Q:
The roots of my potted Agapanthus are growing through the drainage holes. When does a container plant become too potbound?
Madeleine Hiscox, Prospec, CT
Agapanthus campanulatus.
A:
June Huston, home demonstration gardens designer at the Missouri Botanical Garden in St. Louis, Missouri, replies: When roots growing out of the bottom of the pot cause the pot to tip over, the roots are too potbound. Agapanthus species and cultivars do require potbound conditions to flower, but overly cramped conditions are not conducive to flowering. With the pot’s soil content probably at a minimum, the plant will dry out more easily, reducing the chances for optimal growth and flower bud production.
To repot in the spring, transfer the undisturbed root ball to a container only an inch or so bigger than the old one, and fill it with potting soil amended with compost. If the root ball remains undisturbed, chances of bloom during the next growing season are increased. A more invasive approach is to cut the fleshy roots with sharp knife and divide the plants among numerous containers. Bloom will most likely be sacrificed for a year or more, until the plants once again become cramped.
After repotting, place the container in its summer home. Fertilize the plant lightly in mid-summer, at half strength, with a water-soluble fertilizer such as Peter’s 20-20-20. A feeding with a granular fertilizer that is higher in phosphate and potash such as 5-10-10 in late summer will provide the plant with the nutrients necessary for winter root production and good health. During winter dormancy, remoisten the soil only when it is dust dry, and provide only low light.
