Hearts are what make up TumbleSeed's health. You can gain a heart by using the HeartSeed four times. Hearts also rarely appear in the Vineguard Shrines, at the end of the bouncing minigame, or as a reward for recycling. Sometimes, shops have them as well and sell them for just about any price.
Whenever the game tries to spawn a seed power that you already have, it will create a heart instead. This means that the more seed powers you own, the more likely it is you will find hearts throughout the world.
The life force of TumbleSeed. The right side of the Vineguard keeps track of the current heart total.