A Sunflower produces sun to help players buy and plant defensive plants. In Plants vs. Zombies, sun is the resource used to pay for most plants, so Sunflowers are the game's central economy-building unit.
Sun falls from the sky during daytime levels, but relying only on falling sun is usually too slow for a strong defense. Sunflowers generate additional sun at regular intervals, letting players afford Peashooters, Wall-nuts, defensive explosives, and other plants more quickly. Their low cost makes planting several early Sunflowers a common opening strategy.
Sun is not the same as seeds, coins, or stars. Seeds are used to unlock and select plants through the game's progression systems, while coins are collected as a separate reward currency. Stars are associated with level progression and certain unlocks. Sunflowers also have variants in later Plants vs. Zombies games, but the original Sunflower's defining role remains producing sun for the player's lawn.