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Gardening in Containers

by Jane C. Martin, Extension Agent, Horticulture    

Many gardeners do not have the space or energy to garden on a grand scale. They may be limited by the boundaries of their property, or limited by time and energy. But, they can still have a garden that is colorful, productive, and adds to the enjoyment of life. Why not garden in containers? Many containers can function as suitable gardens with slight modification; a simple hanging basket, window boxes in an assortment of styles and materials, an old clay pot to an even older and more decrepit army boot, a worn out bathtub or a discarded toilet - you are limited only by your imagination and creativity!

Choosing a Container

Containers are available in a wide range of materials and styles to match the style of the house, garden, or planting scheme. These containers can range from expensive jardinieres to old, cast-off containers that were stashed and forgotten in the attic. Explore the possibilities; scour garden centers and pottery shops for potential container candidates, but also explore your property for items that might serve as unique containers.

Consider these characteristics of containers to make sure they are suitable for your garden:

• Non-porous or porous - Glazed pots, plastic, metal, and glass containers are non-porous and hold moisture longer, but are not as "natural" as some other materials. Clay pots are porous and will lose moisture quickly in dry weather. Wooden window boxes, planters, and baskets are often lined with plastic sheeting to protect the wood, and thereby become non-porous.
• Drainage holes - Every container should drain to ensure the health of the plant(s). If holes don' t exist in a container' s bottom, drill a few.
• Susceptibility to frost damage - Some containers can be left outdoors over winter and will not be damaged, while most clay pots will crack or flake.
• Weight - The material from which the container is made and the amount of soil it can hold determine the container' s weight. Some newer pots are made of polystyrene, and are very lightweight and easy to handle and move. Heavy pots may be moved around or kept indefinitely on a wheeled platform.

Container Size

Keep plants in proportion to the container, and the container in proportion to the location. Larger containers are usually placed on the ground for aesthetics, practicality of weight, and mobility. Smaller ones can be placed in raised positions. Also consider ease of plant care; soil nutrients and water are used up more quickly in smaller containers and must be maintained more often than larger containers.

Accessories

Interesting and functional accessories can be used in containers, including:

• Trellises - to support vining or tall growing plants - can be made from twigs, grapevines, bamboo
• Topiary frames - can be fashioned from trellis, old baby gates, or fencing
• "Pot feet" - used to raise pots off the surface allowing for good drainage; usually made of clay

Other Aesthetic Considerations

Old, weathered, metal containers take on a verdigris effect over time; this is similar to the bluish-green appearance that copper acquires when weathered. You can purchase verdigris paint kits in craft stores to paint metal containers, or make your own by using white acrylic paint tinted with brown and green art acrylic colors. Make sure you seal the surface with an acrylic varnish to protect the painted surface. Experiment a little to achieve the look you want.

Clay pots can be weathered artificially by painting them with a thick layer of plain yogurt and leaving them outside to dry for 7 to 10 days before planting. Pots will gradually acquire a greenish patina.

Container Potting Mixes

Because containers offer limited space for roots to thrive, the gardener must supply the best conditions possible. Potting mixes should have the proper structure to provide air and hold water, drain properly to prevent water-logging, and be heavy enough to support the plant.

Typical homemade mixes are made by combining equal parts of packaged "potting soil," sphagnum peat moss, and perlite. If the peat moss is dry, it is a good idea to moisten it before incorporating it in the mix. Do this by soaking it in a bucket of warm water, then squeeze out the excess moisture by hand. Avoid the temptation to fill the pots with native clay soil from the garden because it drains poorly, is heavy, and compacts over time. Native soil can be added as an amendment in some situations, such as to add weight to larger containers.

Potting Soil and Soilless Mixes

Commercially available potting mixes and soilless mixes are usually made up of different organic ingredients including varying proportions of sphagnum peat moss, shredded bark, and sawdust, as well as mineral ingredients such as vermiculite, perlite, calcined clay, and sand. These are typically lightweight and weed and disease free. Many different brands and types of mixes are offered in garden stores, and quality varies widely. There is no regulation by state or federal agencies over the content of these mixes and there are no standards that define what constitutes a good potting mix.

Each ingredient in a soilless mix is chosen to provide a certain characteristic.

1. Peat moss is acidic and holds water very well. It is usually fibrous, brown, and sometimes chunky.
2. Ground bark is cheaper than peat moss and is a by-product of the timber industry. It does not hold water and nutrients as well as peat moss, but may provide better aeration.
3. Vermiculite is derived from a mica-like mineral that is heat-treated to form flakes 20 times larger than their original thickness. It provides some nutrients. It also holds water and added nutrients, which are then readily available to the plant.
4. Perlite is a granite-like volcanic material, crushed and heat-treated to pop into white, hardened particles. It loosens the mix and provides air space.

Mixes for larger containers are rather coarse blends of finely ground and composted fir or pine bark and peat moss. For shrubs and small trees grown in containers, 5% to 10% soil or loam is usually added to support the heavier plant and provide a heavier, more stable container. Dry native soil can be sifted through 1/4-inch mesh and mixed with this potting mix.

Evaluating Potting Mixes

An easy way to evaluate a potting soil mix is to wet it, plant in it, and then try watering the planted container. A good mix should wet relatively easily, and should not compact excessively when wet or when pressed into the container. And of course, it should hold water without water-logging. There should be no large particles over 1/4 inch in size, and a minimum of fine particles under 1/16 inch.

A simple test for the porosity of a potting mix is to add two cups of water to one quart of dry mix. After a few minutes you should be able to drain off about one cup of water, which would give you the ideal mix volume of one-half particles, one-quarter water, and one-quarter air space. An easy test for drainage and porosity is to fill a six-inch pot with mix, tapping lightly on the container while filling it. Pour one quart of water through the pot. The water should filter through in less than a minute.

Additions to Soilless Mixes

Fertilizer is the most obvious addition to potting mix. Use either a water-soluble fertilizer at regular watering intervals, or add a time-released fertilizer such as Osmocote®, to the mix at planting.

• Vermiculite and perlite may also be added to potting mix to add volume without weight.
• Calcined clay is a rigid, odorless mineral that resembles cat litter. Succulents and alpines that require excellent drainage can be grown more successfully if one part calcined clay is added to two parts native soil. If incorporated in large enough quantities, calcined clay particles can keep a mix loose and aerated allowing a deep, sturdy, and healthy plant root structure to form.
• Hydrogels (hydrophilic polymers) are hard, crystal-like polymers that absorb water. In theory, this absorbed water is then slowly made available to plant roots to prevent or delay water stress. Individual particles will absorb between 60 and 400 times their dry weight in water, depending on the specific type.

Caution is advised if you are incorporating hydrogels into the potting mix. Hydrogels are known to break down in the presence of fertilizer salts, so you will probably increase their usefulness by using only slow-release fertilizers and irrigation water low in soluble salts. Also, do not use more than the manufacturer' s recommended rate per container or area; plants have actually popped out of the soil and containers in wet seasons or under heavy irrigation because there was too much hydrogel in the mix.

Plants for Containers

Woody Plants

In general, select woody trees and shrubs that grow slowly, are smaller or dwarf in habit, and are hardy to zone 4 (to -30 degrees F). Slow growers reduce the need for pruning and frequent repotting. Examples might include Japanese maples (Acer palmatum), Kousa dogwood (Cornus kousa), star magnolia (Magnolia virginiana), and Swiss stone pine (Pinus cembra). There are numerous small evergreen and deciduous shrubs also suitable for containers.

Ornamental Grasses

Certain grasses are well suited to container culture, notably those with arching habit and those that manifest a degree of symmetry. A few that are recommended include:

• Carex siderosticta 'Variegata' - Creeping Sedge
• Carex buchananii - Leather leaf sedge
• Helictrotrichon sempervirens - Blue oat grass
• Imperata cylindrica 'Red Baron' - Japanese blood grass
• Pennisetum - Fountain grasses
• Festuca - Fescues
• Miscanthus - Maiden grasses (for larger containers)

Perennials

In general, perennials can be grown in containers if the root systems can be protected over winter. Choose smaller species and those that are not invasive in nature.

Annuals

There are numerous annuals suitable for container culture. They are often mixed in containers, so make sure they are compatible when it comes to light exposure and watering requirements.

Bulbs

Most hardy bulbs grown in containers are "forced." The gardener can pot bulbs in late fall, give them a cold treatment for 12 to 14 weeks, then gradually acclimate them to room temperature and enjoy the blooms indoors in mid to late winter. Check catalogs or your garden center for those suitable for forcing.

There are a few hardy lilies that do well in containers for summer color, including Lilium auratum, L. formosanum, L. longiflorum, L. pumilum 'Black Dragon' , 'Cambridge' , 'Dukat' , and 'Stargazer' , as well as Fiesta hybrids and plants of the Jamboree strain.

Vegetables and Herbs

There are many space-saving vegetables suitable for container culture, and new cultivars are becoming available all the time. Look for "bush" type plants and those that are prolific producers. Check current vegetable seed catalogs for varieties and cultivars that do well in containers and give them a try. Many herbs lend themselves to growing in containers. The smaller herbs, such as thyme, chives, and oregano can be grown grouped in a container, while the larger growing shrubby types, such as rosemary and bay, are best grown singly in a container.

Container Maintenance

Installation

Steps in planting containers are relatively easy. Make sure the container is clean before planting, and soak clay pots in water for a few minutes before planting. Place a small stone, shard of pottery, or coffee filter paper over the pot' s drainage hole to keep the potting mix in. The bottom few inches of large pots can be filled with crushed aluminum cans, plastic milk jugs, or Styrofoam® peanuts to reduce weight and the amount of potting mix needed.

Prepare your potting mix by adding amendments as desired, hydrogels, and slow-release fertilizers. Fill the container with this mix. Planting is then easy. Install plants just as you would plant in the ground. Water the container thoroughly after planting. Planting and watering should cause the media level to drop to one-half inch below the rim.

Maintenance

Once planted, watering will be your most frequent maintenance chore, especially if you are growing plants in clay containers. On hot, sunny days small containers may need watering twice. Water completely so that water drains through the drainage hole and runs off. Water early in the day.

If you incorporated a slow release fertilizer into the potting mix, you may not need to fertilize the rest of the season; some of these fertilizers last up to nine months. You can also use a water-soluble fertilizer and apply it according to the label directions during the season.

Mulch can be applied over the container mix to conserve moisture and moderate summer temperatures. Apply about one inch deep.

Depending on the plants you are growing, you will need to deadhead and prune as needed through the season. Monitor frequently for pests such as spider mites. Pests usually build up rapidly in containers.

End of Season Chores

At the end of the growing season after a killing frost, annual flowers and vegetables can be removed from their containers. It is recommended that the potting soil be added to the compost pile or to native garden soil and not reused in containers the following season. This is because the mix may be harboring fungal pathogens and because it loses its structure over the season with repeated waterings.

Containers should be cleaned at the end of the season to rid them of pathogens before storing. Clay and plastic pots are usually scrubbed with a 10% solution of chlorine bleach in water. Store clay containers indoors over winter where temperatures will not fall below freezing. Because clay is porous, it will hold moisture, and can crack when water in the pot freezes and expands. Non-porous pots can be stored outside safely along with the newer polyurethane-elastomer pots.

Overwintering Plants

There are a few options for overwintering large, hardy plants in containers, which are usually left outside. The challenge is that roots are above ground where they are subject to cold temperatures. Very large concrete containers for woody plants would perhaps provide optimal conditions since soil temperatures would be moderated; using "ordinary" size containers may be asking for trouble. Use a soil thermometer to monitor winter temperatures, making sure soil temperatures do not fall below freezing. If they do, move containers to a garage or shed temporarily for added protection.

Other plants, such as perennials, ornamental grasses, roses, bulbs, etc., that are in smaller pots, can be placed in a trench in the ground and covered with mulch or soil to protect the root systems. Water well into late fall before covering. If plants are in clay pots that could crack over winter, remove the plant from the pot and place the rootball into the trench. If trenching is not an option, group pots together, water well, then cover with a thick layer of straw, compost, mulch, or leaves to protect the root systems against freezing.