Any suggestions?  I have some time, well lots of time actually, we aren't getting together again until July.  So any help would be appreciated!

The first thing I though of is royal to keep it delicate looking and you can get extra strength if you add some gum arabic. I made a biplane once out of *royal so it is plenty strong enough and a pergola is such a straight forward design.

*but I did not have the gum arabic but I probably added cream of tartar

But I should have asked how big are you going to make it?

I haven't even thought of dimensions yet.  The cake itself is going to be pretty big.  But the pergola doesn't have to be overly big as I have to fit an entire garden on this cake.
 

Before you can determine what to use to make the pergola, you need to decide on the dimensions since that will impact the decision.  Are you trying to replicate a pergola that exists in the backyard in the garden area?  That would impress your family the most, I think. 

If you want to make the entire thing edible, the frame of the pergola (pillars and large support beams) could be made from gum paste (depending on the size), or you could make a sugar mold.  When these are dry, you can cover them with chocolate or fondant.  If you want to make the smaller beams more delicate, I agree that royal icing is a great idea.

If you don't care about the frame being edible, them construct the pillars and beams from a solid material then cover that.

Have fun with this one!   Sounds like a great project.  Be sure to post a picture when you get it done.