Cistus and Halimiums grow in degraded Mediterranean habitats known as “maquis” and “garrigue”. The vegetation is periodically burned by wild fire (forest fire).
The Cistaceae have evolved a strategy to take advantage of this. They produce large quantities of very small seeds. Only a small proportion are ready to germinate normally. The rest are dormant and have a very hard seed-coat which must be ruptured before they can germinate.
The tiny seeds are shed when ripe and fall to the generally stony ground, where they easily fall into crevices and can remain dormant for many years. The heat of forest fire, which destroys all above-ground vegetation, ruptures the hard seed coat and the seeds germinate en masse when conditions are right (cool and moist). The ground is carpeted with seedlings of cistus or halimium, which often crowd out seedlings of other plants.
Most species of cistus and halimium flower in their second year, so within two years of a fire they will have produced their seed and replenished the seed bank in the ground, ready for the next fire.
NB. When growing cistus or halimiums from seed there will usually be enough non-dormant seed to provide all the seedlings you may want. If, however, seed is in short supply, the best way to ensure maximum germination is to crack the hard coat of the dormant seeds. Although heat can be used (e.g. baking in an oven), it is much simpler, quicker and more reliable to scarify the seed by spreading it on a sheet of fine-grade sandpaper, then rubbing it fairly gently with another piece of similar sandpaper wrapped around a block of wood. Do not be too gentle. The seeds are quite tough and are not easily crushed.
See my page containing a bibliography on the germination of seeds of Cistus & Halimium