There are two kinds of ceramic plant containers: Pots made of terra-cotta, which is Italian for "baked earth," are reddish-brown and made of porous, unglazed clay. Glazed ceramic containers are kiln-fired at higher temperatures with a surface covered with a glasslike glaze. Sand or gravel mixed with cement and water bonds to form a rocklike substance called concrete that can be molded into plant containers.
Terra cotta is porous, making it difficult to give plants too much water. Plant roots need air; the natural porosity of terra cotta helps them breathe. The clay helps insulate roots from extreme heat during the summer. They are inexpensive, and they have what many consider to be a rustic, natural charm.
Terra cotta cracks and breaks easily. The larger ones are heavy. The porosity of terra cotta is a natural breeding ground for bacteria and fungi; they are more difficult to clean with the passing of time.
Use stiff brushes or fine steel to scrub small and medium-sized pots. After cleaning, bake them at 220 degrees Fahrenheit for an hour, then cool slowly until they reach room temperature. They can also be cleaned with bleach and rinsed well, or cleaned with white vinegar and run through a dishwasher. Pots left in the shade may develop a damp green film of algae that can be cleaned at the end of the growing season.
Glazed Ceramic Advantages
The glassy surface of glazed ceramic pots makes them waterproof. They last longer than terra-cotta pots, and they do not crack or break as easily. They provide good insulation for plant roots in summer heat. They are easy to clean; if they are thoughtfully chosen, the designs and colors of glazed pots can make an attractive addition to flower gardens.
Glazed Ceramic Disadvantages
Glazed ceramic pots cost more than terra cotta pots. Their glazed surface means they hold more water than terra cotta. If a glazed pot does not have a drainage hole, add one; place potting mixtures atop 2 inches of pebbles or rocks. Drain or pour off standing water regularly. Some people believe that colorful glazes on pots distract from the natural beauty of foliage and flowers in a garden; they prefer earthy terra-cotta exteriors.
Concrete Container Advantages
Concrete containers protect plants and soil from extreme heat and cold. Large concrete containers are strong enough to handle the roots of shrubs and trees. They're durable, making them a good choice for parks, schools, plazas and other public locations. Some people like what they regard as the "natural" look of concrete, and use them beside curbs, driveways and at entrances.
Concrete Container Disadvantages
Concrete planters are heavy. They're not good for moving around in a garden or changing locations.