The reasons for growing plants in containers are many: it often provides the sole means for growing herbs or vegetables when tending a garden is simply not possible, or when the available ground is contaminated. Containers can also help to create a mini-microclimate, such as well-drained soil, moist soil or even a water garden. Containers may allow for a habitat adjustment, for instance — growing an arid plant under cover in a climate that receives more rainfall than the plant would tolerate, or providing ample shade on a porch when the little shade is available in the garden or surrounding landscape. Growing plants in a vessel can also prolong the growing season by allowing for portability –- a plant may be moved indoors or to a protected location when the temperatures dip below the plant’s cold tolerance.
In addition, many containers are works of art and can provide accents in the landscape, creating a variety of textures and colors and differing plant heights. The portability of container-grown plants can help fill in bare spots in the garden during seasonal changes. Many plants will quickly overwhelm the garden with their innate exuberance— containers can be used to keep them in more modest proportions. Finally, people in wheelchairs or with special needs around mobility can more easily access larger containers, such as elevated raised beds in wood frames.
Plants preferring wetter soil: Containers can help to hold in moisture and can be used to create a moister microclimate for herbs who appreciate the damper side of life. Avoid terra cotta with such plants as the clay wicks away moisture and the pots dry out quickly. The larger the vessel, the easier it is to maintain moisture. If applicable, use a saucer and apply mulch to hold in moisture. Compost, clay, and peat can be added to the soil to increase water retention. You may consider hooking up a roof water catchment system directly to the containers of these water-loving herbs.
Gotu kola (Centella Asiatica, Apiaceae), calamus (Acorus calamus, Acoraceae), skullcap (Scutellaria lateriflora, Lamiaceae), yerba mansa (Anemopsis californica, Saururaceae), boneset (Eupatorium perfoliatum, Asteraceae), vervain (Verbena hastata, Verbenaceae), meadowsweet (Filipendula ulmaria, Rosaceae), yellowroot (Xanthorhiza simplicissima, Ranunculaceae), and nettles (Urtica dioica, Urticaceae) are herbs with a real appreciation for wet feet. Depending on your climate and garden habitats, containers can help provide the extra moisture needed for their survival.
Plants preferring well-drained soil: Consider adding very coarse sand and/ or pine bark fines (see notes below) to increase the drainage of your soil. Perlite is also an option, although it is less sustainable. Water only when the soil dries out. If you are in a climate with extra rainfall and humidity, you may want to keep these pots out of the rain’s reach. I keep arid climate plants in a sunny spot on my porch or in my greenhouse. Be careful to only water the soil and avoid watering the foliage, as these plants are more susceptible to fungal diseases. Mahuang (Ephedra sinica, Ephedraceae), prickly pear (Opuntia spp., Cactaceae), rosemary (Rosmarinus officinalis, Lamiaceae), lemon verbena (Aloysia citrodora, Verbenaceae), garden sage (Salvia officinalis, Lamiaceae) and white sage (Salvia apiana, Lamiaceae) are a few medicinal plants that appreciate drier soils.
Plants preferring shade/part shade: If you are trying to grow temperate or cool-weather plants in an especially hot climate, providing afternoon shade will often keep the plant happier. If you don’t have shade in your garden, you can grow many woodland medicinals in containers on a shaded porch, deck, or even in the shade of your house. Gotu kola (Centella Asiatica, Apiaceae), Jiaogulan (Gynostemma Penta phylum, Curcubitaceae), wild geranium (Geranium maculatum, Geraniaceae), black cohosh (Actaea racemosa, Ranunculaceae), blue cohosh (Caulophyllum thalictroides, Berberidaceae), and goldenseal (Hydrastis Canadensis, Ranunculaceae), are some of the plants I have seen grow well under shade in containers.
Sensitive herbs: Cold-sensitive perennial herbs can be grown outdoors in a pot during the warmer months and then brought indoors or to a sheltered location when the weather chills. Many of these plants will go dormant during the winter, and can be overwintered in a basement, attic or warmer sheltered spot. In contrast, some of these hot climate plants will require sunlight during the winter: place them in a greenhouse or in front of a south-facing window. This is a lovely option for those of you who live in climates where it freezes infrequently. Lemon verbena (Aloysia citrodora, Verbenaceae), white sage (Salvia apiana, Lamiaceae), lemongrass (Cymbopogon spp., Poaceae), gotu kola (Centella asiatica, Apiaceae), ginger (Zingiber officinale, Zingiberaceae), turmeric (Curcuma longa, Zingiberaceae), bay laurel (Laurus nobilis, Lauraceae), Aloe vera (Aloe spp., Xanthorrhoeaceae) and Citrus (Citrus spp., Rutaceae) are some of the tenders that you may choose to baby a bit.
Retired ceramic bathtubs, sinks, and toilets: These are some of the greenest container options available as they are often destined for the landfill or available for cheap. They are long-lived, resistant to cracking from temperature fluctuations, and often provide a sizeable growing area that holds moisture quite effectively.
Plastic pots: Black nursery pots or plastic buckets are often cheap or free, and lighter than other pots, with an increased ease of portability. The downside of plastic vessels includes their shorter lifespan and the heavy environmental toll involved in their creation. Often landscapers throw away large pots, so reuse helps keep them out of landfills. I have concerns about the possibility of chemicals leaching from the plastic and entering the plants via the soil substrate. Many of the chemicals used in the manufacture of plastics are powerful endocrine disruptors. This is an issue that has kept me up at night with worry about the possible contamination of medicinal herbs grown in plastic containers in my own nursery.
Terracotta: Terracotta, or clay pots, wick away moisture quickly so they need to be watered more frequently than pots made from other materials. These pots will also crack and break with the expansion and contraction of freezing soil, so they need to be emptied and protected from moisture during the winter in temperate climates. Terra cotta pots can sometimes be found used but the new pots are typically imported from Asia. However, there are a few domestic producers still around, but their pots are typically more expensive and not widely available.
Glazed ceramic: These pots hold moisture more effectively than terra cotta pots, and are less likely to crack during the winter. They also provide beauty with a variety of colors and textures. The downside of glazed ceramic includes increased weight and the fact that most are produced abroad, with the attendant environmental costs of transportation and possibly unfair working conditions. However, some purveyors of fair-trade goods offer these types of pots.
Recycled “what have you” vessels: Consider reusing olive oil, large old cans, and coffee tins as containers, but see the above comments on potential chemical contamination; these metal food vessels typically have an inner plastic lining.
Fiber pots: These have become more popular lately, and include pots made from peat, manure, and excess pulp. These containers will typically last one season or less, but have the advantage of being compostable upon retirement, Most of these are manufactured abroad; inquire about the production location.
Wooden containers: Many nurseries sell old wine and whiskey barrels, which offer a considerable planting area and are less likely to dry out than smaller containers. Large raised beds with wooden forms also fall into this category; these are easy to access for folks in wheelchairs or who can’t bend easily. Rot-resistant sustainably harvested wood, such as cedar, cypress, white oak, osage orange, and black locust will greatly increase the longevity of these containers. Often the rough ends or slab ends of logs, sourced from local sawmills, are more affordable than standard straight edged lumber. Avoid pressure-treated wood as its toxins leak into the soil and accumulate in the food or medicine being grown.
Drainage is a big issue in container culture as soil compaction more easily occurs. Add a 12-inch layer of crisscrossed sticks to the bottom of larger pots before adding soil and elevate the container from its saucer or the ground with bricks or large rocks.
Typically garden soil or topsoil is too heavy for container culture, but it can be used with amendments such as pine bark fines, composted leaf mold, aged fluffy manure, and worm castings. Pine bark fines are the ground-up bark of pines and are a byproduct of the pulp and lumber pine industry; they offer porosity and water retention and have a neutral pH. Three parts pine bark fines, one part composted manure, and a touch of lime, organic fertilizer, mycorrhizal inoculant and worm castings makes a wonderful all-purpose soil mix for containers. Please see the previous notes about amending soil mixes to meet a specific plant’s requirements. It is possible to reuse soil for multiples seasons. I take out the top fourth of soil, compost it, and mix in compost, worm castings and a touch of lime to the remaining soil. This annual treatment usually spruces up the soil sufficiently for it to be used for two to three seasons.