Topiary Shapes are a great way to repeat the theme shape of your garden. To make a small topiary tree that sits beside my porch scene, I used two wooden beads glued to a round toothpick. The beads should be 1/8 inch thinner than the diameter you want for your topiary. You could use styrofoam balls or woolen pompoms for this as well.
Don't limit yourself to beads for topiary for interesting gardens. In full scale I have seen topiary tea pots made of box wood, so for your quarter scale scene you could set a dolls house scale tea pot in a small garden pot and cover the tea pot with green railroad scatter to make a teapot topiary. Dabs of two tones of green acrylic paint will work as well.
Preparing the Frame For Your Topiary - Once the beads are securely glued to the toothpick, trim off any toothpick that protrudes through the top of the top bead. Paint the parts of the toothpick that you can see dark brown for the tree trunk. Set the topiary frame aside to dry.
Topiaries can be made with single balls, a ball and a square, a ball with a triangle above it, different sizes of balls or many other shapes. Experiment to see what works for your scene.