In the same way it would make sense to decide on your main course before choosing the side dishes, it occurs to me that I probably should have addressed the "Thriller" category prior to discussing spillers. Then again, I tend to seek out the mashed potatoes on the menu first and then decide which entree would make the best accompaniment so I guess I'm in keeping with my usual M.O.
Now back to our container "recipe" which I so wish I could take credit for but if I recall, it was a gardener and writer by the name of Steve Silk who should have all rights to the patent on the phrase. It was his article in Fine Gardening magazine that really started me on the road to seeing container plantings in a different way.
Recall that the thriller in the mixed container is the plant whose aim it is to grab your attention and reel you in. It is characterized by its height and/or its large and distinct foliage and/or its dramatic bloom. It should be, in some way, visually and structurally compelling.
In my opinion, the mother of all thrillers is the Red Abyssinian Banana plant. I'm sure you'll agree as you consider the picture in the upper left hand corner of this post. The photo was taken midsummer last year and, ultimately, the plant reached about nine feet in height. I had it in a good-sized terra cotta pot so I can only imagine how large it would have gotten had I opted for an even bigger container. It is a tropical, therefore, it cannot withstand the cold but I did not go through the trouble of overwintering it inside as I had absolutely nowhere to put it. However, it is my understanding that given the right conditions, preserving it during the winter months is a fairly easy thing to do provided you have space for a nine foot (or larger) plant. I bought another one this year for only $6.99 at the local nursery and I fully expect a similar performance. Some other thrillers:
Canna--another tropical that you can find early in the season in the form of tubers (its "preplant" form that you plant a few inches beneath the soil surface and it begins to sprout and grow into a mature plant). Or you can wait for the nursery to do that work for you, therefore you are guaranteed that a plant actually results. The canna bloom is spectacular but the banana-like foliage is stunning in its own right, sufficient to warrant a role in any container. Again, it won't survive the winter outside unless you are so fortunate as to live in zone 10 or 11.
Ornamental Grasses--of these, Pennisetum Rubrum or Purple Fountain Grass is my favorite. The foliage and the purple-red feather plumes that the plant produces sway in the most appealing way when the breeze blows. Definitely give this one a try.
Red Cordyline--long, sturdy leaves that are deep red and spiky. This plant makes a terrific centerpiece for a mixed container.
Mandevilla--the thriller can also be a climber. I love mandevilla and I have it in a large pot in which I have also put a sturdy wire trellis. I let the mandevilla climb and I fill the space at the base of the trellis with other plants. It never disappoints. But be sure it gets plenty of sun, lots of water and surround it with other plants that have the same or very similar requirements.
There are lots of plants out there to fill the thriller role. Some others that I've used: Japanese Maple, rose shrubs, purple salvia, colocasia (elephant ear), and in one very small arrangement that I planted, the thriller was angelonia.
Bookshelf: Container Gardening Books