Container Gardening

Materials: Each type of container has merits and disadvantages:

• Clay or terra-cotta containers are attractive but breakable and are easily damaged by freezing and thawing. In Northern areas, most need to be stored in a frost-free location to prevent cracking and are not suitable for hardy perennials or shrubs that will be kept outdoors year-round.
• Cast concrete is long-lasting and comes in a range of sizes and styles. These can be left outside in all weather. You can even make attractive ones yourself. Plain concrete containers are very heavy, so they are difficult to move and not suitable for using on decks or balconies. Concrete mixed with vermiculite or perlite, or concrete and fiberglass blends, are much lighter.
• Plastic and fiberglass pots and planters are lightweight, relatively inexpensive, and available in many sizes and shapes. Choose sturdy and somewhat flexible containers and avoid thin, stiff ones—they become brittle with cold or age.
• Containers made of polyurethane foam weigh up to 90 percent less than terra cotta or concrete containers, yet they look remarkably like their much-heavier cousins. Polyurethane foam containers resist chipping and cracking and also can insulate roots against both hot and cold temperatures, making them a good choice for potting up plants that will stay outside year-round.
• Wood is natural looking and protects roots from rapid temperature swings. You can build wooden planters yourself. Choose a naturally rot-resistant wood such as cedar or locust, or use pine treated with a nontoxic preservative. (Don't use creosote, which is toxic to plants.) Molded wood-fiber containers are sturdy and inexpensive.
• Metals are strong, but they conduct heat, exposing roots to rapid temperature fluctuations. Metal must be lined with plastic for growing edibles.

Preparing Your Containers

Since containers are heavy once they are filled with soil, decide where they will be located and move them into position before filling and planting. If keeping them watered during the day is a problem, look for sites that receive morning sun and are shaded during the hottest part of the day, even if you are growing plants for full sun. Afternoon shade will reduce the amount of moisture plants need.

While your containers must have drainage holes, it's not necessary to cover the holes with pot shards or gravel before you add potting mix. The covering won't improve drainage, and pot shards may actually block the holes. Instead, prevent soil from washing out by placing a layer of paper towel or newspaper over the holes before adding mix. If your container is too deep, you can put a layer of gravel or Styrofoam in the bottom to reduce the amount of potting soil required.

Plain garden soil is too dense for container plantings. For containers up to 1 gallon in size, use a houseplant soil mixture; see the Houseplants entry for a recipe. For larger containers, use a relatively coarse soilless planting mixture to maintain the needed water and air balance. Buy a commercial planting mix or make your own from equal parts of compost, pulverized pine or fir bark, and perlite or vermiculite. For each cubic foot of mix add 4 ounces of dolomitic limestone, 1 pound of rock phosphate or colloidal phosphate, 4 ounces of greensand, 1 pound of granite dust, and 2 ounces of blood meal.

Many of the components of potting soil are lightweight, dust-producing materials that can irritate your eyes, skin, and lungs. In particular, vermiculite can contain low levels of asbestos; compost and peat moss can contain mold spores. When you mix potting soil, observe the following precautions:

• Work outdoors or in a well-ventilated garage or garden shed.
• Wear a dust mask.
• Dampen individual ingredients before mixing them together to minimize the amount of dust released.
• When you're finished, wash your hands thoroughly. If you've been working with vermiculite, be aware that the dust can cling to your clothing. Remove and wash dusty clothing as soon as possible to avoid dispersing asbestos inside your house.

You may want to mix in one of the special super-absorbent polymers—synthetic substances that hold large amounts of water available for plants. They will improve water availability without making the soil soggy. While these products are not naturally occurring substances, they appear to be inert and to have no toxic breakdown products.

Premoisten soil either by watering it before you fill containers or by flooding the containers with water several times and stirring. Be sure the soil is uniformly moist before planting.

Plant in containers as you would in the garden. If you are planting a mixed container, ignore spacing requirements and plant densely; you will need to prune plants once they fill in. For trees and shrubs, trim off any circling roots and cover the root ball to the same level as it was set at the nursery. Firm the planter mixture gently and settle by watering thoroughly. Don't fill pots level to the top with soil mixture—leave space for watering.