--- a/taskcluster/mach_commands.py
+++ b/taskcluster/mach_commands.py
@@ -139,16 +139,22 @@ class MachCommands(MachCommandBase):
                      type=int,
                      default=0)
     @CommandArgument('--owner',
                      required=True,
                      help='email address of who owns this graph')
     @CommandArgument('--level',
                      required=True,
                      help='SCM level of this repository')
+    @CommandArgument('--triggered-by',
+                     choices=['nightly', 'push'],
+                     default='push',
+                     help='Source of execution of the decision graph')
+    @CommandArgument('--target-tasks-method',
+                     help='method for selecting the target tasks to generate')
     def taskgraph_decision(self, **options):
         """Run the decision task: generate a task graph and submit to
         TaskCluster.  This is only meant to be called within decision tasks,
         and requires a great many arguments.  Commands like `mach taskgraph
         optimized` are better suited to use on the command line, and can take
         the parameters file generated by a decision task.  """
 
         import taskgraph.decision

--- a/taskcluster/taskgraph/decision.py
+++ b/taskcluster/taskgraph/decision.py
@@ -105,28 +105,33 @@ def get_decision_parameters(options):
         'head_rev',
         'head_ref',
         'message',
         'project',
         'pushlog_id',
         'pushdate',
         'owner',
         'level',
+        'triggered_by',
         'target_tasks_method',
     ] if n in options}
 
     project = parameters['project']
     try:
         parameters.update(PER_PROJECT_PARAMETERS[project])
     except KeyError:
         logger.warning("using default project parameters; add {} to "
                        "PER_PROJECT_PARAMETERS in {} to customize behavior "
                        "for this project".format(project, __file__))
         parameters.update(PER_PROJECT_PARAMETERS['default'])
 
+    # `target_tasks_method` has higher precedence than `project` parameters
+    if 'target_tasks_method' in options:
+        parameters['target_tasks_method'] = options['target_tasks_method']
+
     return Parameters(parameters)
 
 
 def write_artifact(filename, data):
     logger.info('writing artifact file `{}`'.format(filename))
     if not os.path.isdir(ARTIFACTS_DIR):
         os.mkdir(ARTIFACTS_DIR)
     path = os.path.join(ARTIFACTS_DIR, filename)