To germinate Liriodendron tulipifera seeds, they need a period of cold to simulate their natural conditions. Germination rate varies with temperature and time, and increases with lower temperatures given over a longer period. Moisture is also essential during the pretreatment period, which is called stratification. Several water-absorbent materials can be used to keep seeds moist, such as vermiculite or perlite, paper towels or sand.