diff --git a/README.md b/README.md
index 653e0f0..c655406 100644
--- a/README.md
+++ b/README.md
@@ -123,10 +123,10 @@ You can populate below template according to your requirements and use it as you
 # BACKUP_FILENAME="backup-%Y-%m-%dT%H-%M-%S.tar.gz"
 
 # Setting BACKUP_FILENAME_EXPAND to true allows for environment variable
-# placeholders in BACKUP_FILENAME and in BACKUP_LATEST_SYMLINK that will get
-# expanded at runtime, e.g. `backup-$HOSTNAME-%Y-%m-%dT%H-%M-%S.tar.gz`.
-# Expansion happens before interpolating strftime tokens.
-# It is disabled by default.
+# placeholders in BACKUP_FILENAME, BACKUP_LATEST_SYMLINK and in
+# BACKUP_PRUNING_PREFIX that will get expanded at runtime,
+# e.g. `backup-$HOSTNAME-%Y-%m-%dT%H-%M-%S.tar.gz`. Expansion happens before
+# interpolating strftime tokens. It is disabled by default.
 # Please note that you will need to escape the `$` when providing the value
 # in a docker-compose.yml file, i.e. using $$VAR instead of $VAR.
 
diff --git a/cmd/backup/main.go b/cmd/backup/main.go
index 38651be..300c213 100644
--- a/cmd/backup/main.go
+++ b/cmd/backup/main.go
@@ -157,6 +157,7 @@ func newScript() (*script, error) {
 	if s.c.BackupFilenameExpand {
 		s.file = os.ExpandEnv(s.file)
 		s.c.BackupLatestSymlink = os.ExpandEnv(s.c.BackupLatestSymlink)
+		s.c.BackupPruningPrefix = os.ExpandEnv(s.c.BackupPruningPrefix)
 	}
 	s.file = timeutil.Strftime(&s.start, s.file)