The rise in interest in container gardening comes as people expand their use of outdoor gardens as living spaces. They are applying the concepts of interior design to their "garden rooms," using plants much like they would use furniture, paint and art. Plants and their containers add color and texture and define the function of spaces. Container gardening is also a wonderful way for people with disabilities to enjoy gardening, because the garden can come to the person.
In this era of experimentation, there are few rules for container gardening beyond those necessary to the plants themselves -- an appropriate container that holds the right kind of soil, retains an appropriate amount of moisture and allows the right amount of light to reach the plant.
Empty Vessels -- The concept of "container" has gone far beyond the traditional clay or ceramic pots many people generally think of. Granted, they are a mainstay, but not the only option. All kinds of "found" things can be used, including cast-off and recycled containers made of wood, metal and plastic. Odd cans and containers picked up at yard sales can make conversation pieces in their own right. Hollowed-out logs or rocks with cavities big enough to hold plants can add a naturalistic feel to the garden. And old wheelbarrows and other discarded tools have found new uses in the garden.
Once you find the right container, you will need to prepare it for the plants. Drainage is the biggest concern. Different types of plants have very different water requirements. Cacti and other succulents want soil that does not hold moisture and drains quickly. On the other hand, petunias and fuchsia, two quintessential container plants, need a pot that will hold enough moisture to keep them going even on the hottest summer day. If a pot is porous, such as traditional terra cotta clay, it may be helpful to coat the inside of the pot with a sealer to help slow evaporation.
If the container is to be hung, weight is a major consideration as a large hanging basket, freshly watered, can weigh 30 pounds or more. For the hanging baskets, plastic pots or wire baskets lined with lightweight shredded sphagnum moss or coconut fiber and then soil balance the need for water retention with weight.
Most containers should have at least one drain hole in the bottom of the pot. If the pot has a broad flat bottom like a whiskey barrel, several holes may be necessary. The holes should be about a half-inch in diameter, big enough to allow water to drain but small enough to keep soil from washing out of the pot.
If it is not possible to make a hole in a pot, you can still improve drainage by putting a layer of gravel in the bottom of the pot to hold the excess water.
Down and Dirtless -- The type of "soil" you want to use will be determined by the plants you will be putting in the pots. You can even use a commercial or homemade "soil-less" potting mix made from milled sphagnum moss mixed with 20-grit sand, perlite or vermiculite and a dash of rich compost or fertilizer. The ratio of these elements can be changed depending on the water needs of the plants -- more sand for plants that like it dry and more moss for water-lovers. Do not use ordinary garden dirt, as it is usually too dense for containers. This reduces its ability to hold moisture and makes it hard for plant roots to expand throughout the pot. Garden dirt also harbors pests and diseases you do not want in your pot.
Fill 'em Up -- According to Sunset's Container Gardening, plants that make good candidates for container growing should have a naturally compact growth habit; long (or repeat) flowering season; attractive foliage; multiple interest (flowers, attractive foliage, berries or fall color).
Conversely, plants to avoid have scraggly growth habits; short flowering seasons; lackluster foliage; greedy, dense root systems; and water-guzzling demands.
The plants you choose for your container will depend on what look you are trying to achieve and whether they will be in sunny or shady spots. Annuals and bulbs are most often used for color and seasonal interest. Perennials, including shrubs, small trees and flowering perennials are often used as foundation plantings that provide long-term interest and structure. Mixing the three types of plants creates an ever-changing garden.
At the nursery, look for compact plants with good leaf color. The leaves should be perky, not limp or wilted. Avoid scraggly and crowded plants -- they rarely recover from this to look good. Look for well-branched large plants with no broken stems or damaged bark. Check for root-bound balls by gently popping plants out of their containers. Root-bound plants are much more difficult to turn into good-looking plants in a container. From the artist's point of view, select plants with different growth habits and a mix of leaf and flower sizes, textures and colors.
Staying Alive -- Using perennial shrubs, small trees and plants in containers here in the Inland Northwest can be a challenge. Our normally cold winters with their freeze-thaw cycles can easily kill even hardy plants when they are planted in pots above ground. If you use perennials, shrubs or small trees, you have several options for over-wintering the pots.
If you have a greenhouse or very bright sunroom, you can bring them inside to over-winter in relative comfort. Keep them on the cool side and be careful not to over-water them. Watch closely for diseases and pests. If your plants go dormant and lose their leaves, the pot can be over-wintered in a bright but cool garage or basement.
If the plant is too big to bring inside, you will need to find a way to minimize the freeze-thaw cycles. Try potting your plants in a separate container that is inserted into your large display pot. When fall comes, remove the inner pot and plant it in the ground. Another method is the "Michigan tip." Dig a hole big enough that your plant still in its pot can be laid on its side and the whole thing covered with soil and mulch. In either method, you are taking advantage of the insulating capacity of the soil. When spring comes, simply dig it up and get ready for the new season.