How to Make Artificial Boulders for a Garden

Pour equal parts Portland cement and builder's sand into a large bucket or wheelbarrow. In general, 30 lbs. of cement and sand will create an 8-inch by 12-inch by 12-inch stone. Stir these dry ingredients thoroughly with a shovel or rake.

Add twice as much peat moss to the wheelbarrow as you added of the cement and sand. Thoroughly blend these ingredients together, until they are completely mixed. The moss will provide a natural look and help to absorb moisture.

Slowly add water to the mixture until the whole thing resembles oatmeal; it should be damp enough that it doesn't crumble to the touch, but not runny or waterlogged. Mix in the water until you reach the proper consistency, and then allow the mixture to set for 10 minutes.

Stir your mixture once again and add about ¼ part of acrylic fortifying additive, which is available wherever you found your Portland cement. This will make the fake rock gain a stone-like strength and texture.

Spread your mixture onto a mold for your stone. This mold can be made of wood, wire, plastic or another material, as long as it will hold up to the weight of the cement mixture.

Design your cement as you add it to the mold. Spread the cement around using a mason's trowel, so that it is at least 3 inches thick on all sides. Use the point of the trowel to create veins or other texture marks in the cement. Continue until the mixture has the general shape and appearance of a boulder.

Place your faux boulder into a black plastic trash bag and seal the bag thoroughly so that no air passes into it. You may want to add sticks or straws to hold the top of the bag up from touching the cement, or you could end up with unexpected indentations. However, the bag must be sealed as air tight as possible.

Place the black bag in an area in which it will receive full sunlight for most of the day. The sunlight will cause the cement to sweat and provide an automatic misting system for the curing process. Leave the bag in the sun for at least 48 hours, and then open it and lightly scratch at the surface with your fingernail. If you can still scratch any of the cement mixture away, seal it up again. Once you cannot scratch any away, the project is mostly dry.

Move your mostly-dry boulder into a shaded location at room temperature; keep it in the sealed bag. Check the interior of the bag every two days to see if it is moist; if not, lightly mist the boulder with water to increase the moisture. Allow the boulder to finish curing for at least seven more days; the longer it cures, the stronger it will be.

Remove the boulder from the bag when it has cured long enough. Wear protective gloves, because lime from the cement can burn your hands. Rub down the surface of your false boulder with a small stone to create a stone-like texture and natural wear marks.

Soak the boulder in a tub of clean water for three days, changing the water each day. This will naturally wash away the lime in the cement, which can kill plants near the boulder if not properly treated.

Remove the boulder from the soak after three days (make sure you wear gloves) and allow it to dry fully. Apply stain or paint as you choose to create a stone look. When the paint has dried, seal the entire surface with two coats of concrete or cement sealant.