One of the most beautiful container types for planting is clay—terra cotta—pots. These planters, available in a huge range of various sizes and designs, are visually interesting and provide an excellent home for many plants. Made of baked earth (in Italian=terra cotta), clay has been in use for containers, bricks, pipes, roof tile, and sculpture for thousands of years in both Eastern and Western cultures. One of the most astonishing uses of terra cotta is the collection of 8099 funerary statues of warriors discovered in China in 1974.
There are a number of advantages to using terra cotta. Clay is completely natural. It's porous so it's nearly impossible to overwater plants ... something many gardeners learn to be wary of when using plastic or resin containers. Clay tends to protect above-ground root systems from extremes in temperature too, which is especially important during the warmer summer months. Its porosity contributes to breathability of the container, so roots (which need air) can breath and remain healthy.
Of course clay pots are heavy and subject to cracking and breaking. This sad situation means that it makes sense to invest in good quality pots, then take care of them so you can enjoy them for many years. One downside is that their porosity can make them difficult to clean from season to season, so pathogens can affect your plants if you don't sterilize them.
Terra cotta pots are like any other crockery. They break when dropped and often crack or flake when exposed to repeated freeze-thaw cycles in the winter.
Remove all loose debris and dirt from your pots first. You can use a dish scrubber, fine steel wool, or a stiff brush.
Cleaning pots from year to year prevents passing fungi, bacteria, or viruses. One method, albeit stinky, is to bake your flower pots in the oven at 220°F for an hour or so. That will kill anything still residing on your pots. This method works well with small and medium sized pots. Let them cool slowly ... don't attempt to move them until they are at room temperature. This can put your oven out of commission for quite a while though if you have a lot of pots to do. And it doesn't work for really large pots.
Method Two is to use 1:10 ratio of bleach to water, then soak your pots for about 30 minutes. Rinse well. If you let the pot sit for a couple days and dry out before planting, the remaining bleach residue largely dissipates. Bleach does weaken materials over time, so make sure you work with a very dilute solution. More is not better. Some gardeners clean their pots as well as they can, then run them through the dishwasher with white vinegar.
Because clay is porous, the salts in fertilizers pass through the pot's walls and accummulate on the outside creating a hard white crust. To clean the crust off and restore your pot to its original pristine exterior, make a paste of baking soda and scrub with a soft brush. Fine steel wool also works.
If you notice a damp, green film on pots left in the shade, you have a fine crop of algae. It doesn't hurt the plant so leave it alone. You can clean it at the end of the growing season.
The first step for winterizing clay pots is to empty them of contents (including soil), clean, then store in a shed or garage. Store pots upside down or on their sides, preferably without stacking them. (If you stack, they can get stuck and you could break them trying to separate them.)
For empty, over-sized pots that are too big to move, clean as well as you can, then wrap with layers of bubble wrap. Cover to prevent accumulating water that could collect inside, freeze, and then crack your container.
For large plants in huge planters, you will obviously not be repotting them every year or so, let alone storing them indoors unless you have a greenhouse. To prevent terra cotta planters from falling apart, drainage is critical. Use pot feet to raise the pot off the ground to prevent cracking. "Frost Proof" pots are often safe only in areas where the climate is relatively mild (Florida, Texas, Arizona, or Southern California) or when you've invested in the highest quality pots which have been hand built of exceptional clay and fired at high temperatures (like Impruneta). Such pots (see Seibert & Rice for an example) are beautiful but also very expensive.
To overwinter pots with plants, place them somewhere slightly sheltered—a corner or under an overhang with a southern exposure would be ideal. Mass them together and when the weather turns really wicked, insulate with bubble wrap or something similar to buffer the root system as well as the pot.
Terra cotta pots are made all over the world. The finest pots are made in Italy, but other perfectly serviceable pots are made in Germany, Mexico, China, and the US.
The quality of clay (as with any pottery) is critical to the final quality of the pot. Other factors include construction and the temperature at which the pots are fired. You can tell something of a terra cotta container's quality and durability by rapping it with a knuckle while holding the rim (don't rap it out of your hand or the test is moot). If it says "thud" when you hit it, it's fired at a low temperature. If it "rings", it's probably fired at higher temperatures and more likely to be durable.
Most common are hand-turned or molded pots. Hand-turned pots are made individually by potters and come in a variety of shapes and sizes. As with any craft that is labor intensive, the cost of such pots is relatively high compared to the the molded pots that are mass produced in factories.
Molded pots are less likely to be frost proof and are often made of variable quality clay. Different manufacturers fire at different temperatures. Coarse, low-fired clay pots can disintegrate in four or five years. Others may contain resins or have finishes that supposedly prevent wear or cracking, but should you decide to fire an additional glaze (as some ceramics students have been known to do) the pot could melt in the kiln.
Understanding your clay pots is the first step to protecting them from winter weather, so with proper care, you can enjoy them for many seasons.