diff --git a/backend/app/core/tree_validation.py b/backend/app/core/tree_validation.py
index 38c729fd..de562889 100644
--- a/backend/app/core/tree_validation.py
+++ b/backend/app/core/tree_validation.py
@@ -1,6 +1,8 @@
 """Tree validation helper module for draft/published workflow."""
 from typing import Any
 
+PROCEDURAL_TREE_TYPES = {"procedural", "maintenance"}
+
 
 class TreeValidationError(Exception):
     """Custom exception for tree validation errors."""
@@ -224,14 +226,14 @@ def can_publish_tree(
         errors.append({"field": "name", "message": "Tree must have a name to be published"})
 
     # Validate structure based on tree type
-    if tree_type in ("procedural", "maintenance"):
+    if tree_type in PROCEDURAL_TREE_TYPES:
         structure_valid, structure_errors = validate_procedural_structure(tree_structure)
     else:
         structure_valid, structure_errors = validate_tree_structure(tree_structure)
     errors.extend(structure_errors)
 
     # Validate intake form if present (procedural only)
-    if intake_form and tree_type in ("procedural", "maintenance"):
+    if intake_form and tree_type in PROCEDURAL_TREE_TYPES:
         form_valid, form_errors = _validate_intake_form(intake_form)
         errors.extend(form_errors)
 
diff --git a/backend/tests/test_maintenance_tree_type.py b/backend/tests/test_maintenance_tree_type.py
index 1ba33bd2..0ba3866b 100644
--- a/backend/tests/test_maintenance_tree_type.py
+++ b/backend/tests/test_maintenance_tree_type.py
@@ -50,3 +50,24 @@ async def test_list_maintenance_trees_filter(client: AsyncClient, auth_headers:
     trees = resp.json()
     assert all(t["tree_type"] == "maintenance" for t in trees)
     assert len(trees) >= 1
+
+
+@pytest.mark.asyncio
+async def test_invalid_tree_type_rejected(client: AsyncClient, auth_headers: dict):
+    """An unrecognized tree_type value is rejected with 422."""
+    resp = await client.post(
+        "/api/v1/trees",
+        json={
+            "name": "Bad Type",
+            "tree_type": "garbage",
+            "tree_structure": {
+                "steps": [
+                    {"id": "s1", "type": "procedure_step", "title": "Step",
+                     "description": "Do it", "content_type": "action"},
+                    {"id": "end", "type": "procedure_end", "title": "Done"},
+                ]
+            },
+        },
+        headers=auth_headers,
+    )
+    assert resp.status_code == 422