Thrillers, Fillers & Spillers: How to Design a Container Garden

Choosing a Container

Containers for planting can be made of a wide range of materials from clay (glazed and unglazed) to metal or metal with coco fiber lining to plastic or fiber glass resin as well as wooden. Unglazed clay and coco fiber will dry out faster than plastic or fiber glass, and need more frequent heavier watering. Another factor to consider is drainage which is a must for outdoor containers especially during heavy rains which could leave plants drowning in too much water causing rot. Some plastic pots have a circular mark on the bottom indicating where you can drill a hole easily which will save your plants after a rain. Emptying saucers underneath pots after a rain is a good idea too preventing rot. The size of pot selected is also an important factor. If there is not enough soil the pot will dry out quickly and need more frequent watering as well the plants will become pot bound. If the pot is too big the soil will hold more moisture and the plants could rot if over watered, though as a general rule of thumb it is a good idea to have a bigger pot than a smaller one to avoid over crowding. Containers with narrow openings should be avoided. Consider the height of the planter if thinking about using different types of trailing plants. If the container is really tall it doesn't need to be filled with soil the whole way to the bottom to reduce the weight of the final product. You  could fill it half way with foam packing chips or old cell packs crumpled up to take up room but stay light enough to move. Remember you still want it to be somewhat heavy to avoid blowing over in high winds.

Choosing Soil

In containers for annuals the best choice of soil is a potting soil mixture since they contain perilite and vermiculate which help with moisture retention and soil drainage. They are primarily peat moss based and "soilless" so substituting a top soil or black earth would not be advisable since they would not function the same in a pot and poor plant growth would result. We generally recommend a Pro-Mix potting soil since it is a sterile planting medium and the homeowner should not get weeds from it or other plant diseases. It also contains Mycorrhizal fungi which form a symbiotic relationship with the roots of the plant extending the plants natural root system helping it take up nutrients and water. In exchange the fungi receives carbohydrates from the plant which is not harmful and in fact the plants are healthier because of this relationship. If you wish to add compost to the soil use a 2/3 potting soil to 1/3 compost mixture. Sheep manure or earthworm castings would make good soil amendments and are odorless. Remember to leave a 1" or more gap between the soil level in the pot and the top lip of the pot, this will allow room for watering and prevent over flow of mucky water. It is a good idea to moisten your potting soil before planting so you know water has reached the bottom of the container. Before planting as well you can mix Soil Moist into the potting soil if you want to use crystals that absorb water and gradually release it to the plants, preventing the soil from drying out quickly though they cannot be used on food crops. Be careful not to use too many since they can cause the soil to overflow out of the pot after a heavy rain when they become over saturated.

Thrillers, Fillers and Spillers

To make a successful container garden there are three elements to consider the thriller plant, the filler plants and the spiller plants.

The thriller plant is the focal point of the pot and should be selected first with the other elements relating to it. A thriller plant should be the tallest plant in the composition, generally they are upright and could have showy flowers, dramatic foliage or a different shape. Examples are Purple Fountain Grass, Cordyline, Canna Lilies, Dalhia, Dracena, Coleous, and Millet. In a planter that would be viewed on all sides in a round pot placing the thriller in the center is best. If the planter will be viewed from the front only you can place the thriller towards the back of the pot.

The filler plants are just as the name implies something to hide the bottom stocks of the thriller and weave through it. Generally the filler has a different texture than the thriller so if the thriller is coarse the filler is fine or vise versa. In addition to providing a texture contrast the filler adds colour to the planter it could be similar colours to the thriller (monochromatic) like different shades of purples or contrasting colours like purple and yellow. They should be planted around the thriller and don't have to be all the same type of plant some could be flowering and some could be foliage. The size of your pot will determine how many plants you will need without overcrowding since they will fill in over the summer. Examples are Begonia, New Guniea Inpatients, Osteospermum Daisy, Saliva, Persian Shield, Nemesia, Diascia, Petunia, Heuchera, Lamium  and Lantana. 

The spiller plants do just that and will spill over the edge of the planter softening the edge of the planter. The spillers are planted closest to the edge of the pot so they can tumbled towards the ground. These plants should tie into the colour scheme of the thriller and spiller and carry on the theme of those selections with colour, texture and shape of leaf or flower. Examples of spiller plants are Bacopa, Golden Creeping Jenny, Sweet Potato Vine, German Ivy, Swedish Ivy, Lobelia, Baltic Ivy, Fan Plant, Bidens, Verbena, Wave Petunia and Million Bells.

Sun and Shade

It is extremely important to study the area where the plants will be located during the summer to determine how many hours of direct light they will receive and also the time of day when this light is experienced. Six hours of direct light is considered to be full sun and morning sunlight is considered part sun as would evening sunlight. Bright shade is 4-6 hours of morning sun, dappled light is filtered through the canopy of trees, lastly dense shade is north facing or obstructed by structures. If you are gardening in shade or partial shade you can gain interest in a container by experimenting with texture, colour in leaves and sizes of leaf not just colour from flowers. Sun plants will not flower in shade and shade plants can be burnt from too much sun so it is important to plant the right plant in the right place.    


Perennial plants can be used in containers as well as annuals to give extra interest with their leaves. The only draw back is that they will not flower as profusely as an annual would. Plants such as Heuchera, Hosta or Lamium are three great perennials for the shade that would work  well in a shade planter. Some perennials that work in containers such as Creeping Jenny that will trail over the edge of a pot should not be transferred to the garden afterwards because it is so invasive once established, but in a container it is wonderful.  

Fall Planters

Keep in mind in the fall that with trees and shrubs are loosing their leaves and perennials dying back to the ground hardscape elements will become more evident, such as walkways and railings. As well the siding of your home will be more dominate so colour combinations that worked in the summer may not work in the fall and could clash. You will want to choose plants that can tolerate a frost or two such as Ornamental Kale, Chrysanthemums, Pansys, Sedum or Purple Fountain Grass. Pick colours that suit the natural colours of autumn, like oranges and reds. You could incorporate branches as well as grasses to give height to your arrangement. 

If you want to plant a evergreen or other shrub in a container make sure that it is two zones colder than we are in Nova Scotia. We are Zone 5 so anything at Zone 3 would have the best chance of over wintering in a container. The choice of the pot is important too. Ceramic pots, glazed or unglazed would crack with the freezing and thawing of the soil mass over the winter months and a big investment could be lost. Consider using fiberglass, iron, thick plastic or a stone planter if you want the container to survive the elements in our climate.  

Colour Wheel & How to Use Colour in Your Garden

The first thing to know about colour is that it is based on three primary colours red, yellow and blue. Arranging the primary colours in a circle on a wheel is a logical way to understand how secondary colours such as green, purple and orange are formed when they are mixed with the primary colour next to it. Tertiary colours  are formed when a primary colour and a secondary colour are mixed with each other forming Yellow-Orange, Red-orange, Red-purple,  blue-purple, blue-green and yellow green these are hues.

Colour harmony is created by using certain colours taken from different parts of the colour wheel. If you pick three colours that are next to each other on the wheel you have an arrangement of analogous colours such as yellow-green, yellow and orange-yellow. Another type of arrangement is complementary colours which is when you place two colours that are opposite each other on the colour wheel side by side this creates a positive type visual tension with contrast. An example is purple and yellow which are opposite each other on the wheel.

There are warm and cool colours. Warm colours include shades of red, orange and  yellow. These colours often times will excite the spectator and cause a warm feeling. Cool colours include shades of green, blue and lavender. These colours tend to have a calming effect on the viewer. Think about what feeling you want your visitors to have while entering, roaming and relaxing around your property. Cool colours like purple Bacopa, Lavender and green Potato Vine in containers around the patio will produce a serene mood. Electric colours like yellow Dahlia, orange Zinna and red wave Petunia will produce a feeling of excitement and warmth. Combinations of warm and cool colours tend to be more typical with most homeowners but it is good to keep in mind the way our minds respond to colour theory emotionally.   

When using colour in your garden pick a theme of about four colours to use throughout to minimize visual confusion. These could be analogous colours such as reds, red-purples, blue and blue-purple. Like a red Geranium, purple Potato Vine, purple Bacopa, blue Salvia in a container. You could select complementary colours such as blues and oranges to create organized confusion like a orange Marigold with Celosia, Blue Lobelia and blue Salvia. You want to minimize the number of different colours to create a sense of unity like an interior designer would do in a home. Repetition of those selected colours will create unity which is one of the principals of landscape design. That doesn't mean you need to plant all the same type of plant, pick different flowering plants with similar coloured blooms that would be enough repetition and provide some variety as well while remembering you want a thriller, filler and spiller.  

If you are planting in the shade you want to use light coloured annuals such as white, light pink or pale blue so that the blooms don't get lost in the darkness of shade. That doesn't mean that you cannot use dark colours just make sure there is a variety of different intensity of colours with some lighter intensity colours used to pop and create contrast. A light green Coleus could surround deep red New Guinea Impatient.

Contrast between hardscape features such as fences, siding of your house or patio pavers is essential so avoid an analogous colour relationship. So if your fence is red, planting a container with red geraniums in front of it might become lost but if you planted white or pink they would stand out due to contrast. Having some red in the planter though might be a good idea to create a sense of harmony in the entire feel of the garden. Likewise if the fence is white you wouldn't want to plant white Petunia cascading down the planter a darker more vibrant colour like purple or hot pink would stand out better. Or if using a light colour choose something other than white but a less intense hue of purple.

Containers become a focal point in the garden or on the deck so it is important that they are dynamic and full of contrast between the thriller, filler, spiller while having an overall feeling of unified harmony. This feeling of harmony is not just created with colour the foliage of the annuals is an important factor as well with different shades of green such as  blue-green, yellow-green, white and green. There are purple, orange, red and silver  foliage plants. Don't overlook different colours of foliage when planning your container they can be just as striking and dramatic as well as longer lasting than blooms. They also can tone down the intensity of strong flower colours making them less outrageous.

Caring For Your Planter- Fertilizing

Since container plants have such a small amount of soil accessible to their root systems this affects their requirements for both nutrients and water. So plants in pots require more fertilization due to the small soil volume. At the time of planting you can use a quick start liquid fertilizer to help the roots of your annuals get established. These formulas are higher in phosphorous which will stimulate the roots. Three weeks after planting your annual containers it is a good idea to start a fertilizing routine for flowering plants. There are many options available and a choice between synthetic and organic.

Synthetic fertilizers can be water soluble, granular or slow release. Water soluble are mixed into your watering can with water and poured into your pots of flowers (Plant Prod, Miracle Gro) Granular is mixed into the soil around the plants carefully without disturbing the roots (Pink). Slow release fertilizers only need to be applied once and come in a granular form that is sprinkled over the soil around the plants (Nutricote). Organic fertilizers contain natural nutrients often that come from a varity of sources to provide balanced macro-nutrients and mirco-nutrients that will help build soil health while nourishing the plants (Gaia Green Power Bloom, Neptune's Harvest). Regardless of what type of fertilizer you select following the directions on the product label is extremely important to avoid over fertilization which can harm the plants. While under fertilization isn't as harmful as over your plants will not perform to their full potential. A lack of nitrogen will make them yellow (chlorotic) and a lack of phosphorous will cause a lack of blossoms. 

Caring For Your Planter-Watering

Since the annuals are planted above ground and cannot seek out water they are dependant on you to deliver their water requirements to them. While they cannot call out for water they will wilt if they become too dry or the tips of the leaves can turn brown from inconsistent watering.

So how do you know how much water to provide? This all depends on the location whether it is full sun or shade. In full sun plants will use much more water compared to the plants in the shade that don't have all that intense heat everyday. Wind is another factor which will dry plants out faster, something to consider with hanging baskets which can be watered everyday almost. The larger the plant is the more water it will use to sustain itself and the more actively growing the plant is the more water it will consume in a day. For example large tomato plants are extremely heavy drinkers. Of course the type of weather we are experiencing at the time is also a factor watering more on a hot summer day, watering less during a day with showers, and no watering on a day with heavy rain. Some plants don't like an abundance of water, like Portulaca which prefer dry soil and plants like Coleus will rot if they are kept too wet. A major consideration too is the type of soil that you are planting in as well and whether or not you add compost to the soil which will make it harder for the soil to absorb water once it has dried out.

Watering is an activity best done in the early morning before the heat of the summer day sets in so the plants are able to absorb more water and less water will evaporate into the air. When you water in the heat of the day the beads of water on the leaves can act like magnifying glasses and burn the leaves causing spotting on the otherwise healthy leaves. Many people water in the evening since it is cooler and they have more time to spare but this can cause a beneficial environment for fungal diseases to spread with moisture remaining on the leaves since there is no heat from the sun to dry them. 

Caring For Your Planter-Dead Heading

An essential practice for any annual garden is dead heading removing spent blooms, but it isn't just the colourful petals that need to be removed it is also the seed head itself, the green portion underneath the petals. If that is left the plant will go to seed and stop flowering since it has completed it's life cycle and feels that it has fulfilled it's purpose to spread and continue that type of plant in the world. Dead heading will keep the plant blooming the whole season long, combined with using a flowering plant fertilizer.  

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