There were some recent (at the time I’m posting this they’re recent, but you might want to check the dates and latest official information before replying to them) about clearing zombies out of areas, what the Turned are, and (what feels like one of many suggestions on the topic of) hordes attacking, which inspired these ideas.
They're not essential to understanding what I'm writing here, but I thought I would provide links anyway:
Reclaiming lands
Zombies Navmesh
Massive hordes every X days
Oh, and here’s links to the Trello cards where Nelson described the Turned:
https://trello.com/c/yQ1d5wmc/9-turned
https://trello.com/c/vIp1lGpS/45-excursions
The Turned as described by Nelson at this point in development are said to come from an alternate reality, and to form growths on buildings which spawn “monsters or creatures.” If these were the regular way for the Turned to spawn, then destroying them could be a much better way for players to establish a safe area than using safezone radiators.
The spawners might come back by regrowing from what’s left, hordes of the Turned arriving from their dimension and attempting to create new growths, or spawners simply teleporting in from their dimension.