Banana seeds usually germinate after a few weeks, sometimes not until after 1-3 months. Temperatures between 28-30 °C are ideal for germination. Due to the rapid growth, banana plants will later need plenty of water and fertilization.
Store-bought bananas do not contain any usable seeds because they have been bred over the years as triploids, thus contain an additional set of chromosomes. The banana seeds do not ripen, but are merely contained as small black dots in the seedless fruit. In commercial cultivation, bananas are propagated by rhizome cuttings, formed by the banana tree and then planted to grow a new banana plant.
Germinable seeds can be gathered from wild bananas, or you can order a seed packet. There's a distinction between fruit bananas, most of which are hybrids, and ornamental bananas which do not form edible fruit. Depending on the cultivar, banana seeds can reach a diameter of up to 1 cm.