diff --git a/src/polyfills/custom-elements.js b/src/polyfills/custom-elements.js
index 4278de9d1f06..80c5e44ef0d9 100644
--- a/src/polyfills/custom-elements.js
+++ b/src/polyfills/custom-elements.js
@@ -227,11 +227,6 @@ class Registry {
      */
     this.win_ = win;
 
-    /**
-     * @private @const
-     */
-    this.doc_ = win.document;
-
     /**
      * @type {!Object<string, !CustomElementDef>}
      * @private
@@ -260,12 +255,11 @@ class Registry {
     this.mutationObserver_ = null;
 
     /**
-     * All the observed DOM trees, including shadow trees. This is cleared out
-     * when the mutation observer is created.
+     * All the observed DOM trees, including shadow trees.
      *
      * @private @const {!Array<!Node>}
      */
-    this.observed_ = [win.document];
+    this.roots_ = [win.document];
   }
 
   /**
@@ -346,7 +340,9 @@ class Registry {
     };
 
     this.observe_(name);
-    this.upgrade(this.doc_, name);
+    this.roots_.forEach(tree => {
+      this.upgrade(tree, name);
+    });
   }
 
   /**
@@ -510,10 +506,12 @@ class Registry {
     });
     this.mutationObserver_ = mo;
 
-    this.observed_.forEach(tree => {
+    // I would love to not have to hold onto all of the roots, since it's a
+    // memory leak. Unfortunately, there's no way to iterate a list and hold
+    // onto its contents weakly.
+    this.roots_.forEach(tree => {
       mo.observe(tree, TRACK_SUBTREE);
     });
-    this.observed_.length = 0;
 
     installPatches(this.win_, this);
   }
@@ -524,10 +522,9 @@ class Registry {
    * @param {!Node} tree
    */
   observe(tree) {
+    this.roots_.push(tree);
     if (this.mutationObserver_) {
       this.mutationObserver_.observe(tree, TRACK_SUBTREE);
-    } else {
-      this.observed_.push(tree);
     }
   }