Materials: Each type of container has merits and disadvantages:
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:
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.