Mason bees seal each nesting chamber with a wall of clay based soil providing protection for the developing bees from predators. Without a nearby source of moist clay soil, mason bees will not nest in your bee house. The correct moisture content is very important because bees need soil that they can easily gather and use. Arid locations, sandy soil, and rooftop gardens may not have the moist soil the bee’s need.
You can ensure that your yard provides a clay mud source.
About 20 feet from your bee house, dig a shovel sized hole and add the package of clay to the south side of the hole near the bottom then lightly mix it with your native soil and moisten. Your soil should stick together when wet and squeezed. Keep the soil moist as long as the bees are nesting.
Each 8 ounce package of mud mix supports 100+ nesting female mason bees.