Mix the plaster according to the package instructions (usually 2 parts plaster to 1 part water). Pour the plaster into your vessel.
Poke two or three seeds into the plaster. Place in a spot where it won't be disturbed while the plaster sets and the seeds germinate (there's enough moisture in the plaster for the seeds to begin germinating, no need to add anything).
Within a day my seeds had swollen and begun to germinate. The force the seed exerted was enough to crack the plaster.
Now think about what happens in a driveway or sidewalk... a crack forms in the surface, seeds blow or fall into the crack, a bit of rain falls and the seeds begin to germinate. The force of the seed germinating and the roots taking hold forces the more cracking. Now there's a larger crack into which more seeds can gather and cause further cracking. If nothing is done to curb the problem, eventually the sidewalk or driveway will be dessimated.
*FYI Plaster of Paris does have a limited shelf life. If it gets too old, it won't set up properly and you'll have a crumbly mess.
**I've mentioned it before, but dried beans (like you'd use to make soup or baked beans) will germinate. They're cheaper to buy in a large quantity than garden seeds and they can be found easily year-round.
