How can I change some tree's radius and length in C# script?

So, I have this little plant stem here. It only has its initial node.

All I would like to do is make it grow in length and radius in Update(). How can I do that?

I made a simple script for example when the object who have the script growing up every second by Time.DelataTime.

    public float GrowUpSpeed;

    public float growScale_X;
    public float growScale_Y;
    public float growScale_Z;
    void Update()
        growScale_X += Time.deltaTime * GrowUpSpeed;
        growScale_Y += Time.deltaTime * GrowUpSpeed;
        growScale_Z += Time.deltaTime * GrowUpSpeed;
        transform.localScale = new Vector3(growScale_X, growScale_Y, growScale_Z);