Indigo is fairly easy to grow from seed, provided it is kept warm. All the species will grow happily on a sunny windowsill but don't really like growing outdoors even during the summer. If you grow indigo under ideal conditions in a heated greenhouse it will grow rapidly, (up to 2m in a season) and will need trimming. You can make indigo dye from the leaves and stems you cut off.
Only grows in a heated greenhouse or indoors in the British Isles. Indigo is perennial, but can be treated as a half-hardy annual. Minimum temperature is 19°C.
By seed. Sow in a seed tray or large saucer pot during late May, covering the seed lightly with compost or a thin layer of vermiculite, then place a sheet of glass over the tray and keep at a temperature of 24°C. Once the seedlings have germinated, prick out into 9cm pots and move on as they grow. They prefer a loam based compost with good drainage and plenty of organic matter, and like full sun.
Softwood or semi-ripe cuttings from established plants can be taken in July and usually root quite easily without the need for hormone powder. They need to have most of their leaves removed for best results.
Water plants well in the growing season, but more sparingly from October to March. Pot on as the plants grow - a well established indigo plant will be happier the bigger the pot, but a 40cm/18in pot is adequate for a plant up to 5 years old. Large mature plants may produce small reddish pink flowers at the ends of the twigs, growing in spikes amongst the leaf axils.
To encourage flowering, feed with a high potassium or tomato liquid fertiliser.
Pruning of older plants should be carried out in spring or midsummer to encourage flowers, removing the old flowering shoots as soon as they have faded, but growing for foliage to make your own dye you can prune from early summer through to September.
Few problems: established plants growing in greenhouses may suffer from scale insects.
Both seeds and plants are available from nurseries.