diff --git a/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java b/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
index 26adfc0..69f7ab0 100644
--- a/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
+++ b/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
@@ -8,6 +8,7 @@ import lombok.AccessLevel;
 import lombok.AllArgsConstructor;
 import lombok.Builder;
 
+import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.ObjectOutputStream;
@@ -122,10 +123,19 @@ public class ForestTrainer<Y, TO, FO> {
 
 
     public void trainParallelOnDisk(int threads){
-        final ExecutorService executorService = Executors.newFixedThreadPool(threads);
-        final AtomicInteger treeCount = new AtomicInteger(0); // tracks how many trees are finished
+        // First we need to see how many trees there currently are
+        final File folder = new File(saveTreeLocation);
+        if(!folder.isDirectory()){
+            throw new IllegalArgumentException("Tree directory must be a directory!");
+        }
 
-        for(int j=0; j<ntree; j++){
+
+        final File[] treeFiles = folder.listFiles(((file, s) -> s.endsWith(".tree")));
+
+        final ExecutorService executorService = Executors.newFixedThreadPool(threads);
+        final AtomicInteger treeCount = new AtomicInteger(treeFiles.length); // tracks how many trees are finished
+
+        for(int j=treeCount.get(); j<ntree; j++){
             final Runnable worker = new TreeSavedWorker(data, "tree-" + (j+1) + ".tree", treeCount);
             executorService.execute(worker);
         }