The universally used kind of container for plants is a pot made of clay, plastic, wood, ceramic, or some other rigid material. Shapes, colors, sizes, and drainage arrangements of such containers are endless in their variations. The important and useful thing about pots is that they provide individual housing for each plant, and each plant can be moved and cared for according to its needs and the gardener's facilities.
There is hardly a flowering or green- or variegated-foliage plant that will not grow in a pot. Individual plants of the kinds recommended for bench growing do just as well in a pot when you want, for example, only a few snapdragons and not arms full. Plants that like to trail, hang, or sprawl will do so in a pot that is suspended. All things considered, pots are the most useful items in your greenhouse, and you will probably acquire a formidable array of kinds, sizes, and shapes.
In time, seedlings and cuttings outgrow their first small containers and must be transferred to larger quarters. Indeed, this repotting goes on all the time with both young plants and those long established in the greenhouse. Let a plant stay in a pot until the pot is crowded with roots. Here's how to check on root growth: water the plant well; hold it in your right hand, place your left hand over the soil with the plant held between your first and second fingers. Invert the pot. Tap the rim smartly against the edge of a bench. The plant will drop out easily into your hand.
If the root ball has taken the shape of the pot and covered the soil with roots, shift the plant to a somewhat larger pot. If it is in a 2 1/2-inch pot, shift to one no larger than a 4; if it is in a 3, shift to a 5. Don't move plants on too fast, for a too-large pot is just as bad as one too small.
Prepare the new pot with drainage material one or more pieces of broken clay pot over the hole and then a layer of gravel or coarse soil. Pour in potting soil above this but not too much, since space must be left for the root ball. Water the plant to be repotted. When moisture has penetrated sufficiently to let you remove the pot, have a look at the root ball. If you are repotting older plants that have been growing a year or more in the same pot, get rid of as much old soil as you can. With a dibble or other pointed object carefully scrape off soil and dead roots. Then hold the plant in place in the partially filled new pot, and add more soil. Firm this around the root ball with your fingers or a stick. As you work, occasionally strike the bottom of the pot against a bench to settle the soil and fill in any air pockets.
Finally, water the repotted plant lightly from above, or sub-irrigate by standing it in a vessel of water until the top soil feels moist. Shade the plant for a few days to offset the possible shock of transplanting. Then return to sunshine unless the plant is a shade-loving type and resume your feeding schedule.
In the course of time, most plants need this repotting to larger quarters. Some won't, however, because they thrive and bloom better when the "shoe fits tight." But they all need nourishment, and you can provide this without repotting: simply replace the upper inch or so of old soil with a fresh rich mixture and augment this with applications of liquid fertilizer about every two weeks in periods of the plant's active growth.
Some of the easiest flowering plants to grow in pots are abutilon (flowering-maple), acacia, Ardisia crispa (coral-berry), azaleas, Beloperone guttata (shrimp-plant), bouvardia, calceolaria, Carissa grandi-flora (Natal-plum), crossandra, cyclamen, Euphorbia pulcherrima (poinsettia), Euphorbia splendens (crown-of-thorns), Felicia amel-loides (blue-marguerite), gardenia, heliotrope, impatiens, Osmanthus fragrans (sweet-olive), primrose, roses, schizanthus (poor-man's-orchid), Senecio cruentus (cineraria), and zygocactus (Christmas cactus).
And here are a few easy-to-grow foliage plants for pots: Aucuba japonica variegata (gold-dust-plant), caladium, Codiaeum variegatum (croton), coleus, Gynura aurantiaca (velvet-plant), peperomia, pilea, and pittosporum.