Using a class and a function from different header files C++ and CMake

I am learning Modern C++ form a course by University of Bonn. I have declared a class in a grass.h file and its implementation in a corresponding grass.cpp file. Similarly in another tools.h-tools.cpp declaration-implementation pair is a function that takes in a reference to the class object defined above. And finally a main.cpp file initialises a Grass object from first pair and passes it to the function from the second pair. I am able to compile main.cpp and get the desired output from these command line instructions:

c++ -std=c++11 -g -c grass.cpp -o grass.o
c++ -std=c++11 -g -c tools.cpp -o tools.o
c++ -std=c++11 -g main.cpp grass.o tools.o -o main

but unable to do so with CMake. I do not know what the command line instructions are called (linking and compiling?).

Here is the code example "grass.h":

#include <string>
#pragma once

class Grass
{
private:
    int green_;
    std::string name_;
public:
    Grass(int green, std::string name): green_{green}, name_{name} {}

    int get_green() const;                        // getter functions for 
    const std::string& get_green_name() const;    // both private members
};

The functions just return the values and are implemented in "grass.cpp".

Then I have a "tools.h":

#include <iostream>
#include "grass.h"

#pragma once

void PrintGrass(Grass& grObj);

And this function just prints values from the getter functions of Grass.

Finally, the main.cpp contains:

int main ()
{
    Grass grObj{10, "ABC"};
    PrintGrass(grObj);
    cout << endl;
    return 0;
}

The CMake error occurs at 100% Linking CXX executable main. The error is in tools.cpp: undefined reference to `Grass::get_green_name[abi:cxx11]()' and undefined reference to `Grass::get_green()'. This means that the the program is unable to find to find the definition of these function, right? The CMake file looks like this:

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_BUILD_TYPE Debug)
set(CMAKE_CXX_FLAGS "-Wall")
set(CMAKE_CXX_FLAGS_DEBUG "-g -O0")
add_library(grass grass.cpp)
add_library(tools tools.cpp)
add_executable(main main.cpp)
target_link_libraries(main grass tools)

How do I make this work with CMake? Thank you very much for your help.