diff --git a/docs/ENV.md b/docs/ENV.md
index e11fe7d..8639627 100644
--- a/docs/ENV.md
+++ b/docs/ENV.md
@@ -12,7 +12,7 @@ You can configure deployment settings by placing special variables in an `ENV` f
 
 ### Node
 
-* `NODE_VERSION`: installs a particular version of node for your app if `nodeenv` is found on the path.
+* `NODE_VERSION`: installs a particular version of node for your app if `nodeenv` is found on the path. Optional; if not specified, the system-wide node  package is used.
 
 > **NOTE**: you will need to stop and re-deploy the app to change the `node` version in a running app.