Polymnia edulis Wedd.
Polymnia sonchifolia Peopp. & Endl.
Silphium edule Baill.
Common Name: Yacon
Smallanthus sonchifolius is a herbaceous perennial plant growing from a tuberous rootstock; it produces a clump of stout stems around 100 - 200cm tall[
The plant can produce heavy yields of edible, sweet tasting tubers and has long been cultivated for these in the Andes. The plant is also grown for these tubers on a small scale outside the plant's native range.[
S. America - Andean regions of Bolivia, Colombia, Ecuador, Peru
Found at elevations up to 3,600 metres in the Andes.
Smallanthus sonchifolius is a plant of higher elevations in the tropics, where it is found at elevations up to 3,600 metres. It grows best in areas where annual daytime temperatures are within the range 12 - 22°c, but can tolerate 5 - 30°c[
]. Top growth is killed by frost, but the dormant tuber can tolerate temperatures at least a little below freezing[
]. The plant prefers a mean annual rainfall in the range 800 - 1,200mm, but tolerates 600 - 1,400mm[
]. In S. America, plants succeed in areas with annual rainfall varying from 900 - 3,500mm[
]. Growth is unaffected by day-length and so the plants can produce good yields of edible tubers in both tropical and temperate zones, so long as there is a sufficiently long growing season of 6 - 7 months[
Succeeds in full sun and in light shade[
]. For best results, this plant requires a warm position in a deep rich soil[
], though it survives even when growing in poor soils[
]. Prefers a pH in the range 5.5 - 6.5, tolerating 5 - 7[
Plants are fast-growing[
One report says that plants take 6 - 7 months to produce a crop from planting out[
], though on our Cornwall trial ground they have cropped quite well with a 5 month growing period[
]. The roots are brittle and must be harvested with care to avoid damage[
]. The harvested roots can be stored for several months[
]. Plants have not been selected for flavour or yield, some roots can be exceedingly sweet whilst others are fairly bland[
Yields of 38 tonnes per hectare have been recorded in South America[
Root - raw or cooked[
]. When first harvested, the root can taste somewhat starchy[
], but it soon becomes sweet, crisp and juicy and is delicious eaten raw[
]. The flavour is further improved by exposure to the sun although some of the crispness will be lost[
]. The root can be eaten like a fruit or diced and added to salads[
]. The skin has a somewhat resinous taste so it is usually removed[
]. The cooked root retains is sweetness and crispness[
]. Individual roots can weigh up to 500g[
]. The nutritional value is low because the root contains a high quantity of inulin, a carbohydrate that the human body cannot utilize[
The grated pulp of the root is squeezed through a cloth to yield a sweet refreshing drink[
]. This juice can be concentrated to form dark brown blocks of sugar called 'chancaca' in S. America[
Leaves and stems - cooked as a vegetable[
]. They contain 11 - 17% protein, dry weight[
Plants might be useful in agroforestry because they succeed under trees[
], though in relatively sunless climes the plants are not likely to do well in the shade of trees[
Seed - sow in containers in a nursery and only just cover the seed[
]. Prick out the seedlings into individual pots as soon as they are large enough to handle and plant them out when 15cm or more tall
Division when the plant is dormant. The plant forms 2 distinct types of tuber. Large tubers, usually on thin roots 2 - 5cm long, are used as storage organs and do not have the capacity to form new shoots. These are the tubers that are usually eaten. Smaller tubers are formed in a cluster around the stem. These form the shoots for the following year's growth and so are the ones that should be used for propagation. Dig up the plants once the top growth has died down. Remove the large tubers for food, cut the main stems back to about 10cm long and store these stems with their cluster of small tubers in a cool frost-free place. Do not let them dry out. Pot them up several weeks before you wish to plant them out and when they come into active growth divide each cluster into individual shoots with their tubers attached and re-pot these. Plant them out when they are growing actively[
Cuttings of basal shoots as new growth begins[
]. Harvest the shoots with plenty of underground stem when they are about 8 - 10cm above the ground. Pot them up into individual pots and keep them in light shade until they are rooting well then plant them out.
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