Most pots found on the market have slightly sloping sides so that the plant and its root ball to be easily removed intact if you want to repot or plant it out. Materials used for pots are the traditional clay for the terracotta pots and plastic. Plastic is now more common, but most of these pots are made from polypropylene, which deteriorates in cold weather. For more resistant pots you should chose pots made of a polypropylene and polythene mix, which do not become brittle in cold weather.
Pots vary in size according to their uses: standard pots, the most common ones found on the market will be as deep as they are broad; pans or seed pans, used for germinating seeds are one third the depth of a standard pot of the same diameter; half-pots, often used commercially for plants with relatively small root balls, are half to two thirds the depth of a standard pot of the same diameter; sweet pea tubes or long Toms are used for very deep-rooted seedlings or for plants that will remain in the pot for some time; whalehide ring pots are designed to be used for the ring culture of tomatoes in the greenhouse; lattice pots with mesh sides are containers specially designed to be used for aquatic plants.
Degradable pots and pellets
These pots are usually made from compressed peat and various other fibers and are good for plants that resent root disturbance. These can be planted out into the bed and the plants roots will grow through the sides and base of the pots into the surrounding ground. There are also peat pellets that can be used for seeds and for rooting cuttings. They must be expanded with water before being used. You can create your own degradable pots from cones made from double-thickness newspaper.
There are two types of seed trays: the traditional wooden seed trays and the newer plastic ones, easier to clean but often more fragile. You can find cheap and flexible but very thin plastic trays which will not last more than a season or strong plastic trays, more rigid and expensive which will last for several years if stored out of sunlight when not in use.
Multi-celled or module seed trays are good for seedlings that hate being pricked out. The tapered sides to each cell allow young plants to be transplanted with only minimal damage to their fragile roots. Module trays are available in plastic, polystyrene and biodegradable paper and in a wide range of cell sizes and numbers, from four to several hundred.