Above: The familiar edible sweet potato - tubers (left) and heart-shaped foliage (right).

 

Sweet potatoes are best known for their edible tubers. Sweet and starchy, the tubers are an essential ingredient in many local desserts. Botanically known as Ipomoea batatas, the sweet potato is a member of the morning glory family, Convolvulaceae and is closely related to the morning glory vine (I. purpurea) and the leafy vegetable, kangkong (I. aquatica). It is not related to the true potato, which is a member of the tomato family, Solanaceae.

A perennial in the tropics, the sweet potato plant is a non-climbing, herbaceous vine that is native to tropical America. Mature plants produce purplish morning glory-like flowers. Most of us are familiar with the traditional sweet potato tubers that have a light brown skin, longish in shape which tapers towards both ends. Now there are varieties from Japan that have reddish purple coloured skins. When cut, sweet potato tubers reveal a myraid of flesh colors - white, cream, yellow, orange, and purple. The sweet potato bears alternate heart-shaped or palmately-lobed leaves, which are not spared from being used as food. The young leaves are stir-fried as a leafy vegetable with chilli and minced dried shrimps.

Sweet potato vines are extremely vigorous growers that quickly cover up the ground surface, capable of smothering weed growth, thereby reducing the need to weed! They are easily established, relatively pest-free and rather tolerant of soggy soil conditions. These characteristics make the sweet potato vine a very attractive groundcover candidate! The indecisive vegetable gardener may want to consider growing the sweet potato as temporary ground cover for a plot. Once a crop has been selected, the vines can be easily cleared and the leaves be used for food. If left long enough, there may be tubers underground waiting to be harvested!

 

Above: Ornamental sweet potato varieties, clockwise from top left - 'Black Heart', 'Blackie', 'Tricolor' and 'Marguerite'.

There are ornamental sweet potato varieties grown exclusively for their decorative foliage. These ornamental varieties may also produce edible underground tubers but they are usually less prolific and tubers are usually smaller and not as palatable as those that have been bred and grown exclusively for their tubers. The colorful leaves of these ornamental varieties can be used to enhance the flowers and foliage of companion plants.

Locally, four varieties of ornamental sweet potato vines can be found - 'Blackie', 'Marguerite', 'Black Heart' and 'Tricolor'. 'Blackie' has greenish or purple leaves that are deeply lobed. 'Marguerite' on the other hand has bright, chartreuse green heart-shaped leaves. 'Black Heart' has also heart-shaped leaves which are dark purple. 'Tricolor' is perhaps the most spectacular variety with variegated, palmate leaves with splashes of green, white, and pink. Unfortunately, it exhibits a less vigorous growth pattern.

Sweet potato plants are worshippers of the sun and they demand a location with full sun to do well. Plants grown in a shady location will produce leaves that are smaller and the leaf-to-leaf (internode) distance will also lengthen. Ornamental varieties will produce less colorful leaves. Tuber yield will also be affected. Sweet potato used as a ground cover should be allowed to sprawl over an open, unshaded area. Otherwise, growth can get sluggish and the vines will not fill up the area fully.

Above: 'Marguerite' (left) and 'Black Heart' (right) being used as a groundcover.

Not exceptionally particular with soil type, sweet potato grows best in a location that has well-drained soil. Heavy clayey soils need to be lighted with organic materials like compost.

Refrain from feeding with fertilizers rich in nitrogen as it will promote excessive foliage growth and depress tuber yield for tuber producing varieties. Sweet potatoes are hungry vines and a balanced fertiliser should preferably be applied at the base of the bed before planting. Potassium is critical for proper tuber production and hence regular applications of a fertilizer with a higher potassium ratio every fortnight would be useful. Some people use crushed eggshells as well.

Vines have a tendency to root at the internodes wherever they touch the ground. Make it a point to lift the vines and throw them back into the plot to prevent rooting. By allowing the plant to root at multiple internodes will bring about the formation of many small tubers at the expense of larger tubers. One should refrain from over-harvesting of leaves for food from tuber producing sweet potato vines as this would depress tuber yield. Harvesting of tubers can commence roughly about four to six months after planting. Foliage and vines are first removed where the young leaves can be used as a vegetable. The tubers are then lifted carefully using a hoe (cangkul) from one side of the raised bed. Right: Roots appearing at the internodes when they touch moist ground.

PROPAGATION

Sweet potato plants are most commonly propagated via stem cuttings. A tip cutting that is roughly 30 cm long can be used and one can request to take a couple of cuttings from any vegetable farm or community garden. Each cutting should be spaced roughly 30 cm apart in a raised bed. Keep the soil moist and it is normal to witness cuttings wilting under extremely hot weather. Once cuttings have rooted, sweet potato vines can be relatively drought-resistant.

To intrigue children at home, one can grow a sweet potato plant using a tuber bought from the local market, a glass jar and several wooden toothpicks. First, stick three toothpicks around the tuber near midway along the sweet potato so as to allow the suspension of the tuber over the jar. The sharper end of the tuber should face downward into the jar. Next, fill the jar almost to the top with water and put it on a bright windowsill. Within a week, the tuber will send roots down into the reservoir of water and new shoots will appear on the upper end of the tuber. It can then be transferred into a hanging pot to allow the vines with ivy-like foliage to grow and then drape over the sides. Alternatively, the entire plant with the tuber can be transferred to be grown in a raised bed.

Sweet potatoes are seldom bothered by pests and diseases. The most serious pests that one can encounter are weevils and beetle grubs that will bore into the tubers. These can be avoided by practising crop rotation and avoiding consecutive plantings of sweet potatoes in a single location. For control of these pests, drenching of the soil with an insecticide is the only remedy. Beetles, caterpillars and stem borers may also cause some damage to aerial growth and these can be eradicated relatively easily by application of a suitable insecticide.

Occasionally, unsightly brownish fungal spots may appear on the sweet potato leaves. These can be prevented by having better air circulation around the leaves and treatment can be carried out with an application of a contact fungicide. Tuber rot can be prevented by growing in better draining soil and using rot-resistant varieties. Unfortunately, these special varieties are not easily available to the home gardener in Singapore.

 

Lee, C.K. 1979. Grow Your Own Vegetables. Singapore: Times Books International.

Tindall, H.D. 1983. Vegetables in the Tropics. London: Macmillan Press.