Many thanks to Transient and Permanent for reminding us about, and providing a link for, The Liberal Christian, edited by the excellent Scott Wells.
I was particularly struck by "Ecclesiastical Dodo Tree, A tale of growth and survival by the Rev. Derek Parker." In it, Rev Parker makes the case that growing the total number of congregations is, in the long term, "one of the most pressing growth issues for the future of Unitarian Universalism in North America." I think that while this statement is true, it is so far into the long term that discussing it today is like the parents of a baby suffering from failure to thrive syndrome worrying about whether the grandchildren will get into a good school- better you should make sure the kid grows up to give you grandchildren at all.
What is needed to plant new congregations? Many things, but enough UUs in the seats to be worth filling out the paperwork for a start. "Wherever two or more are gathered in my name" may be enough to satisfy Jesus, but not the UUA. While there is no minimum number for an "emerging congregation", experience with all other organizations tells me that fewer than a dozen are unlikely to be able to maintain, let alone grow to the thirty required to be a real congregation. Very, very few congregations can afford to lose a dozen of the kind of dedicated, energetic members it takes to build a new congregation, let alone thirty for a recognized congregation. My own congregation is 103 years old, and has only spun off two others, and both of those were in the 70s.
Why do people found new congregations- regardless of religion- anyway? There is one common reason, having moved to a location that doesn't have a church of your persuasion; I imagine most UU emerging congregations are started that way. Two reasons Evangelists have seldom apply to us, I suspect: your congregation has grown so large- 10,000 or more- that you just have to split to stay a manageable size (if that were common, we wouldn't be having this discussion); or the "if you build it, they will come" form of evangelism, deliberately moving somewhere that doesn't have a church and building one to convert the heathens. There aren't a lot of UUs with that sort of evangelical spirit; and even if there were, would it work? As I noted above, the whole discussion is occurring because we did build them, and they didn't come. Why else would someone leave their congregation, friends, and fellowship to start another? A reason common to both Baptists and UUs in my experience is having a major congregational vote, the losing faction leaving to found a new congregation of like minded. I know a Baptist church from my childhood, and at least two UU congregations in this city that were founded this way. I don't think that's necessarily a healthy way to grow.
I think we need to look at why youth raised UU don't stay, why once enthusiastic new members leave, and why visitors don't return. I think we need to face the fact that half our membership is only here for the RE, and will leave as soon as their kids are grown. I think we need to look at the public image we project with advertising like "Is God getting in the way of going to church? Come to the church that doesn't have or want one!" Once we solve these issues, we can think about growing the number of congregations; but planting new congregations without first growing the total number of Unitarians will only result in making the average congregation smaller... and as the average right now is scarcely a hundred, (if you look at the list on the official website, you'll find hundreds with membership under 75, many with membership of 50 or less), we'd risk them becoming too small to survive.