When container-grown plants receive the right amount of light, moisture, and nutrients, they are usually among the most care-free plants in your garden. Because we keep them in places where we see them often and interact with them daily as we check their needs for water, small problems can be spotted quickly and fixed straightaway.
One problem that's unique to plants grown in pots is salt buildup in the soil, evidenced by whitish deposits on the rim of the pot and sometimes on the soil's surface as well. Some salts are always present in municipal water supplies, and if potting soil becomes too salty, the plants will not be able to take up the moisture and nutrients they need. Accumulated salts can also burn stems and leaves.
To correct this problem, make a habit of flushing containers every few weeks by flooding them with water; drench them again an hour or so later. Like a cleansing bath, this operation dissolves accumulated soil salts, although you may still see residue on containers. In winter or whenever containers are empty, take the time to scrub away crystallized salt deposits with warm water and a stiff brush.
Pinching and Grooming
Every blossom counts in the small world of a container garden, so it's important to do everything you can to encourage the development of bud-bearing stems. With annual flowers, pinching or clipping off old blossoms is crucial to prolonging their flowering. And, when entire stems appear to have borne their last bud, clipping them off will often force out a new flowering stem.
When deadheading old blossoms or removing old stems, always use scissors or pruning shears rather than tugging at the plants with your fingers. Even modest pulling can injure roots, particularly when plants are growing in containers filled with light-textured potting soil.
Also, don't be afraid to control the shape of your plants when you discover ways that pruning might make a plant healthier or more attractive. Many gardeners elevate this process into a true art form, as seen in rightly sculpted topiary and in dramatic bonsai. If you like working with long-lived vines, shrubs, and trees, the same pruning methods used for plants growing in the ground, such as thinning, heading back, and shearing, can be used to shape plants growing in containers. In addition, it's not always practical to move a plant into a larger pot. To save space in the garden, as plants become rootbound, they can be unpotted and root pruned, then potted up in the same container again.
Monitoring Roots
Large plants grown in containers seldom attain full size because their root area is restricted. While most of the best plants for containers willingly accept this limitation, it's important to unpot the plant and check its roots when it seems unable to take up water or simply stops growing. Crowded roots are the most common problem, but occasionally insects or diseases are to blame.
To look at the roots of a container-grown plant, allow the soil in the pot to dry out, then lay the pot on its side and tap it gently. Jiggle the plant to remove it from the container without pulling too hard on the main stem. If the roots are badly matted, spiraled around the inside of die pot into a tight mass, or so thick that you can hardly see the potting soil, it's time to repot the plant.
If you are "potting on," or repotting in a new, larger container, select a pot slightly larger than the existing one but still in proportion to the plant's size. Fill the pot with fresh soil, loosen the plant roots by teasing them away from the soil ball, and continue planting as you would with a new specimen.
If you want to put the plant back in the same container, you'll need to trim the roots. Lay the plant on a bed of newspaper or a plastic sheet. Gently pull away some of the soil, hold the plant upright, and slice from top to bottom with a sharp knife all around the outside of die root ball to cut off old roots and stimulate new growth. Fill the pot with fresh soil and replant as you would a new specimen.
Missing roots indicate that a soil-borne disease has invaded, and there is usually no cure. Dispose of the plant and soil in the garbage, thoroughly clean the container by scrubbing with a solution of warm, soapy water and household bleach, rinse the pot. and start over by filling the pot with commercial, sterile potting mix and setting in a healthy plant.
When Winter Comes
The first few autumn frosts spell the end for most annuals grown in containers. They are best dumped onto the compost heap, potting soil and all. Then give the empty pots a good scrubbing with solution of soapy water and household bleach, and let them dry before storing them for winter. Ideally, you should store the pots in a place where they will not freeze, but pots that are kept dry and protected from ice and snow usually withstand cold without cracking or splitting.
If you have been growing potted hardy perennials, trim back the dead foliage with sharp pruning shears and move the containers to a place where it is cool enough to foster dormancy but not where plants are subject to constant freezing and thawing. Depending on your climate, it may be sufficient to place the pots against a wall of your house or beneath a bench, where they'll be protected from wind. If your storage space is limited to a deck or a patio, place dormant potted perennials in an insulated foam picnic cooler tucked into a protected corner and cover the pots with several inches of loose, fluffy mulch, such as evergreen boughs. You can also use plastic bubble wrap as pot insulation. Either wrap it around the outside of individual, planted pots and secure it with a water-resistant tape, or stuff die bubble wrap between and over the top of an entire group of stored containers and secure it with tape.
Hardy shrubs and trees usually need to remain outdoors in winter, because prolonged exposure to warm, dry indoor air is as stressful for them as hard freezes. Move them to a place where they won't be tortured by winds and be prepared to move them indoors for short periods of time in extreme cold. Even if light levels are low, it's usually better to store hardy evergreens in a cool garage or storage building over winter than it is to bring them into heated indoor spaces in the winter. Keep the soil around the roots lightly moist, by watering once a month, but do not apply fertilizer to these or any other dormant plants until early the following spring, just as they begin to send up new leaves.
Give similar treatment to hardy bulbs that you pot up in the fall. Most daffodils, hyacinths, and tulips need at least 6 weeks of chilling in early winter, after which they can be moved to a less frigid place as they commence growth. Wait until you see green shoots poking through the soil before you shift them to a spot on a warm windowsill indoors.
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