Collard greens produce a large-leaf vegetable during the cool months of spring and early summer. Heat causes the plants to bolt, sending up flower stalks and ending the productive stage of the plant. The plants grow quickly, reaching a spread of up to 2 feet at maturity. Collards grow well as a container plant if you have limited garden space. Plant the greens singly in 5-gallon pots or plant up to four plants in a 25 to 32 gallon planting tub.
Fill the container to within an inch of the rim with potting soil. Alternatively, mix equal parts compost and potting soil together.
Set the container in a full sun area. Water the potting soil until water drains from the bottom of the pot.
Sow three seeds in a single pot or sow two rows of seeds, spacing the seeds 8 inches apart, in a tub. Cover the seeds with a 1/4-inch layer of soil.
Mist the soil with water if the surface begins to dry out before germination. Seeds typically germinate within seven to 10 days.
Thin the plants once the collards are about 6 inches tall. Leave one plant in a single pot and thin tubs so there are two rows of two plants spaced equally apart.
Water the container every one to two days throughout the growing period. Container-grown collards require irrigation when the top inch of soil feels dry.
Fertilise the collards once weekly with a half-strength dilution of a balanced soluble fertiliser. Dilute the fertiliser at the rate recommended on the package for the size of container you are using.
Harvest the outer leaves from the collard greens when the plants are 10 to 12 inches tall. Leave the inner leaves on the plant to continue to grow. Harvest the entire plant before temperatures rise above 26.7 degrees Celsius and before the plant bolts.
Prolong the growing season by moving the container to a cooler area of the garden to prevent bolting. Collards also grow as a fall crop. Plant in early fall once daytime temperatures drop below 26.7 degrees Celsius.