Keep all three meters in their green bands to grow. Tap when a meter runs low for combo points, never overfill, and squash pests.
You are looking after one plant at a time. It has three meters: Water, Sun and Feed. Each meter slowly drains on its own, and each has a green band. Tap the Water, Sun or Feed button (or press 1, 2 or 3) to add to that meter. Each tap adds 20 to Water or Sun and 24 to Feed.
While all three meters are inside their green bands, the plant grows and its health recovers. The longer they stay green, the faster it grows, up to twice as fast after 8 seconds. If a meter is outside its band, health drops. If you tap a meter past the top of its band, you lose 3 health and your combo. Tapping a meter when it is low gives combo points. If health reaches zero, the plant wilts and you can try that plant again.
There are six plants: Daisy, Tulip, Sunflower, Rose, Cactus and Orchid. Each one has different bands and drains at its own pace, so a cactus needs very little water. From the second plant on, pests appear and drain health until you tap them. From the third plant on, heat waves make water drain twice as fast and cloudy spells make sun drain more than twice as fast. Bloom all six to finish the garden.