Container Gardening II: What to Grow & How

• By Christine L. Appert
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• June 2019-Vol.5 No.6
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• 0 Comments

Last month we talked about choosing containers, preparing your soil and siting your containers.  Now it’s time to start planting.

Deciding what to plant and developing a scheme are unquestionably the most appealing aspects of container gardening. The options seem almost endless! Even after you narrow your options to a category of botanicals, shrubs/small trees, herbs, or vegetables, your decisions may still involve some thought and planning depending on your space, time, budget, and aspirations. This article provides an overview of some basic concepts to get started. If you are feeling overwhelmed with all the possibilities or need more inspiration, consult with online resources and library books. The Jefferson Madison Library has a collection of materials devoted to container gardening with abundant pictures and design suggestions.

Botanicals and Ornamentals

Young Japanese maple. These trees can survive in containers outside if given proper care. Photo: Cliff, lic.CC-2.0.

Small trees and shrubs such as a Japanese maple (Acer palmatum) or yucca (Yucca filamentosa, ‘Golden Sword’) can live successfully in a well-insulated container and remain outside through the winter. Some varieties of roses can also be cultivated in containers. Select a hybrid that is compact and relatively small (expected height of 24 to 30 inches) and disease-resistant (especially to black-spot). Large tropical plants and small citrus trees can spend the summer outside in containers, but will need to come inside for the colder months.

Grouping flowers with ornamental grasses and foliage creates an attractive container garden. Choosing plants with similar light, temperature, and water requirements contributes to the health and maintenance of the container’s contents. Use your creativity and imagination to incorporate color, texture, proportion, and shape into the presentation. A “thriller, filler, spiller” design is one concept to try. Place one plant in the center to ‘thrill’ the eye and draw attention, ‘fill’ the planter with an accenting color, and pick one or two plants to ‘spill’ over the sides to soften the appearance.

A “Thriller, Filler & Spiller” combination. Photo: Darryl Mitchell

A Thriller, Filler, Spiller for Shade or Partial Sun   Pictured at left is a recently-planted flamboyant centerpiece with a colorful coleus ‘Twist and Twirl’ (Plectranthus scutellarioides), contrasted with dahlias (Dahlia pinnata) which will bloom in vibrant shades of yellow, and the thin leaves of two ‘Silver Sand’ cushion bushes (Calocephalus brownii),  surrounded by ‘Silver Falls’ dichondra (Dichondra argentea) for a showy display of delicate leaves flowing out of a large container. 

One approach is to exchange plants as the seasons progress. In the early spring, containers are filled with pansies, violas, primulas, lobelia, dianthus, snapdragons, daffodils, and tulip bulbs. Summer is heralded with lively hues and the necessity to work with heat-tolerant plants. For instance, SunPatiens® flourish in sun, heat, and humidity. Marigolds, zinnias, geraniums, petunias, miniature sunflowers, and dahlias brighten containers through fall. Tropical plants, such as caladium, banana, mandevilla, and canna bulbs will explode with color and foliage during the summer but require some TLC if you want to try and preserve them through the winter. Favorite choices for the fall season include mums, calendulas, pansies, and ornamental kales.

Vegetables

Urban dwellers and gardeners seeking a way to create a pocket-sized vegetable garden may find containers offer a flexible solution. The appeal of fresh-picked produce has contributed to the development of specially-bred “pot-friendly” tomatoes, eggplants, cucumbers, and other vegetables. These container-specific varieties are not dwarf or miniature; instead, they are usually more compact and capable of producing a plentiful harvest.

Guidelines for planting vegetables in a container are similar to those established for a garden bed. Popular choices include tomatoes, lettuces of all types, collard greens, leafy greens, peppers, and eggplant. Cucumbers, sweet potatoes, and other plants with vines can do well as long as they have room to sprawl with support. Once you choose what to plant, attention should be given to the planting schedule, container selection and preparation, spacing of plants, light, water and nutrition requirements, and maintenance.

Peas coming up in a ceramic container. Photo: Katie Schumm, NC-ND 2.0.

For instance, if you decide to grow tomatoes, you will want to select a variety that thrives in a container. Determinate variety tomatoes — such as ‘Celebrity’, ‘Tasmanian Chocolate Organic’ or ‘Plum Regal’ — are characteristically compact and bushy. Indeterminate hybrid varieties, such as ‘Firecracker Hybrid,’ offer options for other tastes. A single tomato plant requires a five-gallon planter placed to get eight hours of sun a day. Timing for planting seeds or starters should be followed with consideration for the plants’ susceptibility to extreme high temperatures — over 95º F.  Container tomatoes require supports, frequent watering, persistent fertilization, and vigilant monitoring for pests and diseases.

To help you get started, the chart below shows the minimum container size and spacing needed for planting vegetables. Consult the Virginia Cooperative Extension website at https://ext.vt.edu/ or one of the references at the conclusion of this article for specific growing information for each type of plant.

           Minimum Container Size and Spacing Needed for Vegetables in Containers

Excerpted from:

North Carolina Extension Gardener Handbook, North Carolina State University,https://content.ces.ncsu.edu/extension-gardener-handbook/18-plants-grown-in-containers#section_heading_8772. 

Herbs

A window box brimming with an array of flavorful culinary herbs, such as chives, Italian parsley, and thyme is an irresistible addition to any kitchen garden. Some herbs are natural companions for vegetables or botanicals. For example, it seems that basil, planted with tomatoes, acts as a deterrent for white flies, tomato hornworms, aphids, mosquitoes, houseflies, and asparagus beetles. If you are working with children, herbs offer an enticing way to introduce the dual pleasures of gardening and cooking while appealing to their senses.

Basil, thyme, and rosemary do exceptionally well in containers.

Herbs requiring well-drained soil and tender plants needing to be overwintered indoors are particularly suitable for container gardening. Among the most commonly recommended and grown are variegated sage, purple or golden sage, parsley, Greek oregano, rosemary, marjoram, basil, thyme, chives, and summer savory. Also, chamomile, lemon balm, feverfew, lavender, and a variety of mint flavors can be grown for tea or medicinal purposes. Growing mint in the confines of a container can be helpful even if you could plant it in the ground. The container will keep it from spreading and overtaking the garden bed.

Typically, herbs thrive in sun or part shade. They can be started inside and moved outside relatively early in the spring. Soil management is critical. Herbs do not tolerate squelchy conditions and some plants, such as rosemary, prefer a coarse, sandy-textured medium. Snipping sprigs encourages growth and a continued source of leaves to harvest and enjoy

Container Care

Once the containers are planted, you will have the joy and fulfillment of watching them flourish and produce. Of course, this does not come without an investment of time and effort in maintaining the well-being of the plants. Following some simple procedures will yield satisfying results!

Watering

Unlike a garden bed that has a natural watershed, a container dries out quickly. Clay or terracotta receptacles absorb moisture from the soil and small pots require watering more frequently. Numerous gadgets to test moisture levels are available; however, your finger is really the best tool. Put your finger in the top inch of soil to determine if it is wet or dry and observe the foliage for severe wilting (needs water) or yellowing leaves (overwatering). It is best to water early in the day and before plants show signs of stress from drought. Keep the foliage dry and apply enough water each time so the whole soil ball becomes moist. Water should start to drip out of the base holes. As emphasized earlier, drainage is crucial. Elevating the container with feet will increase ventilation and prevent the pot from sitting in a pool of water.

Specifically, how much and how often to water depends on several variables:

• location of the container
• time of year
• weather conditions (how much has it rained?)
• length of time container has been planted (stage of plant growth)
• type of container
• variety of plants and soil medium

A wide variety  of watering apparatuses is available to keep containers irrigated. A watering can or garden hose is the most familiar and commonly used. Garden hoses can be accessorized with a wand to assist with hanging planters or hard-to-reach pots. If an outside water source is not available, a lightweight, expanding/collapsible hose can be fitted with an adapter and attached to an inside sink, then pulled outside or to a balcony. Self-watering planters and drip irrigation systems are other options but require monitoring to ensure that overwatering does not occur.

Fertilizing

Leaching, when water removes soluble elements from the soil, reduces the concentration of essential nutrients, such as nitrogen and phosphorous. Rain and frequent watering quickly deplete plants of the nutrition vital for growth. A regular schedule of fertilization will reinforce plant health and longevity by enriching the soil and replacing nutrients.

Slow-release fertilizer consists of a water-soluble product that is encased in a semi-permeable resin coating. When the pellets come in contact with water, small amounts of nutrients are released to the soil for use by the plant. Notably, the time that they last is based on temperature (usually 70º F.). The warmer it is, the faster they release nutrients. A four-to-five-month pelleted fertilizer may only last two months if the temperatures are above 85º F. Some potting mixes come amended with slow-release fertilizer; otherwise, gardeners often add it themselves during planting.

Another option is water-soluble liquid fertilizer. It is mixed with water and applied approximately every two weeks. Frequency can be adjusted based on the overall appearance and growth of the plants. Water-soluble fertilizers have minimal temperature dependence. The choice of fertilizer analysis depends on the kinds of plants growing. For example, high-nitrogen sources would be good for plants grown for their foliage, while flowering and vegetable crops benefit from lower nitrogen and higher phosphorous type products, e.g., 9-15-30. If organic fertilizers are preferred, consider a combination of fish emulsion, kelp meal/extract, greensand, and bone meal. Kelp extracts and meal are very advantageous as they are good sources of micronutrients.

Supporting

Foliage and botanicals with vines, top-heavy flowers, plants with weak stems, and some vegetables require assistance to remain upright or disentangled. Tall plants situated in areas that are subject to heavy rain or high winds may also benefit from supports. If a support is needed, it should be anchored firmly into the soil at the time of planting.

Stakes provide a simple basic support system. They are available in several lengths and made from many materials (bamboo, wood, plastic, and wire). No matter what type is used, stakes should not be prominent or higher than the plant. Twist ties or string can be used to secure the plant to the stake. Care should be taken not to damage the stem by pinching or choking when securing the tie. Additional ties should be added as the plant gains height.

Tomato plants using a wire support system in a wooden planter. Photo: Neefer Duir, CC BY-SA 2.0.

Adjustable metal hoops, “grow-throughs,” cages, towers, grids, and trellises provide more customized configurations. Garden suppliers offer creative unobtrusive solutions or ornamental alternatives to embellish the garden decor. Do-it-yourself possibilities are available from Pinterest. Ultimately, the choice of a support should be based on the anticipated growth habits and vigor of the plant.

Grooming and Pruning

Clearing containers of dead leaves, spent blooms, and other debris is not the most glamorous aspect of container gardening; however, it is a building block of horticulture maintenance. Constant high humidity next to moist soil on warm days, and lack of air movement around closely-set plants, coupled with decaying plant material, is a recipe for trouble. Conditions in unkempt containers are favorable for the spread of fungal diseases, such as botrytis (gray mold). Regular surveillance and clean-up serve to keep the garden’s gems looking and growing at their best.

Pruning promotes healthy plant development and can impact the façade of the container. Pinching and deadheading are techniques routinely used during the growing season. Cutting back is done at different times of the year depending on the purpose.

• Pinching: Snip off the growing tip of the stem using a fingertip. New lateral stems will grow from the stem that was pinched and it will then set buds. This procedure can improve the shape and appearance by encouraging a more compact, bushier plant and reduce tall, leggy spurs.
• Deadheading: Remove old flowers by clipping off the spent blooms. This practice extends the flowering season by stimulating plants to continue blooming. Self-cleaning annuals and perennials that drop their flowers as they fade and do not need deadheading include: Wax Begonia (Begonia semperflorens), Coleus (Coleus blumei),  Edging Lobelia (Lobelia erinus), Asteromoea, (Asteromoea mongolica), Coreopsis (Coreopsis verticillata), and Phlox (Phlox divaricata).
• Cutting back: Prune down from the top or crown with a significant cut, or to the soil level for declining perennials or ornamentals. This technique is usually accomplished during the dormant period and promotes vigorous new growth once the next growing season ensues. Over the spring and summer, flowering perennials and annuals may benefit from cutting back. Perennials have varying prime times. If executed at the right point, this method can prevent plants from becoming woody or lanky. Similarly, annuals can be rejuvenated in mid-summer by cutting back to a stem of three to five inches with only four or five leaf nodes. Petunias (Petunia x hybrida) and impatiens (Impatiens balsamina) respond particularly well to this intervention.

Petunias respond well to careful pruning and cutting back.

Overwintering

In our climate, many plants cannot survive outside in containers during the winter months. The particular conditions of the microclimate, the protection provided by the pot, and the specific plants will determine if shelter is needed for the winter. Damage from the cold can easily occur since the temperature of the container is the same as the air temperature. As the most vulnerable part of the plant, the root system is susceptible to freezing and not as well-insulated as it would be in the ground.

A rule of thumb for selecting plants that might manage the winter is to consider those that are designated for two or fewer USDA hardiness zones below where you live. For the Charlottesville-Albemarle area in Zone 7a, that would be plants assigned to hardiness Zone 5 or lower. Hardy perennials (chrysanthemums or pansies), cold-weather vegetables (kale or collards), a few herbs (rosemary or sage) and most non-tropical shrubs and trees should be able to survive most winters. Move containers to a protected area or near a building and consider an additional layer by wrapping with a frost blanket, quilt batting, or household insulation covered with plastic (to avoid soaking during wet weather). Shredded bark mulch can help regulate soil moisture. Alternatively, cut branches from evergreens, like spruces, pines, and arborvitae, can be placed into the soil all around the base of the plant for added protection. If the temperatures go above freezing, keep the plants watered to help protect against frost damage.

When temperatures drop, plants can sit inside on a sunny window sill.

Tropical plants, many perennials, and most herbs need to move inside for the winter. It is best to begin the process gradually before the heat is turned on inside and well before the before the first frost. With a grow-light or a sunny window location and adequate humidity, you can continue to harvest some herbs. Generally, plants do best in an environment away from the drying air of heating ducts and cold drafts from outside doors or windows. Depending on the specific cultural requirement of the plant, a basement or garage may be the ideal winter abode for dormancy.

Prevention Strategies

As the saying goes, “An ounce of prevention is worth a pound of care.” The combined time, planning, effort, and materials put into a garden warrant guardianship of your investment. A proactive approach will contribute to sustained success and reduce issues associated with disease and pests.

Practice Good Hygiene

Containers, especially large-sized pots, do not always need to be cleaned before re-potting The top soil should be refreshed and amended with fertilizer. If there is evidence of plant disease or if a plant dies, the pot should be completely emptied. Scrub the container thoroughly with soap and water and rinse with a solution of ten percent bleach before reusing.

Survey Containers Regularly

Check the containers, soil, and plants daily. Look for insects, disease, or symptoms of overwatering/dryness. Promptly remove any leaves and stems that show signs of a problem. Examine the soil and area around the base of the plant (collar) for signs of root rot or other fungal disease.

A yucca plant suffering from root rot and overwatering.

Container-grown plants are vulnerable to many of the pests and diseases that occur in a garden bed. As soon as you spot a problem, try to determine a diagnosis and implement an intervention. If you are unsure, have questions, or are puzzled with a challenge, get help! An invaluable local resource is the Piedmont Master Gardeners Horticulture Help Desk. For personalized assistance, call (434-872-4583) Monday – Friday, 9:00 a.m. – 12:00 p.m. or e-mail (albemarlevcehelpdesk@vt.edu) and attach pictures with a description of your observation.  Go to https://piedmontmastergardeners.org/gardening-questions/ for more information about the Horticulture Help Desk and to learn about Mobile Help Desk locations and dates.

Your Best Garden Ever

Whether you start with one simple unit or expand to a collection, make the most of your container experience this season. Follow the steps for preparation, plant selection, maintenance, and prevention for a robust and dynamic garden.  It could be your best!

Resources

“The Art of Container Gardening,” Pennsylvania State University Extension, https://extension.psu.edu/the-art-of-container-gardening

“General Recommendations for Growing Vegetables in Containers,” Pennsylvania State University Extension,  https://extension.psu.edu/general-recommendations-for-growing-vegetables-in-containers

Container Gardening: How to Select, Plant, and Maintain, Illinois Cooperative Extension, https://extension.illinois.edu/containergardening/welcome.cfm

“Container Gardening Planting Calendar for Edibles in the Piedmont,” North Carolina State University Extension. https://content.ces.ncsu.edu/container-garden-planting-calendar-for-edibles-in-the-piedmont

“Container Gardening,” University of Missouri Extension, AgrAbility Project, http://agrability.missouri.edu/gardenweb/Container.html?_ga=2.118169870.608999211.1552242785-1564651425.1552242785

“Success with Container Production of Twelve Herb Species,” North Carolina State University Extension, https://content.ces.ncsu.edu/success-with-container-production-of-twelve-herb-species

“Common Problems in Container Gardens,” University of Maine Cooperative Extension Publication 2765, https://extension.umaine.edu/publications/2765e/

“Growing Vegetables and Flowers in Containers,” Pennsylvania State University Extension, https://extension.psu.edu/growing-vegetables-and-flowers-in-containers

National Gardening Association Learning Library, “Annuals and Perennials for Containers”, https://garden.org/learn/articles/view/2161/?utm_source=nl&utm_medium=mail&utm_campaign=nl_2019-03-02

“Container and Raised Bed Gardening,” Virginia Cooperative Extension Publication 426-020, http://pubs.ext.vt.edu/426/426-336/426-336.html

North Carolina Extension Gardener Handbook, Chap. 18: “Plants Grown in Containers”, North Carolina State Extension, https://content.ces.ncsu.edu/extension-gardener-handbook/18-plants-grown-in-containers#section_heading_8769

Relf, Diane, “Vegetable Gardening in Containers,” Virginia Cooperative Extension Publication 426-336, http://pubs.ext.vt.edu/426/426-336/426-336.html