Plaster of Paris for Dioramas

Plaster of Paris should work ok for your base of a diorama. The tree based off wire is a novel approach. Start looking around the house for other things as well. Especially items that are being thrown away. You'll surprise yourself on how you can salvage items for use into model building. Another possibility for trees are weeds or other plants in the yard. Pull them and let the roots dry out. The root makes an excellent tree base. Spices from the kitchen such as oregano can be used for leaves on the tree and as ground cover. Unraveled twine cut into small bits can be used as clumps of grass. Talcum or foot powder mixed with paint works great for mud. 
Have fun with it and good luck!

Kitty litter makes a great addtion to diorama bases as well for debris.  With plaster you have to plan ahead to get your textures on while the plaster is still wet otherwise it dries pretty smooth.  Here I added a rock from outside, kitty litter and a few weeds 

If you plan on having any contours e.g. slopes or bumps in your diorama base, you can use other things (odd bits of wood or cardboard etc.) to form the bumps, and just use the plaster to cover them & smooth the edges.  This can save a lot of plaster.  It's good to see a young, keen modeller - good for you!

Note also that Plaster of Paris can generate quite a bit of heat while it's curing. Usually not enough to melt anything...but it can actually burn sometimes if you get it on your skin.

And, just as a personal view, be careful with natural products like spice leaves if you live in a high humidity area. Mold can be a real problem with time.

I avoid using anything like seeds because it will attract vermin.

Bill

Teenage Modeler

Is Plaster of Paris fine?

It's very finely ground, yes.

It's fickle, too.  There's a sweet spot between "too wet" and "too dry" that can be hard to get to reliably.  How humid a room is matters in mixing.  Use the proportions listed on the packaging, but "sneak up" to the water amount prescribed.  Easier to fix "not wet enough" than "too wet."

It can have a really short "working" time, too.

Plaster often does not "like" being worked thick.
Remember that it's meant to be spread very thin on walls.

So, you may need several thin layers--and they need to dry completely in between.

I'm not much of a fan of plaster of paris for use on walls much, even with 40 years' experience in building trades.