Cannot figure out what causes "undefined reference to..." error [duplicate]

This question already has an answer here:

• Why can templates only be implemented in the header file? 9 answers

I just started learning the C++ programming language and am trying to implement a binary search tree data structure.

When I try to compile my files with

g++ -c binary.cpp -o binary.o -std=c++11
g++ -c main.cpp -o main.o -std=c++11
g++ main.o binary.o -std=c++11

then I always get some errors saying there is an "undefined reference" to some member function like

main.o: In function `BSTree<int>::insert(int const&)':
main.cpp:(.text.#some-more-text#.+0x16): undefined reference to 
 `BSTree<int>::insert(BSTreeNode<int>*, int const&)'
collect2: error: ld returned 1 exit status

The following code snippets are meant to be a minimalistic example:

binary.hpp:

template<typename T>
class BSTreeNode {
public:
  T key;
  BSTreeNode<T> *left;
  BSTreeNode<T> *right;

  BSTreeNode(const T& key) : key(key), left(nullptr), right(nullptr) {};
};


template <typename T>
class BSTree {

protected:
  BSTreeNode<T> *root;

public:
  BSTree() {
    root = nullptr;
  }

  void insert(const T& key) {
    root = insert(root, key);
  }

protected:
  BSTreeNode<T>* insert(BSTreeNode<T>* node, const T& key);
};

binary.cpp

#include "binary.hpp"

template<typename T>
BSTreeNode<T>* BSTree<T>::insert(BSTreeNode<T>* node, const T& key) {

  if (not node) {
    return new BSTreeNode<T>(key);
  }

  if (key < node->key) {
    node->left = insert(node->left, key);
  } else if (key > node->key) {
    node->right = insert(node->right, key);
  } else {
    throw "Key already exists!";
  }

  return node;
}

main.cpp:

#include "binary.hpp"
#include <math.h>

int main(){
  BSTree<int> bt;

  for (int i=0; i<10; i++){
    bt.insert(pow(-1, i) * i);
  }

Even after an exhaustive search I still can't figure out what's wrong with my code.