Note Number: AG0378
Published: August 1995
Updated: May 2013
Sweet corn is grown in Victoria for the fresh market and for processing. The main production areas are the river valleys of East Gippsland and Koo-wee-rup, but production is expanding into other districts. Sweet corn is distinct from maize because the sugar in sweet corn is not converted into starch as readily as is the sugar in maize.
Crops should be planted once the likelihood of frosts is over and soil temperature is above 12°C.
Sweet corn is a warm climate plant which needs about 80 to 120 frost-free days in southern Victoria from sowing to harvest. It needs warm, sunny weather. The temperatures for optimum germination should be above 18°C. For optimum growth and quality the temperature range is from 24°C to 30°C.
The kernels will not set, especially on the tip of the cobs, if conditions are hot or cold or dry and windy at silking. Hot conditions hasten maturity, the sugar in the kernels turns to starch, and quality deteriorates rapidly.
Sweet corn needs adequate moisture throughout the growing season, especially at the flowering stage. However, always avoid waterlogged conditions.
The popular varieties grown are hybrids but sweet corn grown in Australia is now broken up into two major types: normal and supersweet.
Supersweet Nearly all corn grown for fresh market are supersweet types. These types differ from the traditional normal type for they have a shrunken gene which produces twice the level of sugar as in normal varieties and also reduces further the rate of sugar conversion to starch. The result is a very sweet cob and supersweets are now the major type of sweet corn sold on the fresh market.
Supersweets are also available in bi-colour forms which have white kernels dotted amongst the yellow kernels which give them a more distinctive appearance from the traditional normal types.
Supersweet varieties include Honey Sweet, Goldensweet, Goldensweet Improved, Gladiator, Punchline, Sovereign and Matador. Bi-colour cultivars include Golden Pearl, Crunch, Samurai and Madonna, while white cultivars include Everest and Blizzard. New cultivars are constantly being released and the main varieties grown will continue to change.
Cultivars differ considerably in characteristics such as length of growing season, cob number, cob size and number of kernel rows per cob. The depth, colour and lavour are also cultivar characteristics.
Normal - Is the type of corn traditionally grown in the past such as Jubilee. It differs from maize by having genes which slow down the conversion of sugar into starch and characteristically has a creamy texture when ripe. The main normal variety is Jubilee.
The growth of sweet corn can be divided into 3 main growth stages during which stress or shortage of nutrients or water can affect the yield.
Stage 1 - This is the rapid growth stage and begins at around 4 weeks after emergence and lasts for around 25 days and is followed by tassel emergence. Rapid stem elongation begins and at this time ear size is being determined and any stress on the plant will permanently affect the ear.
Stage 2 - Pollination is the next stage and lasts for around 5 - 10 days when any stress will result in delayed silking and prevent kernel development generally at the tip and even if fertilised the kernel can fail to develop resulting in poor tip fill.
Stage 3 - This is the least critical period and lasts for around 12 days after silking. Stress can reduce kernel fill and determines whether the tip kernels fill even if pollinated.
Soil and fertilisers
Sweet corn thrives in deep, free-draining and self-mulching loams or peaty soils, containing plenty of organic matter. Avoid soils that set or waterlog and light sandy soils that dry out. Organic matter and good soil structure are prerequisites for good nutrition, so corn should follow pasture, legumes
o rates will depend upon soil fertility and previous history. A soil test before planting will provide a guide as to fertiliser requirements.
The soil should be tested for soil pH and for available potassium content. If the soil pH is below 5.5, apply lime to raise the level to 6.5.
As a base dressing, fertiliser can be banded separate from the seed at rates of around 500 kg/ha of 12:12:17 or 600kg/ha of 8:10:10 or equivalent. On less fertile soils rates could increase to 700kg/ha and 900 kg/ha respectively. Triple these rates if broadcasting fertiliser.
Side-dress crops at around 110 kg/ha of nitrogen which is equivalent to 240 kg/ha of urea or 300 kg/ha of Calcium nitrate or Potassium nitrate or 500 kg/ha of 20:0:16. This should be applied in 1 or more side-dressings with the first at around 30 days after emergence when the rapid growth stage begins and all fertiliser for side-dressing should be applied by early tasselling.
Farmers should be aware of the potential dangers of using phosphatic fertilisers with high levels of cadmium. Research has shown that the use of phosphatic fertilisers which contain the heavy metal cadmium as a contaminant can increase cadmium levels in both soil and potatoes. There are legal maximum levels of cadmium allowable for vegetables sold in Australia, however no violations have been recorded for sweetcorn as a result of survey investigations. Fertilisers containing cadmium in excess of 1 mg/kg are required to state the following warning.
"WARNING this product contains cadmium. Continued use of this product in agricultural situations may lead to residue levels in plant and animal products in excess of the maximum level specified by the Australia New Zealand Food Standards Code and the accumulation of cadmium in soils."
It is in the grower's and consumer's interests to minimise the addition of cadmium to soils and agricultural produce. Growers should consult fertiliser suppliers or manufacturers for advice on the cadmium levels of fertilisers they are considering using. There are a number of low cadmium horticultural
fertilisers marketed today. These have lower levels of cadmium than some superphosphate and other fertilisers.
Do not over-work and destroy the structure of the seed-bed. Air is excluded and roots will not develop in soil which is too fine, which runs together and which packs down after rain or irrigation.
Seed and sowing
Seed must germinate well and be vigorous, be free from seed-borne diseases, and be uniform in size and shape. The seed-bed must be moist, crumbly in structure and firm. The seed must not touch the fertiliser.
Supersweet corn produces shrunken kernels as seed and does not have the nutrient reserves of normal varieties. This means that germinating seed of supersweet cultivars is not as vigorous as the normal varieties and management of seedbed, temperature and moisture content is more critical.
Sow the seed 20 mm to 40 mm deep, depending on soil moisture and temperature. In cold weather, seed sown 20 mm deep will germinate earlier and better. A plant space of 900 mm by 250 mm, will give a plant population of about 45 000 per ha. Supersweet seed is shrunken and much lighter, consequently there are more seeds per kilogram.
Pollination is an extremely critical stage and poor tip fill will result if the plant is water or nutrient stressed or if environmental conditions are not favourable. Pollination continues over several days but it is not continuous and stops when the tassel is either too wet or too dry. Hot dry conditions are much more likely to interfere with pollination than wet conditions.
Sweet corn has a high requirement for water and needs 4-8 megalitres per hectare. Water management is very important especially during the critical growth stages when any stress imposed on the plant can affect yield. Particular care needs to be taken to avoid stress around pollination as poor tip fill
will result if the plant is stressed.
Leaf rolling is a symptom of water stress and can occur during the heat of the day even if sufficient water is present in the soil. However if leaf rolling is noticed in the morning and the soil is dry, the plant is stressed and yield is already being affected. It is better to schedule irrigation using a monitoring system to monitor soil moisture levels and irrigate according to need. Some examples of soil moisture monitoring equipment include enviroscan, tensiometers, neutron probe and aquaflex.
Good soil preparation is prerequisite for good weed control. Where couch or other perennial weeds are a problem, these needs to be killed the previous summer.
At least a month before intending to sow the crop, shallow disc or shallow rotary-hoe any pasture or cover crop. After the vegetation has dried and died, plough or chisel plough deeply then let the paddock settle. Just before sowing shallow work the paddock so that as little ungerminated weed-seed as possible is brought up.
Pre-emergence herbicides are best applied in bands over the drill row with spray jets mounted on the back of the seeder. The sprayed band should be about 150 mm wide, thus with a row spacing of 900 mm the rate of herbicide applied to each hectare at crop is reduced to a sixth.
The area between the sprayed bands can be kept weed-free by inter-row cultivation. Scarifying must be shallow to prevent root pruning. Alternatively, a complete cover can also be applied with a boom spray. The weeds may also be controlled after the emergence of the crop.
Under wet cool conditions germination and emergence of supersweet varieties are slow and this should be taken into account when considering herbicide use.
There are several herbicides registered to control weeds in sweet corn both pre and post-emergent.
For the registration status of these products, please refer to Australian Pesticides and Veterinary Medicines Authority (www.apvma.gov.au), your chemical reseller or your local chemical standards officer. Ensure you meet the relevant Maximum Residue Limits (MRLs) for the chemical in the end market, be it domestic or export.
Chemical users must ensure they read and understand all sections of the chemical label prior to use.
Atrazine is currently registered for the control of weeds in sweet corn. It must be used with care because it has a long residual effect and can damage subsequent crops, such as beans. This risk can be addressed by applying the lowest possible amount of chemical via band spraying. After harvest, plough the trash in deep to reduce the risk of atrazine residues. Users of atrazine are required to hold appropriate authorisation, such as an Agricultural Chemical Users Permit (ACUP) issued by DEPI.
Pests and Diseases
Sweet corn grown in Victoria is relatively free of pests and diseases. The main pest is heliothis caterpillar, Helicoverpa armigera (corn earworm). However this pest has developed resistance to many broad spectrum insecticides. There are now several soft insecticides (friendly to beneficial insects that predate or parasitise the caterpillar and or eggs) or biological pesticides. These should be used in conjunction with an integrated pest management (IPM) program to control Helicoverpa. An IPM program would utilise the biological controls Bacillus thuringiensis (a bacteria) or Nuclear Polyhedrosis Virus prior to silking and then 2 applications of Spinosad. There must not be more than 2 consecutive applications of spinosad. Broad spectrum insecticides are still effective on small larvae however once used an IPM program will no longer be effective. Other pests include aphids and in dry conditions mites can cause problems. The main diseases are rust which can be managed by using resistant varieties and Tercicum leaf blight.
Sweet corn is harvested by hand or by machine. The timing of harvest is important because in overmature corn the sugar has converted to starch and is tough to eat. Sweet corn should be harvested at the milk stage when the silks are brown and the kernels at the tip of the cob are 75% full and when squeezed
milky fluid will come out. If the kernels are dimpled the sweet corn is overmature. The cob quality deteriorates very rapidly in hot weather.
As soon as possible after harvest bring the temperature of the cobs down to near 0°C to prevent quality deterioration. Forced air cooling is the main method used to cool sweet corn. Hydrocooling can also be used before packing and will remove field heat quickly and can be used for packed produce. In bulk bins or packed cartons it will take much longer to drop the temperature in a cool room.
Sweet corn will keep without deterioration for about seven days at 0°C and 90 to 100% relative humidity. The husk on the cob will help prevent the kernels drying out and reduce the rate of warming up once the cob is removed from the cool room.
Do not let the temperature fall below 0°C as freezing the cobs will cause the quality to deteriorate.
Cultivars vary with some producing one cob per plant, others up to three. A crop of around 45,000 plants per hectare typically produces a yield ranging from 13-22 tonnes per hectare. Sweet corn is packed in cartons or in bags and is sold by the cob.
For information relating to the safe and appropriate use of chemicals, including management of chemical residues and licensing requirements, call our Customer Service Centre on 138 186 and ask to speak to your local chemical standards officer or visit Chemical Standards.
This Agnote was developed by Rob Dimsey, Farm Services Victoria in August 1995.
It was reviewed by Rob Dimsey, Farm Services Victoria in November 2009.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication