diff --git a/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDesc.scala b/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDesc.scala
index b903c477b13..f60eab6b3e5 100644
--- a/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDesc.scala
+++ b/common/workflow-operator/src/main/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDesc.scala
@@ -20,10 +20,11 @@
 package org.apache.texera.amber.operator.visualization.dendrogram
 
 import com.fasterxml.jackson.annotation.{JsonProperty, JsonPropertyDescription}
+import com.fasterxml.jackson.databind.annotation.JsonDeserialize
 import com.kjetland.jackson.jsonSchema.annotations.JsonSchemaTitle
 import org.apache.texera.amber.core.tuple.{AttributeType, Schema}
 import org.apache.texera.amber.pybuilder.PythonTemplateBuilder.PythonTemplateBuilderStringContext
-import org.apache.texera.amber.pybuilder.PyStringTypes.EncodableString
+import org.apache.texera.amber.pybuilder.PyStringTypes.{EncodableString, PythonLiteral}
 import org.apache.texera.amber.core.workflow.PortIdentity
 import org.apache.texera.amber.operator.PythonOperatorDescriptor
 import org.apache.texera.amber.operator.metadata.annotations.AutofillAttributeName
@@ -54,10 +55,13 @@ class DendrogramOpDesc extends PythonOperatorDescriptor {
   @NotNull(message = "Labels cannot be empty")
   var labels: EncodableString = ""
 
+  // Numeric: scipy compares it against the linkage distances. contentAs names the
+  // boxed class — Option erases its element type, and a blank must not read as 0.
   @JsonProperty(defaultValue = "", required = false)
   @JsonSchemaTitle("Color Threshold")
   @JsonPropertyDescription("Value at which separation of clusters will be made")
-  var threshold: EncodableString = ""
+  @JsonDeserialize(contentAs = classOf[java.lang.Double])
+  var threshold: Option[Double] = None
 
   override def getOutputSchemas(
       inputSchemas: Map[PortIdentity, Schema]
@@ -78,17 +82,15 @@ class DendrogramOpDesc extends PythonOperatorDescriptor {
     assert(xVal.nonEmpty, "Value X Column cannot be empty")
     assert(yVal.nonEmpty, "Value Y Column cannot be empty")
     assert(labels.nonEmpty, "Labels cannot be empty")
-    val strippedThreshold: EncodableString = threshold.trim
-    val isThreshold =
-      if (strippedThreshold.nonEmpty) pyb"color_threshold=$strippedThreshold"
-      else "color_threshold=None"
+    // Unset means None, which is scipy's own 0.7 * max distance.
+    val thresholdExpr: PythonLiteral = threshold.map(_.toString).getOrElse("None")
     pyb"""
        |        x = np.array(table[$xVal])
        |        y = np.array(table[$yVal])
        |        data = np.column_stack((x, y))
        |        labels = table[$labels].tolist()
        |
-       |        fig = ff.create_dendrogram(data, labels=labels, $isThreshold)
+       |        fig = ff.create_dendrogram(data, labels=labels, color_threshold=$thresholdExpr)
        |        fig.update_layout(yaxis_title="Linkage Distance", margin=dict(l=0, r=0, b=0, t=0))
        |"""
   }
diff --git a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDescSpec.scala b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDescSpec.scala
index 448693cab7b..b56ffae273a 100644
--- a/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDescSpec.scala
+++ b/common/workflow-operator/src/test/scala/org/apache/texera/amber/operator/visualization/dendrogram/DendrogramOpDescSpec.scala
@@ -19,6 +19,8 @@
 
 package org.apache.texera.amber.operator.visualization.dendrogram
 
+import org.apache.texera.amber.operator.LogicalOp
+import org.apache.texera.amber.util.JSONUtils.objectMapper
 import org.scalatest.BeforeAndAfter
 import org.scalatest.flatspec.AnyFlatSpec
 import org.scalatest.matchers.should.Matchers
@@ -83,13 +85,41 @@ class DendrogramOpDescSpec extends AnyFlatSpec with BeforeAndAfter with Matchers
     code should include("color_threshold=None")
   }
 
-  it should "generate python code carrying a non-empty threshold when configured" in {
+  it should "generate python code passing a configured threshold as a number" in {
     opDesc.xVal = "coord_a"
     opDesc.yVal = "coord_b"
     opDesc.labels = "label_col"
-    opDesc.threshold = "42.5"
+    opDesc.threshold = Some(42.5)
     val code = opDesc.generatePythonCode()
-    assert(carries(code, "42.5"))
+    // A number, not a decoded string: a string raises inside scipy.
+    code should include("color_threshold=42.5")
     code should not include "color_threshold=None"
   }
+
+  /** Reads the shapes a stored workflow can hold. Without `contentAs` a JSON string
+    * stays unconverted inside the Option and the first use throws.
+    */
+  private def readThreshold(json: String): Option[Double] =
+    objectMapper
+      .readValue(s"""{"operatorType":"Dendrogram"$json}""", classOf[LogicalOp])
+      .asInstanceOf[DendrogramOpDesc]
+      .threshold
+
+  "DendrogramOpDesc.threshold" should "deserialize a JSON number" in {
+    readThreshold(""","threshold":42.5""") shouldBe Some(42.5)
+  }
+
+  it should "deserialize the numeric string a workflow saved before the field was numeric" in {
+    readThreshold(""","threshold":"42.5"""") shouldBe Some(42.5)
+  }
+
+  it should "read an absent, null or blank value as unset rather than as zero" in {
+    readThreshold("") shouldBe None
+    readThreshold(""","threshold":null""") shouldBe None
+    readThreshold(""","threshold":""""") shouldBe None
+  }
+
+  it should "hold a Double, not the raw JSON value" in {
+    readThreshold(""","threshold":"42.5"""").map(_ * 2) shouldBe Some(85.0)
+  }
 }