Ridiculously Easy Giant Mushrooms

Minimal effort, maximum impact! These giant mushrooms are suuuuuuuper easy to put together, but they're surprisingly dramatic and help set the scene for a witchy woodland display.

Expanding spray foam (one large can made 2+ mushrooms for me)

Cardboard tubes from wrapping paper (or big pieces of cardboard rolled into tubes)

Big plastic bags (like recycling or garbage bags)

Bamboo sticks or garden spikes

Spray paint

Large bowls (or kiddie pools or giant buckets -- anything round, and the bigger the better!)

Sharp knife

The giant mushroom caps are just giant bowls (the bigger the better!) filled with expanding spray foam -- but you will absolutely destroy your bowl (and not be able to un-mold your mushroom) if you don't line it with plastic first.

Slice a giant plastic bag open so you have one big sheet. Lighter weight plastic is best, so cheap garbage bags are ideal. Drape it loosely in the bowl and make sure it hangs over the sides all the way around. I'd recommend working outside, but if you can't, put some newspaper or a drop cloth down to protect your floor. Spray foam is a nightmare to clean up.

Starting at the bottom of your bowl, spray a layer of expanding foam all over the inside of the bowl, up to the lip. You don't need to fill the middle, because it's going to expand a lot.

I made two at a time and had plenty of foam for both caps.

The surface of the foam will solidify in an hour or so, but it's going to take a lot longer for the bottom to harden because it's not exposed to air. DO NOT UN-MOLD IT YET!! Don't even peek to see if it's ready for at least 24 hours -- 48 if you want to be safe. If you mess with it too soon, the foam that hasn't set yet might collapse or end up with big air bubbles.

Seriously, just leave it alone.

Once your caps are nice and dry (24-48 hours later!!), remove them from the bowls and carefully peel off the plastic. There will likely be some wrinkles and bubbles, but try not to stress about them. Mushrooms are weird-looking things and I promise the strange texture will work.

You shouldn't need to mess with the top of the cap, since it should be smooth and round from the mold. The underside may be lumpy enough that it needs a trim, though. For carving I use a basic kitchen knife, but be warned: Foam will dull the blade, so you might need to resharpen periodically -- and don't use your favorite fancy chef's knife. Slice away excess foam that bulges out under the cap, maybe angling the inside up where it meets the stem. Mushrooms come in all sorts of shapes, so you have lots of room for creativity here.

As you can see in the second and third pictures above, I made one of my caps in a large bucket instead of a bowl. I didn't like the shape, though, so I had to carve the top of the cap to be more mushroomy. It ended up a lot smaller than I'd hoped and the foam is more open and bubbly.

Next, get your cardboard tubes: Wrapping paper tubes are perfect, but if you don't have any handy you could tape together some paper towel or toilet paper rolls, or take a sheet of cardboard and roll it up, securing it with tape. The strength of the cardboard isn't very important, because the spray foam will solidify around it, but it IS important to have a hole through the middle of it. Cut your tubes a couple inches longer than you want them to be.

Put your cap back in its bowl and line your tube up with the middle of the underside of the cap. Press it gently into the foam to make a mark, and then use your knife to cut a slice around that circle, about an inch deep. You don't need to cut any foam out, just make a groove where the tube will fit. Then slide the tube into the cap, twisting back and forth to push it a couple inches into the foam. It should pull the cap up with it when you pick it up, but it's not secure yet. Next we're sticking it on with more foam.

Lay the caps with stems attached on a sheet of cardboard or plastic. Then, working carefully with the spray foam, put a thin coat all over the top surface of the cardboard tube, working back and forth along it from side to side (if you try to go lengthwise it's more likely to fall off). You'll only be able to do half (well, more like 2/3 because it hangs over the sides) so you'll have to wait an hour or so for it to dry before you can roll it over and do the other side.

If you want the stem to flare out at the base, you'll need a second coat of foam, because you can't pile too much up at a time without it all dripping off onto your dropcloth.

Once it's dry all the way around, trim the stem to remove the bumpy surface of the lines of foam.

The surface doesn't need to be perfectly smooth. Again, mushrooms are weird and I think the foamy texture works well here. If you absolutely want to smooth it, you can round it off a bit with woodworking tools like a rasp or a plane, or even with very coarse sandpaper. It will get messy, though, so be prepared for tiny bits of foam everywhere. Or just embrace the foamy mushroom texture.

Your foam is probably a pretty convincing mushroom color already (considering the brands I've used are off-white, pale yellow or pale blue) so you could leave it as it is. You could also mist it lightly with spray paint to give it a hint of brown or gold. If you want it to look extra poisonous, bold red.

For the red ones I used some wide masking tape and cut circles of various sizes to stick all over the cap. I wrapped paper around the stem to protect it and then I sprayed the cap a nice glossy poisonous red. My mutant bucket mushroom with the weird bubbly texture got misted with brown spray paint (going for a morel-like shade, if not shape). I left another plain because it looked mushroomy enough (the little pointy guy at the back, which was made from foam off-cuts from another project and that's why it's a different shape.)

Drive a garden spike or bamboo stick into the ground and slide the hollow tube inside your mushroom over the top. If you live somewhere with really high winds you might need to drive a tent spike through the base to hold it down, but mine have been outside for a couple weeks in London (where it's always fairly breezy) and haven't moved an inch.

Your mushrooms are weatherproof and will last outdoors for months if you want to keep them around. The surface may eventually get pocked and the foam might go yellowish, but hey -- mushrooms are sometimes pocked and yellow too! If you really want to preserve them for longer, you could always paint them with a layer of monster mud (a mix of latex paint and joint compound -- search online for recipes or see my other Instructable about monster hands for more tips) or masonry paint.

Enjoy! I hope your mushrooms look magical!