There are three main considerations when choosing a container for your container garden.
1. Drainage: The container must have adequate drainage so that air can get to the roots of your plants, unless you are planning on growing aquatic plants.
2. Volume: In general, the greater the container’s volume, the better; a big container is better.
3. Construction material: Involves both function and aesthetics
For the most part, there are four main options for a container plant container: pottery, metal, wood and plastic.
Pottery can be very attractive, but it has a number of significant downsides for the active urban farmer. For one thing, pottery is very heavy. A 2 1/2-gallon glazed pot might weigh 10 pounds empty and the soil will weigh another 20 pounds or so. Add a plant and water, and that’s a container garden you’ll rarely want to move.
Unglazed pottery, such as terra cotta, may be a bit lighter, but its porosity can cause significant water loss through evaporation. That can be OK for dryness-loving houseplants and herbs, but many vegetables are a thirsty bunch.
In addition, pottery is more breakable than you might want for an urban farm that you expect to be heavily trafficked. Ceramic pots are particularly susceptible to cracks in colder zones through freeze/thaw cycles. Metals are suitable for garden containers, from a physical perspective. Metal containers are strong, relatively light and not porous. From a chemical perspective, however, many metals can be problematic. Aluminum, copper and zinc can all be toxic to plants.
Copper looks great, can develop a nice verdigris, and is sometimes used in crop fungicides, but it can also be very toxic to plants — check out the ingredients of root killer.
Brass is comprised of copper and zinc, both dangerous.
Bronze contains copper and sometimes aluminum.
Galvanized steel contains zinc and can be contaminated with cadmium, which may be fine for plants but not so much for you. According to the Centers for Disease Control and Prevention, high doses of cadmium over a short period “severely irritates the stomach, leading to vomiting and diarrhea,” while low doses over a longer term can lead to cadmium buildup, kidney disease, lung damage and fragile bones. Whether or not a metal’s potential harm will actually be a problem depends upon a lot of factors. So unless you’re a chemist and know better, I’d recommend avoiding metal, with possible exception of stainless steel if you can find it.
Wood is relatively tough, light and cheap — all attractive qualities for containers for container gardens. It’s even organic — in the sense that it contains carbon and has been alive, if not in the sense that it grew without synthetic inputs.
The problem is that wood rots. You can buy treated wood or preserve it with chemicals yourself, but then you have to weigh the safety of the additional chemicals in your growing container against the value of prolonging a wood container’s useful life. Half-barrels are often charred to prevent rot, which is preferable to chemical additives if you’re planning on growing food.
Whatever kind of wood container you use, make sure it drains well.
Another solution is to line the inside of the wood containers with something impermeable, which takes us to the next option: plastics. You can even buy plastic liners for those half-barrels to make a fish pond or long-lasting planter.
Plastics are probably the most flexible of container materials for container gardens. They can be extremely light, exceptionally strong and have just about any appearance. Many, but not all, are recycled.
If you are going to use plastic, however, consider making two practices part of your life. First, reuse what you can — those plastic kitty-litter buckets being a perfect example. Even though the plastic itself may be recyclable, reuse eliminates the energy (of manufacture, shipping and sale) and resources used to make the new container you would otherwise purchase, as well as the resources involved in the recycling process itself.
Second, if you do buy plastic containers, try to buy easily recycled kinds, such as polyethylene terephthalate (PET, indicated by the number 1 in the triangle on the bottom of a container) or high-density polyethylene (HDPE, number 2 in the triangle). Whatever you buy, use the container to death.
One caveat: Avoid reusing plastic containers that have contained potentially harmful chemicals, such as solvents. You can reuse ones that have held salts (halite, for example) after you clean them thoroughly.
The simplest plastic container is a bag of soil. Punch a few drainage holes near the bottom of the sides, cut a slit in the top, and plant directly in the bag. You can buy specialized grow pots to facilitate things, but you don’t need them.