Growing sweetpotatoes
Common questions
Stephen Harper, Eric Coleman & Jerry Lovatt, Department of Primary Industries and Fisheries, Queensland.
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Introduction
This information has been extracted from the Agrilink Sweetpotato Information Kit, which provides information on all aspects for growing and marketing sweetpotato in Queensland.
This section contains the most commonly asked questions about growing sweetpotatoes. The answers are as brief as possible. Where this is difficult and more detail is required, readers are referred to other sections of the kit.
Varieties
What variety should I plant?
The orange-fleshed variety Beauregard is the most popular sweetpotato grown in Australia and has supereseded old varieties such as Beerwah Gold and NC-3. The white-fleshed varieties have a smaller market than the orange-fleshed. Northern Star is the most commonly grown red skinned, white-fleshed variety. Kestle is a white skinned, white fleshed variety that has a small market.
What is the difference between dessert and staple types?
The dessert-type of sweetpotato has orange flesh, is moist and sweet and has a lower dry matter content than the staple (white) type. It is preferred for sweet dishes, but is used mainly as a vegetable in Australia.
The staple-type has dry, cream, white or purple flesh and is firm and starchy. It is used as a fresh vegetable, in savoury dishes such as curries, baked with seasoned meat or in stews.
What is kumara
Kumara is the Maori name for sweetpotato.
Planting material
Is plant material quality important?
To help in the management of the commonly encountered commercial diseases DPI&F maintains a collection of virus tested plant material in tissue culture, and a nursery of virus checked sweetpotato plant material. From this nursery seed roots are grown every year and are available for sale to sweetpotato growers wanting to introduce new disease free planting material. The nursery varieties are held in a screened facility to exclude aphids, but it is impossible to guarantee 100 per cent disease free material.
Instead of providing cuttings, DPI&F has
sweetpotato seed roots available for sale from late May through to July. Growers
can purchase these roots and grow their own cuttings as soon as climatic
conditions allow. An added benefit of purchasing roots is that they will keep
producing cuttings, allowing growers to enter more clean material into their
cropping system than previously, at a lower overall cost.
The roots usually produce a minimum of five sprouts
(depending on variety) when first cut and then produce more cuttings as the previously cut tips divide further.
How do I prepare the cuttings?
From the tips of the vines take cuttings that are at least 30 cm long for machine planting and up to 45 cm long for hand planting. It is best to plant them within twenty four hours of cutting.
Planting
When can you plant sweetpotatoes?
Sweetpotatoes can be planted year-round in north and central Queensland, but March to June plantings are preferred in north Queensland to meet higher prices in the spring. In southern Queensland the main plantings are from September to March because cold conditions reduce growth and hence yield.
How are they planted?
Vine tip cuttings or sprouts are planted using a vegetable seedling transplanter. Plant so that at least three nodes are in the ground. Some growers prefer to plant by hand. In most soils a flat orientation will result in superior shape storage roots at harvest. The only time vertical planting is better is when irrigation cannot be applied regularly enough to maintain good soil moisture around the nodes in the first two weeks after planting.
What is the optimum plant spacing?
Optimum spacing will depend on variety and climatic conditions, but about 22,000 plants per hectare is considered ideal. With experience you will be able to work out the optimum spacing on your farm to maximise marketable yield. Spacing is a powerful tool to reduce time to maturity.
Row spacing depends on the equipment available, it is commonly 1.5 m. Plant spacing in the row varies from 20 to 45 cm with 30 cm being most common. High-density plantings require more planting material, whereas low densities produce lower yields with fewer, but larger, storage roots.
Pests and diseases
What are the main pests?
Sweetpotato weevil is the major pest. Soil-borne insects such as white grubs, wireworms and crickets can also cause problems. Leaf-feeding insects include grasshoppers, hawk moth larvae, beetles, leafminer, aphids (which spread virus), silverleaf whitefly and mites. Nematodes can cause severe losses, as can rodents, wallabies and pigs.
How do I control sweetpotato weevil?
Farm hygiene is most important. Use cuttings free from weevil larvae. Destroy crop residues and alternative hosts such as morning glory. Rotation of sweetpotatoes with other crops is essential. Hilling-up and irrigation to prevent soil cracking will help reduce storage root infestation by weevils. Two pesticides are registered to help manage sweetpotato weevil.
What diseases are important?
Apart from virus and phytoplasma (virus-like) problems, diseases are not normally of economic importance in new ground if optimum growing conditions are provided and planting material is clean. However, in land that has previously grown sweetpotatoes, soil-borne diseases, for example soil rot, scurf and nematodes, and vine diseases such as scab, alternaria, cercospora leaf spots and fusarium, can occasionally cause problems.
Are viruses important?
Yes. Yield reductions of at least 15% can be expected if foundation (virus checked) stock for planting material is not renewed regularly. In association with the phytoplasma little leaf, yield reductions of up to 80% can be expected. More information is available in the DPI&F Note Sweetpotato diseases: Use healthy planting material as a strategy for disease control.
How do I know if I have virus?
You will have virus! Viruses are endemic and the only way to manage them is with regular introductions of virus checked sweetpotato plant material.
Weed control
How can I control weeds?
Pre-plant management to reduce weed seeds is the first step, so destroy weeds before they seed. Several herbicides can be applied to the soil to prevent the emergence of some grass and broadleaf weeds. Sethoxydim and fluazifop-P can be used to control grasses early in the crop.
Mechanical weed control is necessary during early growth and hand weeding is normally necessary later in the growth cycle. Sweetpotatoes are a vigorous crop and compete with weeds, making weed control late in the crop generally unnecessary. Large weeds should be pulled out before they seed.
Using chemicals
What is the best spray equipment to use?
Boom sprays are most commonly used, but there is no one best type. Buy spray equipment that suits your situation and budget. Select equipment based on its ability to give good coverage; droplet size is very important, as is the volume of active ingredient per hectare to be applied.
How do I calibrate my spray equipment?
To work efficiently spray equipment needs to be calibrated. You will learn the correct method of calibration at the spray accreditation course. Document the results of this calibration as part of your quality management system and keep it with your spray diary.
How do I know how much chemical to use?
Always read the label. The label on the chemical container gives the registered rate of application and the withholding period. The label also tells you how to use the chemical and how to mix and apply the product safely.
What is an MRL?
The abbreviation MRL stands for maximum residue limit, which is the maximum level of a chemical permitted to be present in a food. It is expressed as milligrams of chemical per kilogram (mg/kg) of the food.
Produce at the markets is randomly tested for pesticide residue. Farmers have been prosecuted when chemical residues were found above the MRL or residues of non-registered chemicals were found.
What is a withholding period (WHP)?
The withholding period is the number of days that must pass between the last chemical application and harvest for human consumption, grazing for livestock or cutting for stockfeed. For postharvest treatments, it is the number of days from treatment to consumption. The WHP will be on the chemical label.
Some chemicals do not have withholding periods for livestock, if these chemicals are used, the crop must not be fed to livestock. The withholding period for livestock consumption may be greater than that for human consumption.
Residues should be below the MRL if the chemicals are used at the registered rate and frequency and the withholding period is observed.
How do I get spray accreditation?
You must attend a ChemCert Farm Chemical User Training course. Course participants complete an open book assessment at the end of the course and successful candidates are then issued with a statement of accreditation by ChemCert. This accreditation is valid for five years so participants are kept up to date with changes and developments.
ChemCert accreditation will assist in the documentation of a quality assurance (QA) program. It is highly desirable for growers to be able to demonstrate safe, responsible use of chemicals.
Do I need training in the safe use of chemicals?
In Queensland you cannot buy certain pesticides unless you have a current ChemCert accreditation, endosulfan can only be supplied to or used by an accredited person.
Most customers now see it as highly desirable for their growers to be able to demonstrate safe, responsible use of chemicals. One of the best ways to demonstrate this is to obtain a ChemCert accreditation.
How should I store my chemicals?
Chemicals need to be stored in accordance with the AS2507-1998 standard. This does not mean you will have to spend a fortune on elaborate storage facilities. You do, however, need to be aware of several safety, environmental and food safety factors whenever you deal with chemicals.
Further information on correct use of chemicals is covered in the chemical user course.
Do I need to keep a diary of spraying records?
Yes, you do! Records of chemical application are now one of the most important pieces of documentation you will need to be able to prove what you have done with chemicals. Most merchants and agents supplying the retail sector now expect you to keep spray records showing at least what was applied, how much, by whom, and when the application took place.
Environmental management
What is an EMS?
An Environmental Management System (EMS) is a systematic approach to managing the impacts that a business has on the environment. An EMS does not dictate levels of environmental performance, however it should enable a business to comply with legislative requirements concerning the environment. It should also build on existing activities such as industry best management practices, industry codes of practice and quality assurance schemes.
What type of EMS do I need?
We do not know when or what type of EMS will be required in the horticulture industry in the next few years. It will depend in part on the markets you are aiming to supply. As a minimum, you should become familiar with the Growcom Farmcare Code of Practice for fruit and vegetables. It was developed by Growcom to assist growers meet their general environmental duty of care under the Environmental Protection Act 1994.
The most widely recognised auditable international EMS standard is ISO 14001. This process standard requires a business to have an implemented, documented environmental policy in place which is available to the public. It also requires the business to have a documented environmental management system in place that is capable of being audited by an accredited third party. ISO 14001 does not prescribe specific levels of environmental performance but it does require a business to be able to identify and have access to all relevant environmental legislation.
In Europe, a protocol based on good agricultural practice called EurepGAP has been operating since the late 1990's. A significant number of Australian horticultural businesses are working towards EurepGAP in order to meet compliance deadlines stipulated by UK/European retailers and importers.
What is Enviroveg?
Enviroveg is a relatively new program from the Australian Vegetable and Potato Growers Federation (AUSVEG). Enviroveg is committed to encouraging vegetable growers throughout Australia to adopt and implement good environmental practices. It includes a self-assessment tool to help growers compare their current farming practices with the environmental best practice activities listed in the Enviroveg guidelines.
Farm safety
What are my workplace health and safety obligations?
Australian agriculture has one of the highest rates of workplace accidents. Your obligation as an employer and farm manager are based on three principles:
. a concern for people and their well-being, including yourself and your family;
. sound business management;
. compliance with current workplace health and safety legislation.
The best way to meet your obligations is to take part in the Managing Farm Safety program developed by Farmsafe Australia. The Managing Farm Safety program is aimed at developing skills in risk management of farm safety - an approach that is consistent with the way other farm business risks are managed. The training course and resource package are based on real data about the major risks on Australian farms, including specific agricultural industries, and takes into account the requirements of current occupational health and safety legislation.
Fertiliser
What fertiliser do sweetpotatoes need?
Although sweetpotatoes have a relatively low fertiliser requirement, this will depend on the soil type and the cropping history of the block. A fertiliser application schedule should be based on the results of a complete soil analysis taken six to eight weeks before planting. This will also indicate if lime or dolomite is required to increase the pH to the optimum of between 5.5 and 6.0. These products should be applied at least four weeks before planting.
Use leaf and sap tests during plant growth to monitor nutrient levels. Excessive amounts of nitrogen applied before storage root initiation will make vines grow too vigorously and reduce yield. Around 100 units of nitrogen and 180 units of potassium are required.
Irrigation
What water quality can I use?
Sweetpotatoes are susceptible to saline irrigation water but poorer quality water can be used on sandy soils with good drainage. Irrigation water should be low in sodium (Na) and chlorides, and have a low conductivity; have it checked by a laboratory. Acceptable conductivity, measured in deciSiemens per metre (dS/m), varies with soil type (Table 1). Conductivities above these figures may cause serious reductions in yield.
Table 1. Water conductivity above which yield may be reduced
| Soil type |
Sand |
Loam |
Clay |
| Conductivity |
3.0 dS/m |
1.7 dS/m |
1.0 dS/m |
How much water do sweetpotatoes need?
Sweetpotatoes need up to five megalitres of water per hectare. Thorough watering is required to establish the cuttings. Following establishment it is important to maintain uniform soil moisture conditions right up to initiation of storage roots. Excessive watering on poor soils may produce large lenticels and root rots whereas irregular irrigation will result in growth cracks, malformed roots and a lower marketable yield.
How do I know when to irrigate and how much to apply?
To manage irrigation accurately you will need a scheduling instrument, such as tensiometers or capacitance probes, for example Enviroscan.
Tensiometers are relatively cheap instruments that measure the availability of soil moisture to plants. Install two tensiometers per site: one with the tip in the main root zone 20 cm deep and one with the tip below the main root zone at about 60 cm deep.
The stage of crop will determine how much the soil can be allowed to dry out, but generally tensiometer readings of between 10 and 30 on clay and sandy loams indicate optimum soil moisture. Readings above 30 indicate the need to irrigate.
Two sets of tensiometers per block or 5 ha generally give adequate indication of soil moisture. Where soil types differ within a block, a set should be used in each soil type.
Capacitance probes are expensive and are usually operated by consultants.
Harvesting
When is the crop ready to dig?
The crop is ready to dig when the bulk of the roots are of marketable size. This can vary from 110 days after planting onwards. Digging normally starts just before the optimum size so that each planting can be finished before the roots grow too large.
Why and how do I remove the tops?
The tops are removed to toughen the skin and for ease of sorting on the harvester or over the digger. Two to five days before harvest remove tops using a flail forage harvester or slasher.
How are sweetpotatoes dug?
Many growers use a modified potato digger with a conveyor attached. This allows some sorting for over or undersized storage roots and defects before the roots go to the shed in plastic bins or bulk crates for washing. This machine is often referred to as a harvester.
Smaller growers use a single-row potato digger to lift the roots, which are then collected by hand into 30 L plastic crates.
Both machines must be operated with great care to minimise root damage. Slower harvesting speeds will help reduce damage. Soil should be moist enough to cushion the storage roots as they are being lifted and to soften any clods.
What about washing?
Roots must be thoroughly washed to remove soil before sale. High-pressure water jets are normally used, but roller brushes may also be necessary in heavier soils. A postharvest spray or dip is recommended to prevent rhizopus rot, scurf and bacterial soft rot and to preserve the root quality.
Marketing
How are sweetpotatoes graded?
Grade standards no longer legally apply in Queensland; however, roots are usually graded into small, medium and large. The medium grade is preferred and is usually divided into large and small mediums, to enhance the appearance of the pack. The most desirable root size on the market ranges from 45 to 90 mm in diameter and 150 to 250 mm long.
How are sweetpotatoes packed?
Sweetpotatoes are usually packed into wilm-lined cardboard cartons, though cartons with a waxed inner perform better, especially if wet roots are packed. Wilm-lined cartons have a wax sprayed onto the inner liner and fluting during manufacture. The minimum weight must be clearly marked on the carton. The standard is 18 kg, however there are a number of other pack sizes specified customers, you need to negotiate with your customers about the pack size they require.
Pack roots of uniform size and shape in the one carton and pack the top layer in an orderly manner to improve the appearance of the pack. Pack over the net weight to allow for shrinkage.
Quality assurance
Why do I need quality assurance (QA)?
Because your customers are, or soon will be, asking for it. The driving force behind this push for quality is food safety and the fear of litigation. There have been enough instances of food poisoning or injury resulting from food consumption, for retailers and other businesses in the food industry to ignore them at their peril. Some of these occurrences have been due to contaminated fruit or vegetables.
New national Food Safety Standards became enforceable from February 2001. Fruit and vegetable growers are part of the food industry and so retailers, wholesalers, processors and other handlers of fruit and vegetables have placed requirements on suppliers to ensure that the products they sell are going to be safe to eat. In future, you will probably not be able to market fruit and vegetables without demonstrating that you have an effective food safety management system of some type in place. Quality management systems provide the framework for this to be achieved.
What quality assurance standard do I need?
If you market directly to a supermarket they will tell you what you need. It will be one of either SQF 2000, SQF 1000, ISO 9002 + HACCP or a supermarket Quality Management Standard.
If you supply to a wholesaler/agent or processor, you will need to be an approved supplier to them if they want to on-sell your product to the supermarkets. The minimum requirement to be an approved supplier to a wholesaler currently varies between wholesalers, but the Freshcare Code of Practice is gaining popularity around Australia as providing acceptable food safety practices to meet the supermarkets' requirements for approved suppliers.
What is HACCP?
The Hazard Analysis and Critical Control Point (HACCP) method is an internationally recognised, systematic approach to identify, evaluate and control hazards to product specifications. HACCP relies on prevention to ensure food is safe to eat, but has also been applied to ensure customers' quality requirements are met.
HACCP was developed from the need to produce safe food for the US space program and is now widely used by all sectors of the food industry as the preferred system to manage food safety. HACCP is a key component of SQF 2000, SQF 1000 and the Woolworths Quality Management Standard. These standards require that HACCP address food safety and product quality.
What is an approved supplier program?
An approved supplier program involves suppliers carrying out practices that will provide assurance to a customer that produce is safe to eat and of acceptable quality. Suppliers need to keep sufficient records to demonstrate that the agreed practices are a part of everyday operations. The customer or someone on behalf of the customer will periodically check that suppliers are carrying out the practices.
Examples of where approved supplier programs are required:
. Growers supplying harvested produce to commercial packers and processors.
. Growers and packers supplying packed produce to wholesalers.
. Growers supplying packed produce to a marketing group.
What is Freshcare?
Freshcare is the name for a national, industry-managed program for certification of approved suppliers. It has been designed to provide independent auditing of a HACCP-based food safety program implemented by growers.
A Freshcare Code of Practice has been developed which lists practices and documentation growers need to implement in order to meet the requirements of a Freshcare audit, and so provide assurance to customers that their produce is safe to eat.
General
Why do we use one word for sweetpotato, not two?
The decision to use one word for sweetpotato was made at an international workshop held in Lima, Peru in June 1994. It was made to help differentiate between sweetpotatoes, Ipomoea batatas, and the common English or Irish potato, Solanum tuberosum, which is a member of a different botanical family.
It is important to differentiate between sweetpotatoes and ordinary potatoes because they can be used in different ways and they require different management for growing and postharvest handling. To many people a sweet potato would be a potato with a high sugar content.
Using sweetpotato as one word should help in marketing sweetpotatoes as a distinct product with different uses to normal potatoes.
What are the differences between sweetpotatoes, English potatoes and yams?
Sweetpotatoes, English potatoes and yams are different and belong to different plant families. Table 2 shows some of the more important differences.
Table 2. Comparison of sweetpotatoes, English potatoes and yams
|
|
Sweetpotato | English potato |
Yam |
| Scientific name | Ipomoea batatas | Solanum tuberosum | Dioscorea spp. |
| Plant family | Convolvulaceae | Solanaceae | Dioscoreaceae |
| Plant group | Dicotyledon | Dicotyledon | Monocotyledon |
| Edible part | Storage root | Tuber | Tuber |
| Taste | Sweet and moist (orange-fleshed) |
Bland and mealy | Dry and starchy |
| Number per plant | 4 - 10 | 2 - 6 |
1 - 5 |
| Growing season | 16 - 25 weeks | 16 - 18 weeks | 24 - 32 weeks |
Are sweetpotatoes worth growing?
Prices can fluctuate dramatically, but there is a seasonal trend with the best prices from about August to January. Sweetpotatoes require specialised equipment. They are labour intensive and planting material must be maintained, making this a crop for a long-term commitment. Growing and marketing costs vary from $10 to $16 per carton.
What is the best soil type for growing sweetpotatoes?
Deep well-drained sandy to sandy loams are preferred, but any deep well-drained soil is suitable.
What growing conditions are required?
Sweetpotato is a tropical crop and warm sunny conditions are ideal. The plants are susceptible to frost; cool conditions reduce vigour and can result in malformed storage roots. The growing season is 16 to 25 weeks, depending on variety and temperature.
What causes misshapen sweetpotatoes?
Low soil temperatures during early development of storage roots cause ribbing. Growth cracks result from fluctuating growth, often caused by varying soil moisture. Early infestations by high populations of nematodes can also cause cracking of the storage roots. Veining (subcutaneous roots) is a disorder associated with older roots and is secondary growth. Poor land preparation, soil type and nutritional disorders can also affect root shape. The presence of disease increases the length to diameter ratio and consecutive generations become longer and thinner as disease levels increase.
What machinery and equipment do I need?
At least two tractors are needed: one large tractor (about 45 to 60 kW) for primary tillage and digging, and a smaller one (about 26 kW) for planting, interrow cultivation and spraying.
Other equipment required includes a planter, interrow cultivators, a chemical boom spray, a digger or harvester for harvesting, and a washer for washing the roots. Some of this machinery will be specific to sweetpotato production while some can be used for producing other crops.
Further information
The complete Agrilink Sweetpotato Information Kit, produced by the DPI&F, provides information on all aspects of growing and marketing sweetpotatoes in Queensland.
The kit includes the following sections:
- Before you start
A checklist of things you need to know before you start growing the crop. - Common questions
The 20 or so most commonly asked questions about growing the crop. - Growing the crop
Our guide for establishing, producing and marketing the crop. - Key issues
Detailed information on the key decisions affecting the crop. - Problem solver
A picture series of the common problems and how to solve them. - Contacts and references
A list of industry organisations, product suppliers, and further reading. - Index
An A to Z index to help you find information quickly. - Your information
A place to store your special information.
For further information on the Agrilink series, see the list of contacts on the Agrilink website.
DPI&F information and services
- To access DPI&F's information and services, Queensland residents can contact the DPI&F Business Information Centre on 13 25 23 for the cost of a local call. The Business Information Centre is open 8 am to 6 pm Monday to Friday (excluding public holidays); E-mail callweb@dpi.qld.gov.au. Non-Queensland residents phone (07) 3404 6999.
- Current national information on agricultural chemicals registered for use on all crops is available on the Infopest CD-ROM. Write to DPI&F, GPO Box 46, Brisbane, Qld 4001, E-mail infopest@dpi.qld.gov.au, visit the Infopest web page, or phone (07) 3239 3967 for further information.
Industry links
- Industry links is a page of links to various sites of interest to horticultural growers.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries, Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 14 October 2005