diff --git a/core/modules/hal/tests/src/Functional/EntityResource/Term/TermHalJsonAnonTest.php b/core/modules/hal/tests/src/Functional/EntityResource/Term/TermHalJsonAnonTest.php
index 95514b7fb1..cb88279514 100644
--- a/core/modules/hal/tests/src/Functional/EntityResource/Term/TermHalJsonAnonTest.php
+++ b/core/modules/hal/tests/src/Functional/EntityResource/Term/TermHalJsonAnonTest.php
@@ -52,15 +52,8 @@ protected function getExpectedNormalizedEntity() {
 
     $expected_parent_normalization_links = FALSE;
     $expected_parent_normalization_embedded = FALSE;
+
     switch($parent_term_ids) {
-      case [TermInterface::ROOT_ID]:
-        $expected_parent_normalization_links = [
-          NULL,
-        ];
-        $expected_parent_normalization_embedded = [
-          NULL,
-        ];
-        break;
       case [2]:
         $expected_parent_normalization_links = [
           [
@@ -83,30 +76,6 @@ protected function getExpectedNormalizedEntity() {
           ],
         ];
         break;
-      case [TermInterface::ROOT_ID, 2]:
-        $expected_parent_normalization_links = [
-          NULL,
-          [
-            'href' => $this->baseUrl . '/taxonomy/term/2?_format=hal_json',
-          ],
-        ];
-        $expected_parent_normalization_embedded = [
-          NULL,
-          [
-            '_links' => [
-              'self' => [
-                'href' => $this->baseUrl . '/taxonomy/term/2?_format=hal_json',
-              ],
-              'type' => [
-                'href' => $this->baseUrl . '/rest/type/taxonomy_term/camelids',
-              ],
-            ],
-            'uuid' => [
-              ['value' => Term::load(2)->uuid()],
-            ],
-          ],
-        ];
-        break;
       case [3, 2]:
         $expected_parent_normalization_links = [
           [
diff --git a/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
index d31155bfa7..9eb65303f4 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/Term/TermResourceTestBase.php
@@ -105,13 +105,6 @@ protected function getExpectedNormalizedEntity() {
 
     $expected_parent_normalization = FALSE;
     switch($parent_term_ids) {
-      case [TermInterface::ROOT_ID]:
-        $expected_parent_normalization = [
-          [
-            'target_id' => TermInterface::ROOT_ID,
-          ],
-        ];
-        break;
       case [2]:
         $expected_parent_normalization = [
           [
@@ -122,19 +115,6 @@ protected function getExpectedNormalizedEntity() {
           ],
         ];
         break;
-      case [TermInterface::ROOT_ID, 2]:
-        $expected_parent_normalization = [
-          [
-            'target_id' => TermInterface::ROOT_ID,
-          ],
-          [
-            'target_id' => 2,
-            'target_type' => 'taxonomy_term',
-            'target_uuid' => Term::load(2)->uuid(),
-            'url' => base_path() . 'taxonomy/term/2',
-          ],
-        ];
-        break;
       case [3, 2]:
         $expected_parent_normalization = [
           [
@@ -334,15 +314,9 @@ public function testGetTermWithParent(array $parent_term_ids) {
 
   public function providerTestGetTermWithParent() {
     return [
-      'root parent: [0] (= no parent)' => [
-        [TermInterface::ROOT_ID]
-      ],
-      'non-root parent: [2]' => [
+      'one parent: [2]' => [
         [2]
       ],
-      'multiple parents: [0,2] (root + non-root parent)' => [
-        [TermInterface::ROOT_ID, 2]
-      ],
       'multiple parents: [3,2] (both non-root parents)' => [
         [3, 2]
       ],
diff --git a/core/modules/taxonomy/src/Entity/Term.php b/core/modules/taxonomy/src/Entity/Term.php
index 26fb973482..439fe400d8 100644
--- a/core/modules/taxonomy/src/Entity/Term.php
+++ b/core/modules/taxonomy/src/Entity/Term.php
@@ -91,17 +91,6 @@ public static function postDelete(EntityStorageInterface $storage, array $entiti
     }
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function preSave(EntityStorageInterface $storage) {
-    parent::preSave($storage);
-    // Terms with no parents are mandatory children of <root>.
-    if (!$this->get('parent')->count()) {
-      $this->parent->target_id = static::ROOT_ID;
-    }
-  }
-
   /**
    * {@inheritdoc}
    */
diff --git a/core/modules/taxonomy/src/TermInterface.php b/core/modules/taxonomy/src/TermInterface.php
index cfba19d5a5..3087a56ecb 100644
--- a/core/modules/taxonomy/src/TermInterface.php
+++ b/core/modules/taxonomy/src/TermInterface.php
@@ -10,11 +10,6 @@
  */
 interface TermInterface extends ContentEntityInterface, EntityChangedInterface {
 
-  /**
-   * Root ID for terms with no parent.
-   */
-  const ROOT_ID = 0;
-
   /**
    * Gets the term's description.
    *
diff --git a/core/modules/taxonomy/src/TermStorage.php b/core/modules/taxonomy/src/TermStorage.php
index 2db0392aea..f13ceb931c 100644
--- a/core/modules/taxonomy/src/TermStorage.php
+++ b/core/modules/taxonomy/src/TermStorage.php
@@ -107,39 +107,13 @@ public function loadParents($tid) {
    * Returns a list of parents of this term.
    *
    * @return \Drupal\taxonomy\TermInterface[]
-   *   The parent taxonomy term entities keyed by term ID. If this term has a
-   *   <root> parent, that item is keyed with ROOT_ID and will have NULL as
-   *   value.
+   *   The parent taxonomy term entities keyed by term ID.
    *
    * @internal
    * @todo Refactor away when TreeInterface is introduced.
    */
   protected function getParents(TermInterface $term) {
-    $parents = $ids = [];
-    // Cannot use $this->get('parent')->referencedEntities() here because that
-    // strips out the '0' reference.
-    foreach ($term->get('parent') as $item) {
-      if ($item->target_id == $term::ROOT_ID) {
-        // The <root> parent.
-        $parents[0] = NULL;
-        continue;
-      }
-      $ids[] = $item->target_id;
-    }
-
-    // @todo Better way to do this? AND handle the NULL/0 parent?
-    // Querying the terms again so that the same access checks are run when
-    // getParents() is called as in Drupal version prior to 8.3.
-    $loaded_parents = [];
-
-    if ($ids) {
-      $query = \Drupal::entityQuery('taxonomy_term')
-        ->condition('tid', $ids, 'IN');
-
-      $loaded_parents = static::loadMultiple($query->execute());
-    }
-
-    return $parents + $loaded_parents;
+    return $term->get('parent')->referencedEntities();
   }
 
   /**
diff --git a/core/modules/taxonomy/src/VocabularyStorage.php b/core/modules/taxonomy/src/VocabularyStorage.php
index 0538e71b49..f06a15759e 100644
--- a/core/modules/taxonomy/src/VocabularyStorage.php
+++ b/core/modules/taxonomy/src/VocabularyStorage.php
@@ -24,7 +24,7 @@ public function resetCache(array $ids = NULL) {
   public function getToplevelTids($vids) {
     $tids = \Drupal::entityQuery('taxonomy_term')
       ->condition('vid', $vids, 'IN')
-      ->condition('parent.target_id', TermInterface::ROOT_ID)
+      ->notExists('parent.target_id')
       ->execute();
 
     return array_values($tids);
diff --git a/core/modules/taxonomy/taxonomy.install b/core/modules/taxonomy/taxonomy.install
index 702ca7ce83..b53767367c 100644
--- a/core/modules/taxonomy/taxonomy.install
+++ b/core/modules/taxonomy/taxonomy.install
@@ -27,19 +27,22 @@ function taxonomy_update_8502(&$sandbox) {
   if (!isset($sandbox['current'])) {
     // Set batch ops sandbox.
     $sandbox['current'] = 0;
-    $sandbox['max'] = $database->select('taxonomy_term_hierarchy')
-      ->countQuery()
-      ->execute()
-      ->fetchField();
+
+    $query = $database->select('taxonomy_term_hierarchy', 'h');
+    $query->condition('h.parent', 0, '>');
+    $query->countQuery();
+
+    $sandbox['max'] = $query->execute()->fetchField();
   }
 
   // Save the hierarchy.
   $select = $database->select('taxonomy_term_hierarchy', 'h');
   $select->join('taxonomy_term_data', 'd', 'h.tid = d.tid');
-  $hierarchy = $select
-    ->fields('h', ['tid', 'parent'])
+  $select->fields('h', ['tid', 'parent'])
     ->fields('d', ['vid', 'langcode'])
-    ->range($sandbox['current'], $sandbox['current'] + 100)
+    ->condition('h.parent', 0, '>');
+
+  $hierarchy = $select->range($sandbox['current'], $sandbox['current'] + 100)
     ->execute()
     ->fetchAll();
 
@@ -74,9 +77,8 @@ function taxonomy_update_8502(&$sandbox) {
   if ($sandbox['#finished'] >= 1) {
     // Drop the legacy table.
     $database->schema()->dropTable('taxonomy_term_hierarchy');
-    return t('Taxonomy term legacy tables dropped.');
 
-    return t('Taxonomy term hierarchy converted to default entity reference storage.');
+    return t('Taxonomy term hierarchy converted to default entity reference storage and legacy term hierarchy table (taxonomy_term_hierarchy) dropped.');
   }
 }
 
diff --git a/core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateTaxonomyTermTest.php b/core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateTaxonomyTermTest.php
index b6cf73f65c..fcf25fac9f 100644
--- a/core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateTaxonomyTermTest.php
+++ b/core/modules/taxonomy/tests/src/Kernel/Migrate/d6/MigrateTaxonomyTermTest.php
@@ -93,7 +93,7 @@ public function testTaxonomyTerms() {
       $this->assertSame($values['vid'], $term->vid->target_id);
       $this->assertSame((string) $values['weight'], $term->weight->value);
       if ($values['parent'] === [0]) {
-        $this->assertSame((int) $term->parent->target_id, TermInterface::ROOT_ID);
+        $this->assertNull($term->parent->target_id);
       }
       else {
         $parents = [];