Concrete sculptures are popular outdoor decorations. They can be found online, at home stores and at local nurseries. Purchased sculptures can be expensive or the wrong size for your needs, but if you are looking for a concrete mushroom, you can make your own at home, for little cost and with little effort.
Use a shovel to dig a smooth, well-rounded hole. The depth and width of the hole depends on the size you want your mushroom cap. Place a plastic garbage bag or piece of plastic sheeting in the hole, and spread it out to the surrounding ground.
Mix concrete according to the package's directions. Pour the concrete into the hole, on top of the plastic, and fill it until it is almost level with the ground.
Use a two-by-four, or other piece of wood, to tap the top of the concrete. This will push out any air bubbles, press the concrete into all the crevices of the bag and make the top layer smooth.
Use a box cutter to cut off the top of a two-liter bottle below the start of the neck and the bottom curves, creating a straight tube. Make sure the bottle is clean and completely dry. For a different-sized mushroom stem, use any other plastic tube.
Place the tube in the center of your concrete filled hole. Let it rest on the surface -- if it is pressed in, you will not be able to remove it once it dries. Fill the tube with concrete.
Allow all the concrete to dry according to the package's directions. Drying times will vary, but in general, overnight should be sufficient.
You may need to run a box cutter down the length of the bottle to be able to remove it. Flip the mushroom over and remove the garbage bag. Display it on a flat, even surface.