A common way to design container gardens is by using plants designated as, thriller, filler and spiller plants. Thinking of container plants this way is a particularly helpful to beginners. It also a great way for anyone to get started in designing exciting plant combinations without getting intimidated by all the choices out there.
Spillers are a layer of plants that 'spill' or cascade over the sides of the pot. They are usually placed closest to the rim of the pot so they will tumble out of the pot. Spillers can be flowering or foliage plants and can be used to add a pop of color or a texture that can be a counterpoint to both the filler and thriller plants.
Sweet potato vines are classic spillers. They come in a wide array of leaf shapes and sizes and colors from bright to almost black. Another common flowering spiller is lobelia. It's frothy blue flowers add both color and texture.
Ivies make great spillers. They are a somewhat structured plant and can be used to give texture, draping down the side of a pot.
While you can use one variety as a spiller throughout your pot, it is often effective to alternate, two or if your pot is large, even three varieties of spiller plants.
Also, some plants can be both fillers and spillers, verbena, bacopa, oregano, and sedum, to name just a few.
Here are some examples of foliage spillers:
Here are some flowering spiller plants:Definition and Suggestions for Filler Plants