diff --git a/simgear/scene/tgdb/VPBTechnique.cxx b/simgear/scene/tgdb/VPBTechnique.cxx
index 4938e60f..b5d4ccfa 100644
--- a/simgear/scene/tgdb/VPBTechnique.cxx
+++ b/simgear/scene/tgdb/VPBTechnique.cxx
@@ -61,6 +61,7 @@ VPBTechnique::VPBTechnique()
     setFilterBias(0);
     setFilterWidth(0.1);
     setFilterMatrixAs(GAUSSIAN);
+    _vegetationConstraintGroup = new osg::Group();
 }
 
 VPBTechnique::VPBTechnique(const SGReaderWriterOptions* options)
@@ -69,6 +70,7 @@ VPBTechnique::VPBTechnique(const SGReaderWriterOptions* options)
     setFilterWidth(0.1);
     setFilterMatrixAs(GAUSSIAN);
     setOptions(options);
+    _vegetationConstraintGroup = new osg::Group();
 }
 
 VPBTechnique::VPBTechnique(const VPBTechnique& gt,const osg::CopyOp& copyop):
@@ -78,6 +80,7 @@ VPBTechnique::VPBTechnique(const VPBTechnique& gt,const osg::CopyOp& copyop):
     setFilterWidth(gt._filterWidth);
     setFilterMatrix(gt._filterMatrix);
     setOptions(gt._options);
+    _vegetationConstraintGroup = new osg::Group();
 }
 
 VPBTechnique::~VPBTechnique()
@@ -2283,21 +2286,24 @@ osg::Vec3d VPBTechnique::checkAgainstElevationConstraints(osg::Vec3d origin, osg
 // intersection with the constraint model.
 void VPBTechnique::addVegetationConstraint(osg::ref_ptr<osg::Node> constraint)
 { 
-    const std::lock_guard<std::mutex> lock(VPBTechnique::_vegetationConstraintMutex); // Lock the _vegetationConstraintGroup for this scope
     _vegetationConstraintGroup->addChild(constraint.get()); 
 }
 
 // Remove a previously added constraint.  E.g on model unload.
 void VPBTechnique::removeVegetationConstraint(osg::ref_ptr<osg::Node> constraint)
 { 
-    const std::lock_guard<std::mutex> lock(VPBTechnique::_vegetationConstraintMutex); // Lock the _vegetationConstraintGroup for this scope
     _vegetationConstraintGroup->removeChild(constraint.get()); 
 }
 
+// Remove all the constraints, which will still be referenced by the terrain tile itself.
+void VPBTechnique::clearVegetationConstraints()
+{ 
+    _vegetationConstraintGroup->removeChildren(0, _vegetationConstraintGroup->getNumChildren());
+}
+
 // Check a given vertex against any vegetation constraints  E.g. to ensure we don't get trees sprouting from roads or runways.  
 bool VPBTechnique::checkAgainstVegetationConstraints(osg::Vec3d origin, osg::Vec3d vertex)
 {
-    const std::lock_guard<std::mutex> lock(VPBTechnique::_vegetationConstraintMutex); // Lock the _vegetationConstraintGroup for this scope
     osg::ref_ptr<osgUtil::LineSegmentIntersector> intersector;
     intersector = new osgUtil::LineSegmentIntersector(origin, vertex);
     osgUtil::IntersectionVisitor visitor(intersector.get());
@@ -2308,9 +2314,7 @@ bool VPBTechnique::checkAgainstVegetationConstraints(osg::Vec3d origin, osg::Vec
 void VPBTechnique::clearConstraints()
 {
     const std::lock_guard<std::mutex> elock(VPBTechnique::_elevationConstraintMutex); // Lock the _elevationConstraintGroup for this scope
-    const std::lock_guard<std::mutex> vlock(VPBTechnique::_vegetationConstraintMutex); // Lock the _vegetationConstraintGroup for this scope
     _elevationConstraintGroup = new osg::Group();
-    _vegetationConstraintGroup = new osg::Group();
 }
 
 void VPBTechnique::addLineFeatureList(SGBucket bucket, LineFeatureBinList roadList)
diff --git a/simgear/scene/tgdb/VPBTechnique.hxx b/simgear/scene/tgdb/VPBTechnique.hxx
index 9e9a2e25..6fa5224b 100644
--- a/simgear/scene/tgdb/VPBTechnique.hxx
+++ b/simgear/scene/tgdb/VPBTechnique.hxx
@@ -91,16 +91,12 @@ class VPBTechnique : public TerrainTechnique
         * for all graphics contexts. */
         virtual void releaseGLObjects(osg::State* = 0) const;
 
-        // Elevation constraints ensure that the terrain mesh is placed underneath objects such as airports
+        // Elevation constraints ensure that the terrain mesh is placed underneath objects such as airports.
+        // As airports are generated in a separate loading thread, these are static.
         static void addElevationConstraint(osg::ref_ptr<osg::Node> constraint);
         static void removeElevationConstraint(osg::ref_ptr<osg::Node> constraint);
         static osg::Vec3d checkAgainstElevationConstraints(osg::Vec3d origin, osg::Vec3d vertex, float vertex_gap);
 
-        // Vegetation constraints ensure that trees don't grow in roads
-        static void addVegetationConstraint(osg::ref_ptr<osg::Node> constraint);
-        static void removeVegetationConstraint(osg::ref_ptr<osg::Node> constraint);
-        static bool checkAgainstVegetationConstraints(osg::Vec3d origin, osg::Vec3d vertex);
-
         static void clearConstraints();
 
         // LineFeatures and AreaFeatures are draped over the underlying mesh.
@@ -178,6 +174,13 @@ class VPBTechnique : public TerrainTechnique
 
         virtual osg::Vec3d getMeshIntersection(BufferData& buffer, Locator* masterLocator, osg::Vec3d pt, osg::Vec3d up);
 
+        // Vegetation constraints ensure that trees don't grow in roads.  Unlike the elevation constraints, 
+        // we only use these internally and during generation of this particular tile.
+        virtual void addVegetationConstraint(osg::ref_ptr<osg::Node> constraint);
+        virtual void removeVegetationConstraint(osg::ref_ptr<osg::Node> constraint);
+        virtual bool checkAgainstVegetationConstraints(osg::Vec3d origin, osg::Vec3d vertex);
+        virtual void clearVegetationConstraints();
+
         OpenThreads::Mutex                  _writeBufferMutex;
         osg::ref_ptr<BufferData>            _currentBufferData;
         osg::ref_ptr<BufferData>            _newBufferData;
@@ -189,12 +192,11 @@ class VPBTechnique : public TerrainTechnique
         osg::Matrix3                        _filterMatrix;
         osg::ref_ptr<osg::Uniform>          _filterMatrixUniform;
         osg::ref_ptr<SGReaderWriterOptions> _options;
+        osg::ref_ptr<osg::Group>            _vegetationConstraintGroup;
 
         inline static osg::ref_ptr<osg::Group>  _elevationConstraintGroup = new osg::Group();
         inline static std::mutex _elevationConstraintMutex;  // protects the _elevationConstraintGroup;
 
-        inline static osg::ref_ptr<osg::Group>  _vegetationConstraintGroup = new osg::Group();
-        inline static std::mutex _vegetationConstraintMutex;  // protects the _vegetationConstraintGroup;
 
         typedef std::pair<SGBucket, LineFeatureBinList> BucketLineFeatureBinList;
         typedef std::pair<SGBucket, AreaFeatureBinList> BucketAreaFeatureBinList;