I understand that it's trying to describe something more organic than building and filling church facilities; something that communicates growth and change, and that the alternative term "church-building" would have attached to it all of the baggage of "church buildings." But "church planting" isn't really a scriptural term.
Yup, I'm aware of Jesus' parables of the sower and the seeds and the soil (Matthew 13, 25:24-26; Mark 4; Luke 8); that Paul would take up the metaphor also (1 Corinthians 9:11; 3:6-8). And while they may well have application to gospel-sowing on a community-wide as well as individual scale, I keep coming back to the impression that - with these metaphors - Jesus and Paul were talking about sowing gospel seed in individual, sin-soiled hearts. Their subject was "soul-planting."
When they spoke about "church-building", they used the words associated with church-building.
"And I tell you that you are Peter, and on this rock I will build my church, and the gates of Hades will not overcome it." ~ Matthew 16:18 - one of only two times Jesus used a word translated "church" or "assembly" in scripture.
Paul even switched metaphors to make his point:
"For we are God's fellow workers; you are God's field, God's building. By the grace God has given me, I laid a foundation as an expert builder, and someone else is building on it. But each one should be careful how he builds. For no one can lay any foundation other than the one already laid, which is Jesus Christ." ~ 1 Corinthians 3:9-11
And neither was talking about brick-and-mortar building(s); they were talking about using stone - rock-solid faith that will not shift in storms of persecution and floods of doubt.
Peter, the one whose pebble-faith was grown to boulder-sized conviction, agreed:
"As you come to him, the living Stone — rejected by men but chosen by God and precious to him — you also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ." ~ 1 Peter 2:1-11 (partial quote here)
Sure, I know that using a stone-masonry metaphor today will also carry with it all the baggage of Masonic imagery, too - but the metaphor is scriptural. It conveys an endurance in storm and persecution; speaks of becoming the altar and "spiritual house" in which "spiritual sacrifices" are made, and talks about building on the foundation of Jesus Christ as God's fellow workers.
When my family and I traveled in Ireland this last summer, we saw - all over the middle part of the island nation - stone walls, some hundreds of years old; others thousands. They marked boundaries, formed sheep pens, walled in gardens. And some out on the barren, wind-blown coastal burrens had no apparent purpose on a soil to rocky to support life of any kind. But the walls persist, carefully hand-laid, stone fitting upon stone.
I have a little bit of fear that when we use the term "church planting," we're bringing with it the very correct impression that as planters we are not responsible for the receptivity of the soil; an impression that can lead to a kind of fatalism and expectation of failure. That is to be expected - on an individual basis - because individual people can reject the seed of the word; let it be snatched out of their hearts by fear of persecution; let it be choked out of their lives by cares and worries of the world.
Churches don't. At least, they shouldn't. They should be built up together, supporting each other, founded on an active faith in Christ that is as enduring as stone. That's why Jesus spoke so much about building on a foundation that lasts:
"Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash." ~ Matthew 7:24-27
So plant the seed, yes. Be grateful when not all of it falls along the path. Praise God when the seed finds root, and the root finds soil, and the soil is moist and ready for it.
Then work with Him to build a wall, founded on rock-solid faith in Christ, to keep out the birds and the weeds and the thorns.