diff --git a/io/io/inc/ROOT/TBufferMerger.hxx b/io/io/inc/ROOT/TBufferMerger.hxx
index 060bcb926e6d7..c3dbcbba251d1 100644
--- a/io/io/inc/ROOT/TBufferMerger.hxx
+++ b/io/io/inc/ROOT/TBufferMerger.hxx
@@ -79,7 +79,7 @@ private:
    std::mutex fWriteMutex;                                       //< Mutex used for the condition variable
    std::condition_variable fCV;                                  //< Condition variable used to wait for data
    std::queue<TBufferFile *> fQueue;                             //< Queue to which data is pushed and merged
-   std::unique_ptr<TFile> fFile;                                 //< Output file, owned by the user
+   std::unique_ptr<TFile> fFile;                                 //< Output file, owned by this class
    std::unique_ptr<std::thread> fMergingThread;                  //< Worker thread that writes to disk
    std::vector<std::weak_ptr<TBufferMergerFile>> fAttachedFiles; //< Attached files
 
diff --git a/io/io/src/TBufferMerger.cxx b/io/io/src/TBufferMerger.cxx
index 24dcc851c74fd..538716f1ad457 100644
--- a/io/io/src/TBufferMerger.cxx
+++ b/io/io/src/TBufferMerger.cxx
@@ -280,6 +280,7 @@ TBufferMerger::~TBufferMerger()
    fCV.notify_one();
 
    fMergingThread->join();
+   fFile->Close();
 }
 
 std::shared_ptr<TBufferMergerFile> TBufferMerger::GetFile()
diff --git a/tree/treeplayer/inc/ROOT/TDataFrameInterface.hxx b/tree/treeplayer/inc/ROOT/TDataFrameInterface.hxx
index 86cee48c6b850..fa0f182ee32fd 100644
--- a/tree/treeplayer/inc/ROOT/TDataFrameInterface.hxx
+++ b/tree/treeplayer/inc/ROOT/TDataFrameInterface.hxx
@@ -12,6 +12,7 @@
 #define ROOT_TDATAFRAME_INTERFACE
 
 #include "ROOT/TActionResultProxy.hxx"
+#include "ROOT/TBufferMerger.hxx"
 #include "ROOT/TDFNodes.hxx"
 #include "ROOT/TDFOperations.hxx"
 #include "ROOT/TDFUtils.hxx"
@@ -1066,26 +1067,33 @@ protected:
       }
 
       {
-         std::unique_ptr<TFile> ofile(TFile::Open(filename.c_str(), "RECREATE"));
-         TTree t(treename.c_str(), treename.c_str());
+         auto df = GetDataFrameChecked();
+         unsigned int nSlots = df->GetNSlots();
+         TBufferMerger merger(filename.c_str(), "RECREATE");
+         std::vector<std::shared_ptr<TBufferMergerFile>> files(nSlots);
+         std::vector<std::unique_ptr<TTree>> trees(nSlots);
 
-         static bool firstEvt = true;
+         auto fillTree = [&merger, &trees, &files, &bnames, &treename](unsigned int slot, Args &... args) {
+            if (!trees[slot]) {
+               files[slot] = merger.GetFile();
+               trees[slot].reset(new TTree(treename.c_str(), treename.c_str()));
 
-         auto fillTree = [&t, &bnames](Args &... args) {
-            if (firstEvt) {
                // hack to call TTree::Branch on all variadic template arguments
-               std::initializer_list<int> expander = {(t.Branch(bnames[S].c_str(), &args), 0)..., 0};
+               std::initializer_list<int> expander = {(trees[slot]->Branch(bnames[S].c_str(), &args), 0)..., 0};
 
                (void)expander; // avoid unused variable warnings for older compilers such as gcc 4.9
-               firstEvt = false;
             }
-            t.Fill();
+
+            trees[slot]->Fill();
          };
 
-         Foreach(fillTree, {bnames[S]...});
+         ForeachSlot(fillTree, {bnames[S]...});
+
+         for (auto &&tree : trees) tree->Write();
+         for (auto &&file : files) file->Write();
 
-         t.Write();
-         firstEvt = true;
+         trees.clear();
+         files.clear();
       }
       // Now we mimic a constructor for the TDataFrame. We cannot invoke it here
       // since this would introduce a cyclic headers dependency.