diff --git a/CollectionTreePlugin.php b/CollectionTreePlugin.php
index 38e8b96..f6c3b8d 100644
--- a/CollectionTreePlugin.php
+++ b/CollectionTreePlugin.php
@@ -13,6 +13,9 @@
  */
 class CollectionTreePlugin extends Omeka_Plugin_AbstractPlugin
 {
+    /**
+     * @var array Hooks for the plugin.
+     */
     protected $_hooks = array(
         'install',
         'uninstall',
@@ -28,10 +31,14 @@ class CollectionTreePlugin extends Omeka_Plugin_AbstractPlugin
         'public_collections_show',
     );
 
+    /**
+     * @var array Filters for the plugin.
+     */
     protected $_filters = array(
         'admin_navigation_main',
         'public_navigation_main',
         'admin_collections_form_tabs',
+        'collections_select_options',
     );
     
     /**
@@ -285,4 +292,17 @@ public function filterAdminCollectionsFormTabs($tabs, $args)
         );
         return $tabs;
     }
+
+    /**
+     * Manage search options for collections.
+     *
+     * @param array Search options for collections.
+     * @return array Filtered search options for collections.
+     */
+    public function filterCollectionsSelectOptions($options)
+    {
+        $treeOptions = $this->_db->getTable('CollectionTree')->findPairsForSelectForm();
+        // Keep only chosen collections, in case another filter removed some.
+        return array_intersect_key($treeOptions, $options);
+    }
 }