It helps if you plant it in good soil -- even more if that soil is stocked with nutrients and growth formulas and other new sciency things designed to help flowers grow. But unless you water the thing, all you have is a fairly expensive pot of dirt.
And it's not just a one-time thing, either. Turns out, the darn things need to be watered all the time. Even once they've started to grow, they still require water for survival. Fail to water, and your lovely flower wilts. Continue to forget, and it dies.
It's true for plants, for pets, and even for people. We need water -- a consistent supply of it! -- in order to survive.
As water is for our physical beings, prayer and learning are for our spiritual -- our real -- selves. It's telling, I think, that Jesus didn't tell us, before being taken back to the Father's side, to make converts, but to make disciples. It's one thing to plant a seed, but we must also encourage our fellow believer to grow stronger and become more like Christ. This is a process, and one that won't end until we end our lives here and come into our inheritance at His side.
Paul tells the church, in Hebrews 2:12-14,
See to it, brothers and sisters, that none of you has a sinful, unbelieving heart that turns away from the living God. But encourage one another daily, as long as it is called “Today,” so that none of you may be hardened by sin’s deceitfulness. We have come to share in Christ, if indeed we hold our original conviction firmly to the very end.
His encouragement here is to edify one another, and to keep one another on the right path. We do this through constant prayer. We do this by constantly seeking to know more about God and His will. This is why Christ called believers to make disciples. It's one thing to plant the seed; but once somebody comes to the faith, it is up to us to help cultivate it.
This is one of the reasons the church is so important. The church is a good soil in which this seed of faith can find what it needs for growth. But, more importantly, the fellow believers help sustain the life of this new faith by a constant stream of encouragement, prayer, and teaching.