diff --git a/devstack-vm-gate-wrap.sh b/devstack-vm-gate-wrap.sh
index f3fe703e..922d502b 100755
--- a/devstack-vm-gate-wrap.sh
+++ b/devstack-vm-gate-wrap.sh
@@ -204,9 +204,12 @@ export DEVSTACK_GATE_GRENADE_FORWARD=${DEVSTACK_GATE_GRENADE_FORWARD:-0}
 # DGGPN=1 means upgrade everything but n-cpu.
 export DEVSTACK_GATE_GRENADE_PARTIAL_NCPU=${DEVSTACK_GATE_GRENADE_PARTIAL_NCPU:-0}
 
+# the branch name for selecting grenade branches
+GRENADE_BASE_BRANCH=${OVERRIDE_ZUUL_BRANCH:-${ZUUL_BRANCH}}
+
 if [ "$DEVSTACK_GATE_GRENADE" -eq "1" ]; then
     export DEVSTACK_GATE_TEMPEST=1
-    if [ "$ZUUL_BRANCH" == "stable/icehouse" ]; then
+    if [ "$GRENADE_BASE_BRANCH" == "stable/icehouse" ]; then
         export GRENADE_OLD_BRANCH="stable/havana"
         export GRENADE_NEW_BRANCH="stable/icehouse"
     else # master
@@ -221,10 +224,10 @@ if [ "$DEVSTACK_GATE_GRENADE" -eq "1" ]; then
     # the roll forward case
 elif [ "$DEVSTACK_GATE_GRENADE_FORWARD" -eq "1" ]; then
     export DEVSTACK_GATE_TEMPEST=1
-    if [ "$ZUUL_BRANCH" == "stable/havana" ]; then
+    if [ "$GRENADE_BASE_BRANCH" == "stable/havana" ]; then
         export GRENADE_OLD_BRANCH="stable/havana"
         export GRENADE_NEW_BRANCH="stable/icehouse"
-    elif [ "$ZUUL_BRANCH" == "stable/icehouse" ]; then
+    elif [ "$GRENADE_BASE_BRANCH" == "stable/icehouse" ]; then
         export GRENADE_OLD_BRANCH="stable/icehouse"
         export GRENADE_NEW_BRANCH="master"
     fi