If you build your straw bale walls without the use of mesh on either side of the wall, you will need to attach the bales to the structure in some other way. The most common is shown in the image to the right and involves cutting 4″ wide x 12″ long (or so) pieces of plaster lath and nailing them to the posts at the top of each course.
The lath is then folded down over the top of the bales and attached with landscape pins. This works well and, again, is necessary only if you do not attach the bales to the structure by way of welded wire mesh or stucco wire sewn from one side of the bale wall to the other.
A newer option for attaching the bales is the use of expansion foam in the voids created around the notches at each post location. By shooting in a little expansion foam, the bales will be attached to the structure in a way sufficient to withstand movement during the plastering phase; however, this is not a structural connection, so you must not rely on it to withstand any lateral or out of plane forces applied to it. The foam is a faster way to connect the bales if all you need is a temporary fix while you wait for the plaster to lock everything in place.