diff --git a/cylc/flow/scheduler.py b/cylc/flow/scheduler.py
index 7449e7d274a..2cf8e40d671 100644
--- a/cylc/flow/scheduler.py
+++ b/cylc/flow/scheduler.py
@@ -467,6 +467,7 @@ async def configure(self, params):
             self.config,
             self.workflow_db_mgr,
             self.task_events_mgr,
+            self.xtrigger_mgr,
             self.data_store_mgr,
             self.flow_mgr
         )
@@ -1734,6 +1735,8 @@ async def main_loop(self) -> None:
                     self.pool.queue_task(itask)
 
             if housekeep_xtriggers:
+                # Spawn parentless clock-triggered
+                self.pool.spawn_parentless_clock_triggered()
                 # (Could do this periodically?)
                 self.xtrigger_mgr.housekeep(self.pool.get_tasks())
 
diff --git a/cylc/flow/task_pool.py b/cylc/flow/task_pool.py
index 92905becf1e..082714bb7aa 100644
--- a/cylc/flow/task_pool.py
+++ b/cylc/flow/task_pool.py
@@ -79,6 +79,7 @@
     from cylc.flow.data_store_mgr import DataStoreMgr
     from cylc.flow.taskdef import TaskDef
     from cylc.flow.task_events_mgr import TaskEventsManager
+    from cylc.flow.xtrigger_mgr import XtriggerManager
     from cylc.flow.workflow_db_mgr import WorkflowDatabaseManager
     from cylc.flow.flow_mgr import FlowMgr, FlowNums
 
@@ -98,6 +99,7 @@ def __init__(
         config: 'WorkflowConfig',
         workflow_db_mgr: 'WorkflowDatabaseManager',
         task_events_mgr: 'TaskEventsManager',
+        xtrigger_mgr: 'XtriggerManager',
         data_store_mgr: 'DataStoreMgr',
         flow_mgr: 'FlowMgr'
     ) -> None:
@@ -108,6 +110,7 @@ def __init__(
         self.task_events_mgr: 'TaskEventsManager' = task_events_mgr
         # TODO this is ugly:
         self.task_events_mgr.spawn_func = self.spawn_on_output
+        self.xtrigger_mgr: 'XtriggerManager' = xtrigger_mgr
         self.data_store_mgr: 'DataStoreMgr' = data_store_mgr
         self.flow_mgr: 'FlowMgr' = flow_mgr
 
@@ -744,6 +747,7 @@ def spawn_to_rh_limit(self, tdef, point, flow_nums) -> None:
             return
         if self.runahead_limit_point is None:
             self.compute_runahead()
+        is_clock = False
         while point is not None and (point <= self.runahead_limit_point):
             if tdef.is_parentless(point):
                 ntask = self._get_spawned_or_merged_task(
@@ -752,10 +756,18 @@ def spawn_to_rh_limit(self, tdef, point, flow_nums) -> None:
                 if ntask is not None:
                     self.add_to_pool(ntask)
                     self.rh_release_and_queue(ntask)
+                    if (
+                        ntask.state.xtriggers
+                        and set(ntask.state.xtriggers.keys()).intersection(
+                            self.xtrigger_mgr.wall_clock_labels
+                        )
+                    ):
+                        is_clock = True
+                        break
             point = tdef.next_point(point)
 
         # Once more (for the rh-limited task: don't rh release it!)
-        if point is not None and tdef.is_parentless(point):
+        if point is not None and tdef.is_parentless(point) and not is_clock:
             ntask = self._get_spawned_or_merged_task(
                 point, tdef.name, flow_nums
             )
@@ -1753,6 +1765,18 @@ def force_trigger_tasks(
 
         return len(unmatched)
 
+    def spawn_parentless_clock_triggered(self):
+        """Spawn successor(s) of parentless wall clock satisfied tasks."""
+        while self.xtrigger_mgr.wall_clock_spawns:
+            itask = self.xtrigger_mgr.wall_clock_spawns.pop()
+            # Will spawn out to RH limit or next parentless clock trigger
+            # or non-parentless.
+            self.spawn_to_rh_limit(
+                itask.tdef,
+                itask.tdef.next_point(itask.point),
+                itask.flow_nums
+            )
+
     def sim_time_check(self, message_queue: 'Queue[TaskMsg]') -> bool:
         """Simulation mode: simulate task run times and set states."""
         if not self.config.run_mode('simulation'):
diff --git a/cylc/flow/xtrigger_mgr.py b/cylc/flow/xtrigger_mgr.py
index 4512d97b7c8..1d1482d7317 100644
--- a/cylc/flow/xtrigger_mgr.py
+++ b/cylc/flow/xtrigger_mgr.py
@@ -211,6 +211,11 @@ def __init__(
         # Signatures of active functions (waiting on callback).
         self.active: list = []
 
+        # Gather named wall-clocks closet to current time.
+        # (no need to check or spawn future clocks of tasks respectively).
+        self.wall_clock_labels: set = set()
+        self.wall_clock_spawns: list = []
+
         self.workflow_run_dir = workflow_run_dir
 
         # For function arg templating.
@@ -316,6 +321,8 @@ def add_trig(self, label: str, fctx: SubFuncContext, fdir: str) -> None:
         """
         self.validate_xtrigger(label, fctx, fdir)
         self.functx_map[label] = fctx
+        if fctx.func_name == "wall_clock":
+            self.wall_clock_labels.add(label)
 
     def mutate_trig(self, label, kwargs):
         self.functx_map[label].func_kwargs.update(kwargs)
@@ -380,7 +387,7 @@ def get_xtrig_ctx(self, itask: TaskProxy, label: str) -> SubFuncContext:
 
         args = []
         kwargs = {}
-        if ctx.func_name == "wall_clock":
+        if label in self.wall_clock_labels:
             if "trigger_time" in ctx.func_kwargs:
                 # Internal (retry timer): trigger_time already set.
                 kwargs["trigger_time"] = ctx.func_kwargs["trigger_time"]
@@ -420,10 +427,12 @@ def call_xtriggers_async(self, itask: TaskProxy):
             itask: task proxy to check.
         """
         for label, sig, ctx, _ in self._get_xtrigs(itask, unsat_only=True):
-            if sig.startswith("wall_clock"):
+            if label in self.wall_clock_labels:
                 # Special case: quick synchronous clock check.
                 if wall_clock(*ctx.func_args, **ctx.func_kwargs):
                     itask.state.xtriggers[label] = True
+                    if itask.tdef.is_parentless(itask.point):
+                        self.wall_clock_spawns.append(itask)
                     self.sat_xtrig[sig] = {}
                     self.data_store_mgr.delta_task_xtrigger(sig, True)
                     LOG.info('xtrigger satisfied: %s = %s', label, sig)