The mushroom plant that starts small and grows to produce more suns is Sun-shroom.
Sun-shroom is a nocturnal producer introduced in the original Plants vs. Zombies. It costs less than a Sunflower and initially creates small suns, making it especially useful in nighttime levels, where ordinary sun production is limited. As it grows through its stages, the value of the suns it produces increases.
A common mix-up is confusing Sun-shroom with Sunflower or Twin Sunflower. Sunflower produces regular suns from the beginning, while Twin Sunflower is an upgraded form that produces two suns at a time. Marigold, meanwhile, is mainly associated with producing coins in the Zen Garden rather than generating battlefield sun.
The mushroom’s behavior also fits the game’s broader night-and-day system: most mushrooms sleep during daytime stages unless awakened with Coffee Bean. Sun-shroom is therefore both a resource plant and part of the original game’s distinctive mushroom mechanics.