diff --git a/environments/undercloud.yaml b/environments/undercloud.yaml
index 58506c8217..d460c96efb 100644
--- a/environments/undercloud.yaml
+++ b/environments/undercloud.yaml
@@ -119,8 +119,8 @@ parameter_defaults:
   IronicForcePowerStateDuringSync: false
   IronicInspectorCollectors: default,extra-hardware,numa-topology,logs
   IronicInspectorInterface: br-ctlplane
-  IronicInspectorSubnets:
-    - ip_range: '192.168.24.100,192.168.24.200'
+  # IronicInspectorSubnets:
+  #   - ip_range: '192.168.24.100,192.168.24.200'
   IronicProvisioningNetwork: 'ctlplane'
   IronicRescuingNetwork: 'ctlplane'
   ZaqarMessageStore: 'swift'
@@ -155,12 +155,20 @@ parameter_defaults:
   #     - ip_netmask: 192.168.26.0/24
   #       next_hop: 192.168.24.1
   ControlPlaneStaticRoutes: []
-  UndercloudCtlplaneSubnets:
-    ctlplane-subnet:
-      NetworkCidr: '192.168.24.0/24'
-      NetworkGateway: '192.168.24.1'
-      DhcpRangeStart: '192.168.24.5'
-      DhcpRangeEnd: '192.168.24.24'
+  # A dictionary of Undercloud ctlplane subnets.
+  # NOTE(hjensas): This should be {} in this environment file, otherwise it may
+  # results in values set here being merged with the values set in
+  # undercloud.conf. See Bug: https://bugs.launchpad.net/tripleo/+bug/1820330
+  # Example:
+  #   UndercloudCtlplaneSubnets:
+  #     ctlplane-subnet:
+  #       NetworkCidr: '192.168.24.0/24'
+  #       NetworkGateway: '192.168.24.1'
+  #       DhcpRangeStart: '192.168.24.5'
+  #       DhcpRangeEnd: '192.168.24.24'
+  #       HostRoutes:
+  #         - {'destination': '10.10.10.0/24', 'nexthop': '192.168.24.254'}
+  UndercloudCtlplaneSubnets: {}
   UndercloudCtlplaneLocalSubnet: 'ctlplane-subnet'
   MistralDockerGroup: true
   PasswordAuthentication: 'yes'
diff --git a/releasenotes/notes/fix-uc-ctlplane-subnet-defaults-merged-5a6061b29618f5cf.yaml b/releasenotes/notes/fix-uc-ctlplane-subnet-defaults-merged-5a6061b29618f5cf.yaml
new file mode 100644
index 0000000000..d55d2db93d
--- /dev/null
+++ b/releasenotes/notes/fix-uc-ctlplane-subnet-defaults-merged-5a6061b29618f5cf.yaml
@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixes an issue that caused a subnet to be wrongly created on the Undercloud
+    provisioning network based on environment default values. If the default
+    ``ctlplane-subnet`` was renamed in ``undercloud.conf``, the defaults for
+    ``ctlplane-subnet`` in ``environments/undercloud.yaml`` was merged with the
+    subnets defined in ``undercloud.conf``. See `bug 1820330
+    <https://bugs.launchpad.net/tripleo/+bug/1820330>`_.