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Success with Mixed Containers Using Perennial and Woody Plants (B 1418)

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Matthew Chappell and Bodie Pennisi
Department of Horticulture, Extension Specialists

Dating back to the ancient Hanging Gardens of Babylon, the art and practice of container gardening has enabled gardeners to grow plants virtually anywhere. Increased development has pushed urban gardeners to maximize impact in limited spaces and with limited resources, making container gardening even more popular. Gardeners who practice container gardening have many options: growing vertically, on (as opposed to in) compacted industrial soils and even where there is no soil (Figure 1). This publication offers information on types of plants suitable for mixed containers, with an emphasis on perennial and woody species and cultivars, as well as aesthetic qualities, cultural conditions and placement within the container.

Figure 1. Container gardening allows homeowners
and businesses to “grow vertically”
in spaces not typically reserved for planting.
A good example is this window planter in
Charleston, S.C. – a city known for magnificent
mixed planters and window planters. Figure 1. Container gardening allows homeowners and businesses to “grow vertically” in spaces not typically reserved for planting. A good example is this window planter in Charleston, S.C. – a city known for magnificent mixed planters and window planters.

Why Container Garden?

Combining ornate pots with plant themes creates a living element of self-expression and personal design. Mixed containers are heavily used in retail plant nurseries and commercial properties as focal points to draw the attention of consumers and clients. Novice gardeners can enjoy ready-made containers (also called container gardens, or combos). For more adventurous and knowledgeable gardeners, there are myriad plants and containers to choose from. Mixed containers can combine annuals, perennials, woody shrubs and even small trees. As few as three different plants can be planted to achieve the desired effect. In general, small combo containers (including baskets) contain annual plants that are rotated with seasonal color (e.g., cool-season annuals followed by warm-season annuals). Larger containers are often planted with perennials, shrubs and/or small trees in addition to annuals. Regardless of the source or plants selected, container gardening can add visual interest to draw the eye, direct traffic and provide focal points that can either grab attention or screen unsightly landscape or architectural elements.

Terms
Annual A plant that completes its life cycle in less than one year. Cool-season annuals are planted in fall (e.g., pansy, snapdragon, ornamental kale). Warm-season annuals are planted in late spring (e.g., petunia, zinnia, vinca). Annuals as a group are unable to withstand both summer heat and winter cold (Thomas, 2012).
Herbaceous Perennial A plant, including many bulbs, that completes its life cycle in two or more years and can withstand both summer and winter conditions (e.g., daylily, hosta, canna) (Thomas et al., 2012). Herbaceous perennials typically lose their above-ground foliage in winter.
Tender Perennial An annual that happens to survive the winter in Georgia; however this definition is very weather-dependent. Some tender perennials may be annuals seven out of 10 years! These species are of tropical origin (e.g., pigeon berry (duranta), sun coleus, elephant ears). Tender perennials may be species that grow to be shrubs and even small trees in their native habitat; however, when grown without winter protection, they typically lose their above-ground stems and behave as temperate zone perennials.
Shrub A perennial plant with multiple woody stems that are retained during winter (e.g., pieris, loropetalum, cotoneaster).
Tree A perennial plant with woody stems, typically larger than a shrub, and with a single or multiple trunks (e.g., Japanese maple, crape myrtle, dogwood).
Evergreen perennial, shrub or tree A plant that retains its foliage year-round (e.g., hellebore (Christmas and Lenten Rose), Aspidistra (cast iron plant), broadleaf (camellia) and needled evergreens (juniper)).
Deciduous shrub or tree A perennial plant that loses its foliage in winter (e.g., native azalea, crape myrtle).

Design Principles

Themes

Figure 2. Wholesale growers are
using mixed containers to provide
customers with an instant effect
while increasing the value-added
profit associated with mixed containers,
such as this Monrovia®
combination container. Figure 2. Wholesale growers are using mixed containers to provide customers with an instant effect while increasing the value-added profit associated with mixed containers, such as this Monrovia® combination container.

The first and most important step in designing a successful mixed container is creating a theme. The opportunities for container creation are abundant and only limited by the gardener’s creativity and experience. Themed containers allow the designer to stay on track when selecting the plants and for that reason it is always best to begin container gardening by creating a unifying theme, be it a color, texture, use or size of plant material. In addition to the popular “Thriller, Spiller, Filler” mixed containers (Silk, 2004), more specific and daring themes are becoming popular. Some examples of themed containers include food-based containers, such as “spaghetti” containers that combine tomatoes with herbs or “pizza” containers that include tomatoes, eggplants, peppers and basil. Mixed containers featuring woody ornamentals are also becoming more prevalent, as they offer the combination of seasonal color with a strong foundation that can be used for structure or screening.

For commercial nursery producers, the benefit of producing mixed containers is the resulting value-added product (Figure 2). Themes in a pre-planted mixed container can be sold at a higher profit margin than if the plants are sold individually. Customers, particularly those without the time or inclination necessary to create a mixed container, also benefit by reaping the rewards of a pre-planted, themed container. The attraction for both provider and consumer is simple: “one pot, one price.”

Mixed containers that combine annuals, perennials and woody ornamentals will exponentially increase plant selection possibilities (Figure 3). Annuals are less expensive and can provide color year-round due to the ability to change out plants seasonally (Thomas, 2012). For these reasons, annuals have been more popular in mixed containers; however, perennials can add lasting foliage colors and textures not found with annuals and can be used to add height to a container. Perennial bulbs can also be used to add seasonal color similar to annuals (Thomas et. al, 2012). Woody ornamentals can add height, screening ability, structure, and unique foliage and bark textures to a mixed container. Both perennial and woody plants also offer permanence to mixed containers that may appeal to those who want the long-term beauty associated with mixed container plantings without the work associated with seasonally changing out plant material.

Figure 3. This mixed container
uses a combination of winter annual
plants (parsley and pansies)
with the evergreen shrub Gardenia
‘Frostproof.’ The winter annuals will
be replaced in spring with summer
annuals to provide continual color. Figure 3. This mixed container uses a combination of winter annual plants (parsley and pansies) with the evergreen shrub Gardenia ‘Frostproof.’ The winter annuals will be replaced in spring with summer annuals to provide continual color.
Figure 4. In this container, pansies
(an annual plant) are used to fill
space before the perennial variegated
English ivy becomes established
and fills the container. Figure 4. Figure 4. In this container, pansies (an annual plant) are used to fill space before the perennial variegated English ivy becomes established and fills the container.


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Using perennials and woody ornamentals in mixed container plantings can provide year-round beauty if the plants are properly cared for. One major obstacle to using herbaceous perennials or woody ornamentals is price; these plants will initially cost more and will need extended time to grow and blossom to their full potential. If a mixed perennial and/or woody ornamental container is the goal, plan ahead to give plants the necessary space so that they can thrive at their mature size. This could be where using annuals comes into play. When there are spaces in a perennial and woody ornamental mixed container, annuals can be placed in the open spaces for fast color (Figure 4). Annuals can also be replaced seasonally and taken out completely once the perennials and woody ornamentals reach a mature size.

Texture

Plant texture is an important factor when deciding which plants to use in a mixed container. Plant texture refers to the size, shape and surface of plant foliage and flowers. Plant textures range from fine and delicate to harsh, coarse and bold. Additionally, plant texture is not just based on the physical feeling or visual appearance of a plant at one specific time. The texture of a plant constantly changes depending on the time of day, light, shadow, distance that the container will be viewed from and maturity of the plants. These factors will result in a constant flux in appearance that makes container gardening even more exciting, and yet challenging because balance must be achieved when combining plant textures. Too many fine-textured plants can be perceived as fuzzy or blurry, especially during twilight hours or from a distance. Plants that are too bold or coarse can appear overwhelming and abrasive, particularly when viewed closely. Contrast is needed between textures within containers or among closely spaced containers to create balance.

Figure 5. Fine-textured Liriope spicata,
violas and ferns are used in the shorter
container. Coarse-textured Aspidistra elatior
(right container) and Fatsia japonica and
Aucuba japonica (top container) are used in
the taller containers to provide a contrast in
texture and height. Figure 5. Fine-textured Liriope spicata, violas and ferns are used in the shorter container. Coarse-textured Aspidistra elatior (right container) and Fatsia japonica and Aucuba japonica (top container) are used in the taller containers to provide a contrast in texture and height.

There are a few simple things to remember when choosing plants based on their texture. Broad-leafed plants create boldness and add weight to a container. On the other hand, fine-textured plants, such as grasses or conifers, are soft and often invite closer inspection and tactile exploration of the foliage or flower(s) (Figure 5). This concept of attracting or directing a viewer based on texture can dictate where the container is placed in relation to structures, entrances or other focal points. If the container is placed near a doorway where it will be seen up close, for example, then a ratio of twothirds fine textured plants and one-third coarse textured plants is recommended to create a sense of welcoming. If the container is to be viewed from a distance, two-thirds of the plant materials should be broad and coarse, leaving one-third fine-textured.

Color

The use of color, be it from foliage or flower, mirrors that of texture in that it can be used to direct movement or attract attention (Figure 6). Most individuals perceive colors as either “cool” or “warm.” Cool colors, such as white, green and blue, are usually associated with water, sky and forest, and they evoke relaxed and calm feelings. Warm colors, such as red, orange and yellow, are associated with heat, fire and the sun. Because of this, they demand attention and evoke excitement. Purple can be either a cool or warm color, depending on what other colors appear adjacent to it in a landscape. When purple appears near blue, it is perceived as a cool color. When it is placed near red, purple is seen as a warm color.

Figure 6. In addition to a textural
contrast between the plants
in this container, there is a striking
color contrast between the Agave
americana (upright green plant),
Loropetalum chinense ‘Purple Pixie’
(weeping purple foliage plant) and
Spiraea japonica ‘Limemound’ (lime
green foliage plant). At the entrance
of this building, such a striking color
combination draws attention and
invites inspection. Figure 6. In addition to a textural contrast between the plants in this container, there is a striking color contrast between the Agave americana (upright green plant), Loropetalum chinense ‘Purple Pixie’ (weeping purple foliage plant) and Spiraea japonica ‘Limemound’ (lime green foliage plant). At the entrance of this building, such a striking color combination draws attention and invites inspection.

It may seem odd that colors can affect people’s moods, but this has long been studied by psychologists and is used in many fields, from interior design to packaging to clothing. For example, reds increase heart rate and passionate responses. Blue, on the other hand, reduces heart rate and evokes a sense of tranquility. Green is associated with a relaxed and refreshed mood, and is linked to new beginnings. Yellow is associated with happiness and increased mental clarity. In small doses, greys can magnify the effects of other colors and “bind” other colors into a harmonious whole. In large quantities, though, it could lead to a drab and tired-looking container. For a more detailed discussion on the uses of color, see UGA Cooperative Extension Bulletin 1396 Landscape Basics: Color Theory (Chappell et. al., 2011).

Plant Growth in Containers

Plant Density and Growth Vigor

Using the correct plant density in mixed containers with perennials and woody plants is critical for success because these plants will remain in the container for extended periods. Also, because perennial plants (both herbaceous and woody) will take two or more seasons to reach mature size, it is recommended to include annual plants in the container to achieve a “full” look.

Similar to planting in a landscape, plants should always be evaluated by their mature size and growth vigor before being selected for use. Plants should always have the mature size indicated on the label, so this is something to look for unless you are already familiar with a particular plant. The field of ornamental horticulture is so dynamic that new cultivars of popular species may be introduced each year, with new growth habits that are different from related plants. An example is Mexican petunia ‘Katie’ (Ruellia brittoniana cultivar ‘Katie’), which grows to a maximum height of 6 to 8 inches compared to the species Mexican petunia (Ruellia brittoniana), which grows to 3 feet in height with equal spread.

Customers may want to learn names of the plants in the mixed planting; nurserymen could provide a larger and more durable plaque with plant names as a value-added feature and as a way to advertise their business. It is a good idea to keep the label and/or pot (with printed signage) that the plant came in so you can refer to it later if the need arises. When unsure about a plant variety it is best to find out as much information about the plant as possible by asking retail center staff or paying close attention to online descriptions. Most reputable online retailers offer helpful advice about various uses of plants, including container culture. It is always better to be too conservative than to end up with a plant that will either outgrow or be outcompeted by the other plants. Overgrown or unbalanced plants can create a lot of frustration for a consumer who has invested so much time and money in a failed container. For the nursery owner, marketing unbalanced mixed containers can cause customer dissatisfaction that would need to be corrected or risk losing the customer altogether.

It is especially important to recognize that plants vary in their growth vigor. Some are slow growers; others are fast growers. If planted side by side the latter can outgrow the former and literally push it out. However, these growth habits can be used to one’s advantage: fast growers should be combined with other fast growers to quickly fill out the pot and successfully compete with each other. Some species exhibit aggressive growth within the first season and can rapidly outcompete slower-growing species (e.g., canna); others may initially grow slowly but form a large clump called a colony (e.g., Mexican petunia) that eventually may outcompete slower-growing plants. One way to keep aggressively growing species in check is to plant them in their original container (or a slightly larger one) – in effect, double-potting.

Other benefits of fast-growing plants include quick establishment and relatively low maintenance (at least initially), which makes them more forgiving of less-than-optimal growing conditions. In fact, many of these plants are recommended for novice consumers due to the higher success rate.

As a customer selecting plants to build a mixed container, a good starting point is to use the two-thirds principle. In other words, select plants that fill the surface area of the container at two thirds their mature spacing. This would be particularly applicable for annual plants that have relatively short life spans with lots of growth. At two-thirds spacing, the plants will be slightly crowded at maturity, forming a very dense arrangement, but not so crowded as to cause detrimental competition. This concept can be accomplished in many ways, allowing the mixed container to take on a uniqueness limited only by the creativity of the gardener. As a general rule, plants with growing points closer together will have denser foliage, while plants with growing points more spaced out will be less dense.

Plant Height and Container Balance

Figure 7. The use of height in mixed planters
can effectively create a mature garden appearance. Figure 7. The use of height in mixed planters can effectively create a mature garden appearance.

Plant height is arguably the most important factor when creating a mixed container (Figure 7). A standard method of container gardening uses a system of placing tall plants (including woody ornamentals) in the back, medium-sized plants (including woody ornamentals and perennials) in the middle, and shorter annuals and perennials in the front. This method of arranging the tallest plants at the back and stepping down gradually to smaller plants is great for corner spaces or against a flat surface. There has also been a shift towards using the “spiller, thriller and filler” concept with one large central plant in the middle of the pot (thriller), several smaller plants around the base of the thriller plant (filler), and several small plants that flow over the rim of the pot (spiller). In instances where the container will be viewed from all sides, this technique can be simple and effective.

The creative and adventurous home gardener should not be afraid of using a more random approach to adding unusual or unbalanced height, texture or symmetry to mixed containers. For example, an asymmetric container balance can bracket a doorway, inviting people into a designated area or business. Additionally, one could use various inanimate objects to add height and interest to mixed containers. Dried woody stems, driftwood, rocks and other various objects (e.g., garden art, statues, birdhouses, welcome signs, flags, etc.) can be used in place of plants to create both height and texture. Garden art can be replaced throughout the seasons and used to commemorate holidays and special celebrations.

The bottom line is that creative use of balance and objects in mixed container gardens can spice up an otherwise monotonous container planting, particularly in areas where container gardening is common (e.g., apartments, patios, commercial entrances).

Cultural and Maintenance Requirements

Cultural Requirements and Growing Environment

Figure 8. In these two mixed planters,
the shrubs all have similar
water and light requirements. (Left
to right) The Nandina domestica,
Aucuba japonica and Camellia japonica
all grow best in partial to full
shade environments with moderate
soil moisture (but not wet soil).
By grouping these plants together,
watering and other maintenance is
less confusing for the gardener. Figure 8. In these two mixed planters, the shrubs all have similar water and light requirements. (Left to right) The Nandina domestica, Aucuba japonica and Camellia japonica all grow best in partial to full shade environments with moderate soil moisture (but not wet soil). By grouping these plants together, watering and other maintenance is less confusing for the gardener.

Regardless of the type of plants selected, it is imperative to not only provide the optimal growing conditions for individual species but also to combine species with similar light and moisture requirements in the same container (Figure 8, Tables 1 and 2). Pay particular attention to sun exposure and moisture, as nothing will stress plants more than not having adequate light and water. Stressed plants will not only underperform in terms of growth and flowering but also become susceptible to pests and diseases. The all-important principle “right plant in the right place” should be practiced in mixed containers. As a rule of thumb, low-light plants (also referred to as “shade-loving”) should be combined with other low-light plants (e.g., impatiens, hosta and aucuba). Conversely, full-sun plants should be combined with other full-sun plants (e.g., petunia, whirling butterflies and Knock-Out® rose).

Location of the mixed container will also determine what type of plants to include. If it will be in full sun, one must adhere to the similar light requirements rule. On the other hand, if the container will be shaded at least part of the day, more flexibility is allowed in plant selection.

During the hot summer, the “kind” of sun also becomes a factor. As experienced gardeners well know, morning sun is cooler than afternoon sun. Hence, one can push the envelope by using a low-light plant in a mixed planting with a highlight plant if that plant would receive only morning sun and if the plant would receive optimum moisture. In effect, the shade-loving plant may be stressed but that stress can be minimized with more frequent irrigation.

Maintenance of Mixed Containers

Nutrition and Water. If the plants selected for a mixed container are planted at the correct density and selected for the proper exposure (i.e., sun versus shade), maintenance will be minimal. The most important task when caring for a container is irrigation. Newly planted containers typically only need watering every two to five days, depending on the weather, with more water required during hot days with low humidity. After a few months, when the plants have become established, containers will likely need watering on a daily basis during active growing periods (spring to fall) and once per week in dormant or slow-growing periods (winter). How much water is needed will be determined by the size of the container, but as a general rule, irrigate until water starts coming out of the bottom of the container. At that point watering more will only waste water and actually wash fertilizer out of the pot.

Fertilization is the next key to maintenance. Controlled-release fertilizers are recommended for mixed containers, particularly those containing shrubs and trees. Controlled-release fertilizers are designed to release fertilizers over a much longer period of time than water-soluble (liquid) fertilizers or conventional fertilizers such as granular 10-10-10. For example, by fertilizing in spring with a controlled-release fertilizer that lasts 12 months, only one application should be needed per year. If a six-month fertilizer is selected, you should use this product in April and June. Fertilization is not recommended after August because plant dormancy can be delayed, putting plants at risk for major cold damage during the winter.

Pruning. Pruning may be required in mixed containers. Pruning is used to shape plants and to remove leaves or branches that are competing for space. Unlike watering and fertilizing, pruning is a personal choice based on individual aesthetics and desires. However, several basic rules for pruning exist. First, only prune spent flowers (after the bloom) on spring-flowering shrubs AFTER they bloom. Pruning these plants before they bloom means you are removing that year’s flowers. Second, prune summer- and fall-blooming shrubs and trees (that bloom in July or later) during the winter, as they will set blossoms on new growth the following year. On shrubs and trees that do not bloom, only prune in January and February. Last, use pruning to remove dead or diseased tissue, but disinfect your tools between plants to prevent the spread of disease from one plant to another. The last maintenance task is optional, which is to protect your container from cold weather during winter months.

When selecting plants for a mixed container, refer to the “USDA hardiness zone” for each species or variety (http://planthardiness.ars.usda.gov/PHZMWeb/). This is a number that ranges from five to nine for most plants marketed in Georgia, with lower numbers being more cold hardy and higher numbers being less cold hardy.

Plants grown in a container are exposed to colder root temperatures than those grown in-ground, so it may be necessary to protect containers that have plants with hardiness numbers of eight or higher. To protect containerized plants, they can be moved directly next to a home or other heated structure, mounded with leaves or other mulch to the top of the container or brought inside (e.g., an unheated garage) when temperatures drop below 20° Fahrenheit (-6.7° Celsius). However, do not bring plants into a heated structure for the entire winter as they need cold weather to properly grow and flower the following year.

In summary, gardening with mixed containers provides opportunities for all levels of gardeners to experience success with differing plant materials, container types and plant types and is an excellent method of adding visual interest to draw the eye, direct traffic or provide focal points for both landscapes and buildings.>

References

Chappell, M., B. Davis and B. Pennisi. 2011. Landscape Basics: Color Theory. University of Georgia Cooperative Extension Bulletin 1396.

Silk, S. 2004. Thrillers, Fillers & Spillers. Fine Gardening 97: 44-47.

Thomas, P. 2012. Flowering Annuals for Georgia Gardens. University of Georgia Cooperative Extension Bulletin 954.

Thomas, P., G. Wade and B. Pennisi. 2012. Flowering Bulbs for Georgia Gardens. University of Georgia Cooperative Extension Bulletin 918.

Tables

Click here to download Table 1. "Selected perennials that lend themselves to container gardening" and Table 2. "Shrubs and small trees that lend themselves to container gardening."

Published on Mar 21, 2013