Introduction
The sweet potato (Ipomea batatas) is a member of the morning glory or Convolvulaceae family. It is not related to the common potato. The plant has long trailing slender stems and is perennial, but is treated as an annual in cropping.
The tuberous root has high food value, fibre and energy. It is rich in sugar and vitamin C and contains good quantities of vitamin A, vitamin B, calcium and iron.
Climate
The sweet potato is a semitropical plant that grows best between 20 and 30°C. A minimum frost-free growing season of four to six months is needed, with a minimum of cool, cloudy weather. Plant growth is restricted below 10°C and plants are physically damaged at 1°C.
Soils
Sweet potatoes grow best on well-drained sandy loams, although sandy soils produce good crops if well fertilised and watered. Heavy and swampy soils tend to produce rough, misshapen roots.
The pH of the soil measured in calcium chloride should ideally be 5.8 to 7.0.
The yield of roots declines rapidly as salt levels rise. The soil conductivity should be less than 25 to 50mS/m.
Cultivate to provide 20 to 30cm of well worked soil. Deep ripping with a tined implement may improve soil drainage and root shape.
On sandy soils, the crop can be planted on flat or ridged ground. Ridged ground facilitates machine harvesting. On heavier soils and in situations where waterlogging may occur, grow the plants on 20 to 25cm high ridges.
Rotation
Sweet potatoes should be rotated with other crops to prevent build-up of diseases such as scurf and to manage root-knot nematode. Plant sweet potatoes on the same soil only once every two to four years.
Planting material
Sweet potatoes are vegetatively propagated from cuttings. Select storage roots from healthy, high yielding plants. Good quality sweet potatoes may be bought in the markets for planting material, although market agents and growers may not know the names of the varieties.
Choose roots from a uniform consignment. They should be well formed, with bright skins and no trace of disease. To plant one hectare, you will need 400 to 500kg of seed potatoes. About 1000 tip cuttings are produced from 20kg of roots.
Preparing cuttings
New growers should make a seed bed in a warm area by planting roots end-to-end in rows 5 to 10 centimetres apart and 3 to 4 centimetres deep. In Perth, do this in August to September. Clear plastic may be placed over the plants.
In other areas, wait until the soil temperature is above 15°C. Take cuttings from the seed bed when the shoots are 15 to 20cm long. Cuttings should not be contaminated with soil. Some leaves may be removed from the base of the cutting, but this is optional.
Cuttings may be rooted in water before planting, but this is not essential. Place bundles of cuttings in 4 to 5 centimetres of water for 48 hours. Do not immerse the tip of the cutting in water or the growing point will rot. Rootlets may be just visible at planting, but should not be longer than two millimetres.
Established growers take cuttings from the best plants of the previous year’s crop. They may protect these plants with shade cloth in winter and early spring.
Do not pull slips from the old tubers as they may carry root diseases, especially scurf. It is preferable to cut off shoots (cuttings) with secateurs which have not been in contact with soil and are free of scurf.
Cuttings are normally planted by hand but a mechanical transplanter may also be used. Plant cuttings with 5 to 7 centimetres of the tip exposed and with the lower half horizontal in the ground. Space rows at 90cm, with 25 to 35cm between plants in the row. Plant closer if experience shows that a variety will produce roots which are too large at a wide spacing. Another system is to plant on ridges 1.2m apart formed by banker discs, with double rows at 0.3m and 0.5m within the rows.
Sweet potatoes can be planted at Carnarvon for most of the year, but growers should target periods of low market supply to maximise financial returns. For this purpose, plant between July and September in Carnarvon for harvest from December to February, and in March for harvesting from August to September.
In Broome, sweet potatoes are mainly harvested from December to March.
In Perth, plant from late September to late December and harvest from February/March to May/June.
Varieties
Several different types of sweet potato are available in Western Australia. New varieties are being released regularly.
Contact seed suppliers to determine the best variety for your location and cropping time.
Irrigation
Soil must be kept moist after planting to ensure good establishment of the cuttings.
On sandy soils, daily irrigation of established plants is recommended, although this will vary with temperature, wind speed and the stage of crop development. Yields and quality are seriously affected if the crop is stressed when the harvest roots begin to develop about 50 to 60 days after planting.
Water quality is important and the conductivity of water used for irrigating sweet potatoes should be less than 170mS/m.
Do not overwater as plants mature as this may cause rotting and skin cracking.
Plants are usually watered by overhead sprinkler, but may also be watered by trickle irrigation. The latter must be removed before harvesting. There have been no trials on the watering requirements of sweet potatoes in Western Australia.