This is a great way to make cool trees for a theatre production or a more mythical type film. I wouldn't recommend it for close up shots in a film though unless you're a very skilled painter.
NOTE: you will need a few days to work on this because the spray foam takes a long time to dry properly, and you need to spray it section by section.
OTHER NOTE: try to do this outside so when you have to spray paint it's not a huge hassle to bring it in and out of your workspace.
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Step 1: Supplies
Here's what you need:
NOTE: the amount you need of each material will depend on how tall or wide you want the tree. I'll tell you exactly what I used and how I built it, but you may need to adjust for your needs.
-2 big pieces of cardboard tubing, one being slightly thinner then the other so it can slide nicely in and out the other (one pictured above)
-at least 6 cans of spray expanding foam (also pictured above)
-dark and light brown matte spray paint
-fake moss (brown and/or green)
-trash bags or a large roll of paper
-hot glue gun
Step 2: Set Up the Tubes
Lay the tubes on a drop cloth. Slide the slightly smaller tube into the bigger one until you have the desired height. Then take your drill and put four screws around the tube to connect the two pieces.
Step 3: Spray Foam
Keep the tube laying down. If you stand it up, the foam won't stick well, and it will fall off and go to waste. Draw a pencil line around where you want your roots to start when you put them on later. Grab a can of spray foam and carefully read all the directions. This is not its intended purpose, so you need to make sure you know how it works.
So with it still laying on its side, start spraying strips down the entire side (except for where the roots will go). It will look weird at first, but give it a few minutes, and you'll see how the foam expands and starts to look like knots in a tree.
You want as much texture as possible here, so as you're spraying, if you have small pieces that are leftover hanging on the tube from the can, wipe it right off on top of another strip to make it look like an extra knot. Once you start working, you'll see what I mean. You'll get into a rhythm and start building your own growth patterns and textures.
Do this section by section, because if you don't give the foam enough time and you role it over, it's gonna get squished. You may need a few hours between each section. If you're pressed for time, you can do what I did and balance one end on a chair so you have it at and angle and can keep spinning it without it hitting the ground.
Step 4: Roots
Once the foam on the main part of the trunk is dry and hardened, stand it up. Take the chicken wire and start making a kind of tent around the bottom to shape the roots. Try to not make it even so it looks more realistic. For example, maybe take an extra piece of chicken wire and make it look like a longer root extending farther away before it plunges into the ground.
When your shape is figured out, take your trash bags or paper and cover all the chicken wire, like its and actual tent.
Take your spray foam and continue the patterns you've already created down into the roots so you can't see the seams between the wire and the tubes.
Step 5: Paint
Take your dark brown paint and give the whole thing a solid coat of that. Now the texture you've just created leaves a lot of nooks and crannies to hit, so make sure you get detailed and hit all of them.
Once that's done, get the light brown paint and take a few steps back. Lightly spray it towards the tree. You don't want solid spots, you just want to use this as shading and to mess with the color. Up close it should look like you're just speckling it.
Step 6: Moss and Ivy
This part is all decorating.
Take your moss, break it up into pieces, and hot glue it all around the tree. Just make sure it looks natural, which isn't too hard to do with moss.
The fake Ivy usually comes in strips. If this is the case, glue the top of it up high on the trunk and wrap it down as you go.
Step 7: Set It Up
The best way to set this is actually bury it in a bit of dirt, or if you're using it in a theatre setting, take some brown fabric and surround the bottom, mimicking dirt.
If you want branches for the top, sculpt them out of chicken wire, cover up most of it in paper, and make the same pattern with the foam. Then use the wire on the end to hang them off the top of the tube.