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Photo: Decorating flowers in a pot. Photo: Hanging flower bed. Photo: Basket with mixed flowers.

Cooperative Extension Fact Sheet FS1215

Outdoor Container Gardening with Flowering and Foliage Plants

  • Renee Perst, Master Gardener, Hunterdon County
  • Bruce Crawford, Program Leader, Home and Public Horticulture
  • Joseph Gyurian, Horticultural Consultant, Somerset County

Introduction

Any site with ample light and accessibility for watering is perfect for container gardening with flowers and foliage plants. Many plants that can be grown in a garden can be grown in a container. Container gardening is ideal for anyone with limited gardening space or an inability to tend to a traditional garden. Container gardening allows for creativity in a small area. It can bring a garden of any size to the doorway, balcony, terrace, rooftop, windowsill, or practically any outdoor location.

Design

When creating a mixed flower and/or foliage planter, the key is to select plants that will all thrive with similar sun, temperature, and nutrient requirements. Choose your location, and then match the plants to it. Be adventurous. Experiment with different combinations. It is possible to mix annuals, perennials, vegetables, houseplants, herbs, and grasses into your creation, as long as their cultural requirements are similar. Avoid choosing slow growing and vigorous plants for the same container, as one will overpower the other by the end of the growing season. If possible, select plants that are easy to care for and will perform well over a long period. Flowering plants are typically the group of plants that come to mind first, but plants with colorful or architectural foliage are equally as important and often provide a longer period of interest.

Scale is a very important factor to consider when designing. A tall plant will need a container with a wide base for stability, while a cascading plant will need a pot high enough for it to drape over the sides. In fact, designing a container is very similar to designing a home garden, just on a smaller scale and with an eye more towards color and texture. Design themes can be based on using a common color, a specific combination of colors, or simply a repetition of one single plant. All these themes can create a beautiful display when paired with a container whose color compliments that of the plants. Color catches the eye and grabs your attention. Blues, violets and greens soothe and are cool and serene. Reds, oranges and yellows create warmth and brightness. The use of contrasting textures can also create a wonderful container. In the image at right photographed at Longwood Gardens, the use of the upright arching foliage of the Bromeliad (Aechmea chantinii ‘Harvey's Pride’) presents a nice contrast to the slender flowing foliage of the Mexican Feather Grass (Nassella tenuissima).

A container can be designed within itself if it is sufficiently large. However, a more creative design often results with several pots laid out together with a cohesive planting design that bridges all the containers (as pictured below at Longwood Gardens). Consider at least three pots of different heights and diameter to provide an attractive layered appearance. Just as with a garden, repetition of plants among the containers is essential for a unified and relaxing appearance. Obviously, the layout of the plants and pots will also depend upon the angle from which the pots will be viewed or if they are to be viewed from all directions.

The same concept of repetition is true for window boxes as well, since all the boxes should sport the same combination of plants, although the order that they are arranged can vary. For instance, all pink begonias with cascading ivy or trailing Torenia, regularly spaced, would create a very simple yet pleasing effect when repeated throughout all the windows of a dwelling.

A common and very successful design recipe for containers is called "thrillers, fillers, and spillers." The thriller serves as the focal point for the container; the spillers are plants that trail over and soften the edges, while also serving to lead your eye down to lower containers; the filler forms the body of the container, often highlighting the thriller. In some instances, such as in the image of the Bromeliad and Mexican Feather Grass, a plant can serve as both the filler and the spiller.

If you are creating a composition of several containers, use one or possibly two thriller plants among all the containers. Just as in a home landscape, too many feature plants can make the composition ‘nervous.’ Typically, the thriller would go into the largest of the containers, and various fillers and spillers would be repeated throughout the remaining vessels. However, gardening is supposed to be fun and there is no ‘law’ that says you cannot mix it up with perhaps the thriller going into a broad, low container! Resist the urge to place the thriller in the center of the container and to ring it with plants. By placing it off-center, there is more room at the front for fillers and spillers and then the design won’t magnify the symmetry of the container.

Good thriller plants have dramatic bold foliage, like Banana (Musa species) or spear-like foliage as the previously mentioned Harvey’s Pride Bromeliad or Stripped False Agave (Furcraea foetida ‘Mediopicta’). Filler plants often have a rounded form such as Lantana or Dragon Wing Begonia, and spillers like Fan Flower (Scaevola) or Sweet Potato Vine (Ipomea) have long trailing stems. The possibilities are endless and experimenting with these ideas will allow you to create a unique and personally-inspired container.

Containers

Many items can be used to grow plants as long as they will hold enough soil and provide necessary drainage. This is a chance to be really creative. An old leaky watering can, tubs, crates, buckets, toys, baskets, or an old boot are just some ideas. Any water-tight container could hold a small water or bog garden. The traditional terracotta clay, plastic, resin, metal, or untreated wood containers also make wonderful choices.

Whatever the choice, always start with a clean container. If using a previously used pot, wash it first with a solution of one part household bleach to nine parts water. (For example, one-half cup of bleach to 4.5 cups of water.) Since bleach is caustic, wear rubber gloves and avoid spilling on your skin. Bleach also emits harmful fumes, so mix the solution outdoors or in a well-ventilated room.

Most beginning gardeners underestimate the size of the container needed. Be sure to keep in mind the mature size of the plants and their growing habits. A small pot will look completely out of scale as plants fill out and mature, and the watering needs will be challenging. There should be enough room in the planter for at least 3 inches or more of soil under the root balls and for the final soil level to be about 1–2 inches from the top edge to allow for efficient watering.

Remember that containers have different properties. Terra cotta or metal pots dry out very quickly in the summer heat, while glazed ceramic, plastic or fiber, do not. Thus, a terracotta pot might be a better choice for shade, while ceramic or fiber might be better for sun. The type of plant is also a consideration in choosing a container. Cacti and succulents herald from drier desert areas and perform much better in porous terra cotta pots while those from the rainforest adjust easily to the additional moisture held in ceramic pots.

If you are growing a hardy perennial in a pot, such as Hosta, place the pot in a blanket of leaves or mulch in a shaded location for the winter to reduce the impact of freezing and thawing on the container. It could also be placed in an unheated garage and given a light watering every two weeks or so. Finally, always consider the weight of the finished container. Plastic containers tend to be lighter than ceramic or clay pots. Placing a heavy pot on casters is a good idea, allowing for easy positioning and moving.

In all cases, adequate drainage is a must. Plant roots exchange gases as part of their life cycle so plants need air around the roots. When the water does not drain readily from the container, it continually fills the soil air pores and the roots cannot "breathe" or exchange gases. The roots will eventually die from a lack of oxygen. Adding more holes to any pot that does not drain quickly will help, especially if they are added to the sides of the container along the bottom, so that water flows away from the container. Never allow the container to stand in water. Raising a pot off the ground with specially designed ‘pot feet’ or even bricks will help most drainage problems and allow needed air circulation as well.

Soil

A good container soil drains freely, is lightweight, and holds adequate moisture. Most gardeners find that pre-packaged "soil-less" potting mix works best. Typical potting mix components may include peat moss, vermiculite, perlite, bark, coir fiber (ground coconut hulls) and polystyrene, but not natural soil from the ground. The disadvantage of a potting mix is that its porous nature allows it to dry out quickly. The addition of packaged pasteurized compost to the soil-less mix at a ratio of one part compost to three parts mix will provide a low level of slow release fertilizer and allow better water retention. Though helpful, adding compost is not required. Soil from the garden is never recommended. It is too dense, compacts easily and will not allow proper drainage and aeration. It may also introduce unwanted elements, such as fungi, soil-borne insects, and weed seeds to your container.

Always thoroughly moisten the potting mix before placing it in the container for planting. Place the potting mix in a separate large tub-like container. Gradually add small amounts of water while stirring the soil to thoroughly moisten the mixture. When finished, the mix should feel uniformly moist but not be saturated and should readily crumble in your hand. The organic compounds in the mix absorb water and expand, and when there is adequate space, retain air pores among the soil particles. This moistening procedure before placing the potting mix in the planting container retains the air porosity in the soil that plant roots require.

Planting

Fill the planting container with the moistened mix to within 1–2 inches of the top. There is a common misconception that placing gravel, small rocks or other large grained material in the bottom of the container improves drainage. Actually, better drainage through capillary action is achieved by maintaining an even soil particle size from top to bottom. To preserve the air spaces, avoid compacting the potting mix when placing it in the planting container. Plants are placed much closer in containers than when planted in the ground—with an 18" diameter pot accommodating up to three plants.

When all plants are added, distribute soil so that all plant root crowns are consistent with the level of the container soil. Avoid sunken or protruding root balls. Add additional soil if needed, pat down lightly, and water. The soil level will drop a bit when the plants are watered in and settled, but this will be minimal if the soil was properly pre-moistened.

Watering

Proper watering is essential. Plants in containers dry out much more quickly than plants in the ground. Never underestimate how much a large planter, filled with mature plants, can "drink" on a hot day! However, too much water can be just as fatal as not enough, especially if the soil is poorly drained. Watering is tricky because it is very weather-dependent. Do not water by the calendar as several days may pass during a cool wet period when no water is required, only to be followed by hot, dry days when the containers will need water 1–2 times per day. The amount of sun, soil volume, size, type, and location of the planter all will determine when water is needed. Often, the exposed soil surface may be dry but the roots, just inches below, are not.

Moisture meters, available at most garden centers, are a great help when deciding when water is needed, however once you understand how plants visually communicate drought stress, they really are not needed. Especially handy for hanging baskets and smaller containers that are prone to drying out more quickly are water retention crystals. They are packaged as a dry, granular material that is mixed with the soil prior to planting according to manufacturer’s specifications. Once hydrated, it expands and holds water for the plants to slowly absorb. Once the crystals are added to the mix, they are effective for one year.

Fertilizing

Proper fertilizing is another key element to keeping your plants looking their best. Potting mixes can vary widely in their composition and nutrient content so it is up to the gardener to thoroughly read and understand the label. Because container plants need frequent watering, existing nutrients are washed away with every watering. The addition of slow-release fertilizer (various blends are available) is easy and practically foolproof. Mixed into the soil at planting time or scratched into the soil surface, this product will fertilize your plants for an extended period of time, but not all season, since the rate of release increases with warm temperatures and adequate moisture. By late summer, your containers may start to look ‘hungry’ and require an additional application of slow release fertilizer or a water-soluble fertilizer (15-30-15) that is applied once every two weeks while watering. It is important to remember that when it comes to fertilizing, more is never better!

Light

Understanding the terms used to refer to the amount of light, such as "full sun" or "partial shade," is critical in deciding where your plants will grow well. Many survive in places where they do not receive the proper lighting, but if they receive less light than required, they will become etiolated (stretched out, with poor form), fail to bloom, and become easily diseased.

If using a container to brighten a dark or very shaded area of the garden, have two of the same planters and rotate them weekly. Provide the proper light requirements during the off week.

Maintenance

As the plants grow and mature, cut back and remove faded flowers, a process called deadheading, to retain a pleasing size and shape. It will make the plant look better, promote a more compact growth habit, and encourage the plants to bloom longer.

Create your own vision. Container gardening with flowers is an easy, adventurous, flexible, transportable, and compact way to indulge your green thumb. The only limit is your imagination.

Useful Plants for Containers

Key:
S Sun
PS Partial Sun
SH Shade
PSH Partial Shade
A Annual
P Perennial

Foliage Plants

Thrillers:

  1. Agave species (Century Plant), (A, S)
  2. Colocasia (Elephant Ears), (A, S to PS)
  3. Cordyline, Dracaena (often called Spikes), (A, S to PS)
  4. Furcraea foetida 'Mediopicta' (Stripped False Agave), (A, S to PS)
  5. Musa (Banana), (T, S to PS)
  6. Phormium tenax (New Zealand Flax), (A, S to PS)

Fillers:

  1. Begonia Rex-cultorum (Rex Begonia), (A, PSH)
  2. Hosta (Hosta), (P, PS to SH)
  3. Ornamental Grasses
    1. Carex buchananii (Buchanan’s Sedge), (A, S)
    2. Nassella tenuissima (Mexican Feather Grass), (A, S)
  4. Plectranthus scutellarioides (Coleus), (A, S)
  5. Plectranthus argentatus (Silver Spur Flower), (A, PS to S)
  6. Senecio cineraria (Dusty Miller), (A or P, S)
  7. Strobilanthes dyerianus (Persian Shield), (A, S to PS)

Spillers:

  1. Dichondra argentea (Silver Falls), (A, S)
  2. Hedera (English Ivy), (A or P, S to SH)
  3. Heuchera (Coral Bells), (P, PS to PSH)
  4. Ipomoea batatas (Sweet Potato Vine), (A, S to PSH)
  5. Lamium maculatum (Spotted Dead Nettle), (P, PSH to SH)
  6. Lysimachia nummularia (Creeping Jenny), (P, S to SH)
  7. Muehlenbeckia (Creeping Wire Vine), (A, S to PS)
  8. Plectranthus coleoides (Swedish Ivy), (A, PS, PSH)
  9. Vinca major (Perwinkle), (P, S to PSH)

Flowering Plants

Thrillers:

  1. Abutilon x hybridum (Flowering Maple), (A, PS to PSH)
  2. Canna (Canna hybrids), (A, S to PS)
  3. Senna didymobotrya (Popcorn Plant), (A, S)
  4. Tibouchina heteromalla (Princess Flower), (A, S)
  5. Tibouchina urvilleana (Glory Bush), (A, S)

Fillers:

  1. Angelonia angustifolia (Angelonia), (A, S to PS)
  2. Begonia (Begonia hybrids), (A, S to PSH)
  3. Bidens ferulifolia (Fern-leaved Beggarticks), (A, S to PS)
  4. Heliotropium arborescens (Heliotrope), (A, S to PS)
  5. Impatiens (Impatiens hybrids), (A, PS TO PSH)
  6. Impatiens hawkeri (New Guinea Impatiens), (A, S to PS)
  7. Lantana (Lantana hybrids), (A, S to PS)
  8. Pelargonium (Geranium), (A, S)
  9. Torenia fournieri (Wishbone Flower), (A, PS to PSH)

Spillers:

  1. Bacopa monnieri (Bacopa), (A, S)
  2. Calibrachoa (Million Bells), (A, S to PS)
  3. Dorotheanthus bellidiformis Mezoo™ (Mezoo™ Trailing Red Iceplant), (A, S to PS)
  4. Lobularia maritima (Sweet Allyssum), (A, S)
  5. Petunia (Petunia hybrid), (A, S to PS)
  6. Scaevola aemula (Fan Flower), (A, S to PS)
  7. Torenia fournieri (Wishbone Flower), (A, PS to PSH)

Selected References

May 2020