Thriller, Filler, Spiller: How to Layer a Container Garden

What Thriller, Filler and Spiller Mean

Thriller, filler, spiller is a three-part formula for a mixed container. The thriller is one tall, upright plant that acts as the focal point. Fillers are rounded, mounding plants that fill the middle of the pot and make it look full. Spillers are trailing plants that hang over the rim and soften its edge. Choose one or more plants for each role, give all of them the same light and water needs, and the pot has height, body and a finished edge.

University extension guides describe the roles in nearly the same words. Penn State Extension says thrillers add height, drama and a vertical element. University of Illinois Extension describes fillers as round, mounding, lower-growing plants, and spillers as plants that hang over the side of the container. Mississippi State University Extension suggests starting with one plant for each role.

Where Each Plant Goes in the Pot

The UC Master Gardeners of El Dorado County lay out the planting order. Fill the pot three quarters full with moist mix. Plant the larger thriller first, in the center or toward the back. Work outward with the fillers, end with the spillers at the front, then fill in with potting mix and water.

Where the thriller sits depends on how the pot is viewed. Penn State puts it in the center when the container is seen from all sides and in the back when it is seen from one side. Fillers go in front of or around the thriller, and spillers go close to the edge. Rutgers takes a different view and advises resisting the urge to center the thriller and ring it with plants. Its reasoning is that an off-center thriller leaves more room at the front for fillers and spillers and keeps the design from magnifying the symmetry of the container.

A full pot needs fewer plants than it seems to on planting day. University of Georgia Extension says three or four plants from 4-inch pots are sufficient for a 5- to 7-gallon container.

Thriller, Filler, Spiller Ideas for Full Sun

Rutgers defines full sun as at least six hours of direct sun each day. Most of the plants below come from the sun column of the UC Master Gardeners handout. Geranium, ornamental grasses and sweet potato vine come from the University of Illinois lists, and several others also appear in the Penn State and Mississippi State articles.

Role	Full sun ideas
Thriller	Canna, angelonia, cordyline, celosia, pentas, tall ornamental grasses
Filler	Petunia, ageratum, dusty miller, gomphrena, lantana, geranium
Spiller	Sweet potato vine, calibrachoa, verbena, bidens, lantana

Some plants fit more than one role. The UC handout lists lantana as a spiller, while Penn State and Rutgers list it as a filler. University of Illinois Extension gives lantana a mature height of 12 to 24 inches, rates it for full sun, and notes that it tolerates heat and humidity. That makes it a sound choice for a hot, sunny spot.

Sweet potato vine needs room. Illinois describes it as a very vigorous full-sun plant that likes the heat and will spread great distances, so pair it with a thriller and fillers big enough to hold their own.

A simple full-sun recipe using only plants from these lists: one canna at the back, lantana and petunias in the middle, and sweet potato vine with calibrachoa at the rim.

Ideas for Shade and Part Shade

Rutgers defines partial shade as 2 to 4 hours of direct sun per day and shade as less than 2 hours. The UC handout adds that most shade plants still need 2 to 4 hours of sunlight daily and that morning sun is best.

Role	Shade ideas
Thriller	Caladium, coleus, upright fuchsia, ferns, hosta, astilbe
Filler	Begonia, impatiens, torenia, Persian shield, viola
Spiller	Creeping Jenny, lobelia, sweet alyssum, ivy geranium

Height tells you which role a shade plant can play. Iowa State University Extension lists caladium at 12 to 30 inches tall, tall enough for a thriller, while wax begonia and torenia (wishbone flower) both stay at 6 to 12 inches and work as fillers.

Match Light and Water Before You Buy

A combination that looks right at the garden center can fail by midsummer if the three plants want different conditions. The UC handout’s rule is to select plants with similar needs for light, water, soil and fertilizer. Penn State says the same about location: decide whether the pot will sit in sun, partial sun or shade, then choose plants whose requirements match it.

In practice, count the hours of direct sun where the pot will stand and compare the number with the Rutgers definitions: six or more for full sun, 3 to 6 for partial sun, 2 to 4 for partial shade. Then pick all three roles from the same table above.

Pot, Mix and Planting Depth

Drainage comes first. University of Georgia Extension calls adequate drainage holes the most important consideration when choosing a container, and Illinois advises making holes in any container that lacks them. Skip the layer of gravel. According to the UC handout, gravel at the bottom lessens root space and shifts the saturated zone upward, a point we cover in full in rocks in the bottom of a pot.

Size matters for summer annuals. Georgia notes that small pots restrict root growth, which limits top growth. Rutgers asks for at least 3 inches of soil under the root balls.

Fill with a soilless potting mix. Illinois describes these mixes, made of peat, vermiculite, bark, coconut coir or perlite, as generally the best option for containers. Leave room at the top for water: Rutgers puts the soil surface 1 to 2 inches below the rim, and Georgia says to fill to within 2 inches of the top. If you plan to refill last year’s pots, see the steps for reusing old potting soil.

Watering, Feeding and Midseason Upkeep

Water by feel. Illinois says to water when the top inch or so of the mix is dry, and Penn State suggests pushing an index finger about two inches into the soil to check. Water thoroughly, until it starts to trickle out of the drainage holes, and do it in the morning when temperatures are cooler. Rutgers notes that containers will need water 1 to 2 times per day on hot, dry days, and our guide to how often to water container plants in summer goes into the details.

Soilless mixes are usually low in nutrients, so the plants in them may need fertilizer at some point, as Illinois puts it. Rutgers says containers may start to look hungry by late summer and suggests either more slow-release fertilizer or a water-soluble fertilizer applied once every two weeks while watering. Georgia gives an interval of every two to three weeks during the growing season for a liquid soluble fertilizer. For the reasons behind those intervals, see why container plants need more fertilizer.

Grooming keeps the three layers in proportion. The UC handout advises deadheading flowering plants frequently, pinching shoots to shape them, and clipping back harder as plants grow. A vigorous spiller such as sweet potato vine is a likely candidate for clipping once it starts to cover the fillers.