If I remember correctly, code "requires" a breaker in the main panel to feed the sub panel. Your 30 amp breaker protects the wiring that runs from the main panel out to the sub panel in your green house. If you were ever to cut the wire with a shovel or something, that breaker is what would prevent the whole system from shorting out.
The 15 amp breakers in your green house protect the wiring for your individual light and outlet circuits.
Your configuration is normal, and I have the exact same setup powering a sub panel in my shed.
However, unlike the main panel, the neutral bus should be isolated from the ground bus in the sub panel. You should also have an additional ground rod installed out at the greenhouse to suppliment the ground at the subpanel.
The US power grid uses "alternating current" (AC). The electrons flow from positive to negative, then reverse to flow backward from negative to positive (rising and falling in a sine wave). In the US the current alternates back and forth like this 60 times a second (60 hz). So technically, the electricity is flowing in both directions.
From a practical standpoint you have a supply (the power grid, generator, etc.), and a load (lights, heaters, radios, computers, etc.). Without the load, current isn't flowing either direction. :)
Anthony Watson watsondiy.com mountainsofware.com