Sweet Potato

Information


Introduction

The tropical American vine, sweet potato (Ipomoea batatas) is in the botanical family Convolvulaceae.  Despite its name it is not actually a potato but a swollen underground root.

Think of a plant that has a high food value, is easily grown, matures quickly, produces a lot of food for the planting space used, has good keeping qualities, and tastes nice. It sounds too good to be true. Yet the sweet potato has all of these qualities.

Sweet potato yield starch, flour, glucose and alcohol and are especially rich in Vitamin A.  Yellow and orange varieties of the sweet potato root contain a high amount of Vitamin A. Other pale-fleshed varieties contain much less of this important vitamin. Sweet potato has an increasing role as an important health food due to high nutrient content and its anti-carcinogenic and cardiovascular disease preventing properties.  All varieties of sweet potato are good sources of vitamin C, B2, B6, and vitamin E as well as fibre, potassium, copper, manganese and iron and are low in fat and cholesterol.

They can be boiled, steamed, baked and fried.  Like pumpkins, they can be used in sweet dishes such as pies, puddings, biscuits, cakes and desserts and are an important constituent in baby foods.  Cooked red or orange fleshed sweet potatoes are sweet, soft and starchy with a flavour that resembles roasted chestnuts and baked squash.  Mashed sweet potato, with a very small amount of milk or coconut or soya milk added, makes a good weaning food for babies. Using your own sweet potatoes from the garden or from a local producer to prepare baby food means you are in more control of what your child is eating at this critical stage of development. You will find also the flavours textures and colours of our range of varieties more varied and distinctly different than generally currently available in stores.

STORAGE ROOT FORMATION AND DEVELOPMENT

Storage roots arise from the underground portion of a vine cutting that is being used as planting material.  Swollen edible storage roots develop from the initial adventitious roots, primarily those at the nodes rather than the callus.  Thick adventitious roots appear to form at nodes and thin roots at inter-nodal regions, generally however no storage roots form at inter-nodal regions.  The first formed adventitious roots have the potential to become storage roots.

Dryness and compactness of soil and an over abundance of N and low soil oxygen leads to increased lignifications and make roots prone to develop into non-storage roots.  The conditions in the first twenty days of rooting influence the subsequent development of storage roots.

So whether thick adventitious roots become fibrous roots, then pencil roots and ultimately storage roots is determined by the interaction of the cultivar and the environment over time.  The development of storage roots can be temporarily or permanently frustrated or blocked at any one of the developmental stages.

Storage roots are the principle carbohydrate storage organs in sweet potato.  Storage root formation is also obviously linked to the canopy which carries out photo assimilates export and leaf photosynthesis. After storage root initiation, development of the roots depends on the increase of cells and the development of starch granules in the cells.

Propagation methods

Sweet potato is propagated vegetatively by cuttings. In tropical climates sweet potato grows as a perennial, in temperate climates as an annual plant. Asexual propagation of sweet potatoes can be from storage roots, shoot tips and stem cuttings.  The cuttings are best rooted at around 20-25°C and 95% relative humidity in a moist, but well drained media. Rooting takes less than 10 days from sticking. At FitzGerald Nurseries we use motherplants from virus free certified tissue culture production. Sweet potatoes in our region are not propagated from storage roots but from mother plants produced by cuttings as it requires over a ton of storage roots to produce enough slips for just one acre.  However in warmer climates alternatives conventional systems are achievable.

Spiralled roots at base must be removed by cutting or breaking away

Planting material (Slips, Plugs and pots)

  1. Rooted slips are around 15 – 20cm in length and can be planted directly in the ground at 5cm – 10cm depth with ideally 2 to 3 nodes underground to optimize potential storage root initiation. As sweet potatoes are sensitive to frost they cannot be planted until the last frosts are over and soil temperatures should be minimum 10°C.
  2. Rooted cuttings in plugs are suitable for the pre-culture in a 10.5 to 12 cm pot for the hobby gardener market or they can be planted directly in the ground in a greenhouse.  Sweet Potato can be planted and grown in 15 litre containers after the last frosts. The young roots of the plugs must not be planted if spiralling so before planting it is necessary to ensure spiralled roots are removed to avoid badly shaped storage roots at harvest. This method is ideally suited for climates with higher risks of late frost and to gain a bit of time in the overall production cycle. It is possible to remove a large percentage of the plugs roots and plant the plug deeply leaving 10 – 15cm of shoot over soil surface. It is important to ensure cuttings and soil is kept moist until new roots develop.

VARIETIES

The above ground part of sweet potato consists of toothed/lobed leaves which are green at all stages (sometimes purple when young) and generally have spreading growth habits. Some have triangular leaves with purple ribs or veins. The storage roots range from elliptic shaped to round or more ovate. Flesh colour varies from orange to yellow to white. Dry matter contents vary across varieties. Some varieties have cream skin and flesh while others red skin and white flesh. The orange skin orange flesh types are the most commonly seen in USA and Europe and contain higher levels of Vitamin A and beta carotene.


PRODUCTION

1.Growth and Development

Sweet potato plant growth can be divided into three phases following field transplanting of cutting;

  • Establishment of a fibrous root system with little leaf growth (10 weeks),
  • Extensive vine growth and storage root development (7 weeks),
  • Storage root bulking with little further vine growth, while leaf area remains constant for the remainder of the growing season.

Cultivars can be classified into short duration (12-17 weeks), medium duration (17-21 weeks) and long duration (>21 weeks) (Production time from storage root initiation to harvest)

Sweet potatoes continue to produce leaves until harvest.  Mutual shading of leaves in the canopy can decrease light interception and bulking.  When sweet potato vines are grown on trellis, in home gardening situations, exposing more of the leaves to sunlight, the storage root yield will be increased.

There are a number of conditions which inhibit storage root formation:

  • Exposure of the root system to light
  • Oxygen deficiency through waterlogged conditions
  • Dry compact soils leads to high numbers of pencil roots
  • High fertility (particularly N) may adversely affect storage root numbers through increased shoot competition for photosynthesis.

2. Water & Drought

Sweet potato is considered to be a drought tolerant crops, it can still produce good yields in production systems suffering from drought stress.  However experiments carried out from 1927 to 1979 have demonstrated that sweet potato benefit from supplemental irrigation where moisture is erratic and unpredictable.  If soil moisture is close to field capacity at time of transplant, the moisture for early establishment is sufficient but additional moisture is required for maximum storage root development.

Drought in the first six weeks will seriously impact plant establishment and yield.  Loamy soils require about 25% field capacity, sandy soils upwards from 50% field capacity.

Flooding leads to poor root initiation and excess water encourages shoot growth over root growth.  Flooding causes some roots to lignify, limiting their potential as storage roots.

3. General Growing Conditions

Crop growing temperature: 20 – 35°C

Storage root formation starts when the days are shortening

Soil: pH 6-6,5, well drained sandy soils

Full sun

4. Pest and disease control

As sweet potato is a food crop biological control is advised. We use biological controls against red spider, aphids, thrips etc….And mice also like sweet potato very much.

5.  Growing Systems

Glasshouse:  soil grown sweet potato under glass with or without use of trellis strings or canes.

Polytunnel:  soil sweet potato trial under fixed glasshouse and polythene tunnel.

Cloches: field grown under Cloches with approximately 0.75meters in height covering 2 ridges.

Field Grown: planting after the last frosts

Plant density: 30-35000 slips/ha (30 cm in the row, 1 metre centres )

Containers/bags: rooted plugs planted in larger containers or bags

15L soil volume per plant in containers

HARVESTING AND STORING SWEETPOTATO ROOTS

1.Harvesting

Figure 2. Sweet potato harvested using digging forks

The sweet potato crop is harvested by hand or mechanically, using a sweet potato chain harvester or diggers in large scale field production.  If irrigated the field is allowed to dry for 2 – 4 weeks before harvesting, then the foliage is removed by shredders or any number of devices that will effectively remove leaves and vines so the harvest can be processed more efficiently and prevent blocking of machines. Harvest before first frosts and take care not to bruise the thin skin of the sweet potatoes to avoid infections.

 

After the foliage is removed from the field by shredders or de-viners, the sweet potatoes are turned up and exposed to sun for drying using a turning plough or shaker harvester in large field scale or digging fork for small scale and hoe production.  Pickers shift through the soil collecting the roots and sort them into various grades.

Machine harvesters for sweet potatoes require large scale production and adaptation of other harvesters such as potato harvesters need to take into account the delicate skin and risk of damage to the flesh.

2. Curing the root

Curing involves holding the root at a temperature of 29 degrees Celsius and a relative humidity if 85 – 90% with proper ventilation for a three to seven day period.  During curing ventilation is used to remove carbon dioxide (CO2) and replenish oxygen (O2).  Under warm weather conditions in more southern regions of Europe curing can take place on the field otherwise curing facilities are required.  Curing enhances the culinary or eating quality of the root, it aids wound healing which reduces the loss due to shrinkage and disease.  Curing also sets the skin of the sweet potato, reducing the chance of skinning during packaging.

3. Grading and Packaging

Sweet potato crops are usually washed and graded based on size, condition and absent of defects. Marketable root weight is between 0.25kg and 1.0kg with 2 grades of 0.25-0.6kg and 0.6-1.0kgs. Top quality roots should be free of soil with smooth undamaged skins.  They should have good even shape with no cracking or insect damage.

4. Storage

The primary goal of modern storage is to maintain the quality of harvested roots by minimising potential problems associated with physiological disorders and disease.  Climate controlled storage facilities are necessary in modern, scaled, sweet potato production operations as it can hold the roots for a year and remain marketable.  Storage facilities are designed to hold the temperature at 13 Celsius and humidity 85 -90% with adequate ventilation requirements for long-term storage.

NUTRITIONAL INFORMATION

Sweet potatoes are:

  • fat-free and cholesterol-free
  • have only 130 calories per serving (one medium potato)
  • a good source of fibre – 3.9 grams
  • an excellent source of:

Vitamin A – 7860 IU/serving
Vitamin C – 29.51 mg/serving
Potassium – 265.2 mg/serving
Calcium – 28.6 mg/serving
Iron – 0.79 mg/serving
Folate – 18.2 mcg/serving

 Beta Carotene (only orange fleshed varieties)

Levels of the above qualities can vary from variety to variety and more detailed analytical work by variety is underway to document our varieties offered.

Sweet potatoes are ideally suited for the human nutrition of:

  • Infants and babies changing over to a solid food diet
  • People with health problems – The nutrients, vitamins and minerals of sweet potato help increase the good and reduce the bad cholesterol, regulate the blood sugars (Diabettes Mellitus type 2), help with a good immune system and reduce the risk of cardiac problems and cancer
  • Older aged people – good source of nutrition and Vitamins and minerals while low on calories

People with weight problems – low carbohydrate vegetable. No fat