diff --git a/src/main/java/ca/joeltherrien/randomforest/Main.java b/src/main/java/ca/joeltherrien/randomforest/Main.java
index bccd125..24d96c8 100644
--- a/src/main/java/ca/joeltherrien/randomforest/Main.java
+++ b/src/main/java/ca/joeltherrien/randomforest/Main.java
@@ -71,10 +71,18 @@ public class Main {
             final ForestTrainer forestTrainer = new ForestTrainer(settings, dataset, covariates);
 
             if(settings.isSaveProgress()){
-                forestTrainer.trainParallelOnDisk(settings.getNumberOfThreads());
+                if(settings.getNumberOfThreads() > 1){
+                    forestTrainer.trainParallelOnDisk(settings.getNumberOfThreads());
+                } else{
+                    forestTrainer.trainSerialOnDisk();
+                }
             }
             else{
-                forestTrainer.trainParallelInMemory(settings.getNumberOfThreads());
+                if(settings.getNumberOfThreads() > 1){
+                    forestTrainer.trainParallelInMemory(settings.getNumberOfThreads());
+                } else{
+                    forestTrainer.trainSerial();
+                }
             }
         }
         else if(args[1].equalsIgnoreCase("analyze")){
diff --git a/src/main/java/ca/joeltherrien/randomforest/covariates/Covariate.java b/src/main/java/ca/joeltherrien/randomforest/covariates/Covariate.java
index 13b3efa..193889f 100644
--- a/src/main/java/ca/joeltherrien/randomforest/covariates/Covariate.java
+++ b/src/main/java/ca/joeltherrien/randomforest/covariates/Covariate.java
@@ -82,8 +82,8 @@ public interface Covariate<V> extends Serializable, Comparable<Covariate> {
          * @return
          */
         default <Y> Split<Y, V> applyRule(List<Row<Y>> rows) {
-            final List<Row<Y>> leftHand = new LinkedList<>();
-            final List<Row<Y>> rightHand = new LinkedList<>();
+            final List<Row<Y>> leftHand = new ArrayList<>(rows.size()*3/4);
+            final List<Row<Y>> rightHand = new ArrayList<>(rows.size()*3/4);
 
             final List<Row<Y>> missingValueRows = new ArrayList<>();
 
diff --git a/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java b/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
index 769046b..31d04e8 100644
--- a/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
+++ b/src/main/java/ca/joeltherrien/randomforest/tree/ForestTrainer.java
@@ -93,6 +93,35 @@ public class ForestTrainer<Y, TO, FO> {
 
     }
 
+    public void trainSerialOnDisk(){
+        // 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!");
+        }
+
+
+        final File[] treeFiles = folder.listFiles((file, s) -> s.endsWith(".tree"));
+
+        final AtomicInteger treeCount = new AtomicInteger(treeFiles.length); // tracks how many trees are finished
+        // Using an AtomicInteger is overkill for serial code, but this lets use reuse TreeSavedWorker
+
+        for(int j=treeCount.get(); j<ntree; j++){
+            if(displayProgress) {
+                System.out.print("\rFinished " + treeCount.get() + "/" + ntree + " trees");
+            }
+
+            final Runnable worker = new TreeSavedWorker(data, "tree-" + (j+1) + ".tree", treeCount);
+            worker.run();
+
+        }
+
+        if(displayProgress){
+            System.out.println("\nFinished");
+        }
+
+    }
+
     public Forest<TO, FO> trainParallelInMemory(int threads){
 
         // create a list that is prespecified in size (I can call the .set method at any index < ntree without
diff --git a/src/main/java/ca/joeltherrien/randomforest/tree/TreeTrainer.java b/src/main/java/ca/joeltherrien/randomforest/tree/TreeTrainer.java
index 9756c79..98292e3 100644
--- a/src/main/java/ca/joeltherrien/randomforest/tree/TreeTrainer.java
+++ b/src/main/java/ca/joeltherrien/randomforest/tree/TreeTrainer.java
@@ -111,10 +111,22 @@ public class TreeTrainer<Y, O> {
                 }
             }
 
+            final Node<O> leftNode;
+            final Node<O> rightNode;
+
+            // let's train the smaller hand first; I've seen some behaviour where a split takes only a very narrow slice
+            // off of the main body, and this repeats over and over again. I'd prefer to train those small nodes first so that
+            // we can get terminal nodes and save some memory in the heap
+            if(bestSplit.leftHand.size() < bestSplit.rightHand.size()){
+                leftNode = growNode(bestSplit.leftHand, depth+1, random);
+                rightNode = growNode(bestSplit.rightHand, depth+1, random);
+            }
+            else{
+                rightNode = growNode(bestSplit.rightHand, depth+1, random);
+                leftNode = growNode(bestSplit.leftHand, depth+1, random);
+            }
 
 
-            final Node<O> leftNode = growNode(bestSplit.leftHand, depth+1, random);
-            final Node<O> rightNode = growNode(bestSplit.rightHand, depth+1, random);
 
             return new SplitNode<>(leftNode, rightNode, bestSplit.getSplitRule(), probabilityLeftHand);