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Unlike Supersweet and Extra-Tender varieties, the limited amount of sugar in the grains of these cobs begins to turn to starch as soon as the cob is harvested. They are for quick consumption otherwise refrigeration is necessary to slow the reaction.
Disclaimer
The descriptions, advice, suggestions and vegetation cycles below are aimed professional growers and are offered in all good faith, for purely informational purposes.
Fore more information please contact us on 01932 862059 or email info@tozerseeds.com
Please use the links below for detailed technical information about Sweetcorn. For further assistance please contact our Sweetcorn specialist, David Rogers
Sweetcorn has always been a Tozer speciality, the company’s first plant breeder Dr Dermot Dawson was breeding sweetcorn in the UK in the 1930’s. Sadly sweetcorn breeding in the UK came to an end in the 1970’s, however the company continues to run extensive trials of new varieties every year both at our head office and with many of the UK’s leading growers.
Sweetcorn is a member of the Gramineae or grass family. As such, the plant is a monocotyledon producing, generally tall, round stemmed plants with leaves at each node. The stem terminates in a male flower known as the tassel. The female flowers appear slightly later in the leaf axils and are referred to as cobs. Each cob has parallel rows of kernels or grain which are paired in even numbers of rows. The number of rows will vary from 10 to 22. From each embryonic kernel, a silky hair extends upwards and out of the cob sheath. Pollination occurs when pollen from the tassel lands on these silky hairs.
Pollination is generally wind induced and occurs most effectively when the sweetcorn is grown in a block. Shortly after pollination the silks on the outside of the cobs turn brown and about 30 days later, the cob will be marketable.
Most cultivars will produce two cobs. The primary or top cob will mature first, followed by a secondary cob lower down. Commercially, it is the primary cob that is important although the secondary cob may be marketable as a lower grade item.
There are various different types (Genotypes) of sweetcorn of which only 3 are of any importance within the UK:
These are the original type of sweetcorn, once harvested their sugars quickly turn to starch meaning that they should be eaten as soon after harvest as possible.
These types of sweetcorn replaced the SU types in most fresh market commercial productions during the1990s. Sh2 cultivars carry genes which increase the sugar content of the kernels by 30%, but perhaps more importantly, maintain this sweetness over a much longer period. It is not surprising that all supermarkets now insist on their growers producing Sh2 cultivars.
Extra Tender sweetcorn varieties began to appear during the early years of this century. They are a form of supersweet but significantly sweeter, they also have a thinner pericarp which means they are less ‘chewy’ to eat.
Supersweet cultivars will also require isolating from all other sweetcorn and maize types in order to prevent cross-pollination and the resulting loss of supersweet characteristic.
N.B: All yellow Sh2 types can be grown together, but Sh2 derivatives such as whites and bi-colours will also require isolation.
All commercial baby corn is imported already processed from countries with low labor costs. It is hand harvested and hand husked from immature corn at or within 1-2 days after silking. A variety of different genotypes can be used.
Ornamental or Indian corn varieties are generally inedible but have a range of colours and kernel sizes. Generally a difficult crop to get right in the UK due to the length of season required to harvest. Please see the Ornamental Corn page for further information
Due to many of the problems covered in the paragraphs below, Sh2 production has polarised into climatically favourable areas of England. The bulk of the crop is now grown in Hampshire, Isle of Wight, Dorset, Essex and Kent. Generally, the further north sweetcorn is grown the shorter the season and the less likely production will be successful.
Sweetcorn can be grown on most soils but the best soils are the medium to light lands with adequate water holding capacity favoured by growers of other intensive crops. Light sandy soils will require irrigation and heavy clay soils may need to be tiled to improve drainage conditions and may slow down the rate of growth. Ideally sweetcorn should be grown on land which is south facing and protected from wind and frost. The optimum pH range for sweetcorn is 5.8-7.0.
The seed of Sh2 cultivars is smaller, lighter in weight and shrunken in appearance (giving this type of corn the name shrunken). For this reason Sh2 cultivars are more difficult to produce than their normal sugary predecessors. Firstly, Sh2’s require at least 2-3˚C extra soil temperature than normal sugary cultivars (15-16˚C as oppose to 12-13˚C). Generally sweetcorn will take about 5 days to emerge from 20˚c soils. Sh2’s, being a shrunken seed; also need to take up at least twice as much water in order to germinate. Being generally lower in vigour, Sh2 seed should be placed no deeper that 4cm and into a well prepared, level seed bed.
Sweetcorn seed is generally treated with a mixture of systemic and contact fungicides, it is considered important to add significant protection to sweetcorn seed as it is far more vulnerable to fungal attack than most other vegetable seed. It is also possible to add a systemic pesticide to the seed. For more information please see Treatments and Formats
Drilling densities generally range from 44,000/ha to 62,000/ha with lowest densities being used for the earliest drilled crops. Sweetcorn is not usually transplanted as the young root system can be sensitive to disturbance.
Drilling generally takes place from mid-April (in some very early areas, drillings are made in late March) under some from of polythene mulch. The earliest crops will emerge from fixed covers from which the corn will require ‘releasing’ at a later stage. Alternatively a moisture degradable polythene system can be used; with this system a one pass operation is possible: A bed former forms a raised bed and moisture degradable mulch is stretched tightly across. A beak drill then drills through the mulch. The hole punctured by the beak drill then allows the seedling to emerge.
Generally, the crop is established on 60 or 75cm rows Research has shown that large, in row spacing. i.e.: 30-45cm results in the best quality cobs, however, these large spacings would require hand thinning, and this is not practical for large areas of corn. In practice, a compromise occurs with the majority of growers choosing to drill to stand at around 20cm.
Due to the odd shape and size of Sh2 seed, vacuum drills are the most suitable equipment for accurate seed placement. Other drill types e.g.: Stanhay are used, but do not achieve accurate results with the long, sharp, angular seed.
Uncovered drillings are generally not a safe option until at least mid-May. Even then, most large producers are also drilling at least some of the crop through plastic, because not only will the mulched crops be earlier, but they also produce
Normal sugary varieties should be isolated from Supersweet and Extra Tender types in order to minimise cross pollination, this can be achieved through distance or time. All sweetcorn crops should be isolated from Maize by at least 75 meters or 3 weeks pollination time.
In order to maximise the length of harvest period it is necessary to utilise different varieties and drilling dates.
The earliest drillings can be made under protection in April, the variety used should be an early one, these varieties are likely to mature in the first half of August. These early varieties are generally used until the beginning of May when they are replaced by more mid-season types. By mid-May it should be possible to drill without protection with final drillings generally taking place during the first week of June. The final drillings will generally be of a late maturing type, these are more suited to maturing in the shorter days of autumn.
Nutritionally, sweetcorn require an abundance of nitrogen throughout the growing period. Up to 150kgs/ha are used for early drillings on light soils. Phosphorus applications should be in the order of 60-80kg/ha. Potassium requirements vary from 80-100kg/ha in fertile soil to 120-150kg/ha on sandy soils. Sweetcorn does not have a high trace element requirement, with the exception of zinc.
Sweetcorn suffers fewer problems than most vegetable crops. Frit fly and cut worms can cause serious damage but can be effectively controlled by insecticidal applications.
Post emergence, it is aphid which causes most problems. Some sweetcorn cultivars seem to attract aphids to a greater extent than others. Aphids present particular difficulties where they attack the cob sheath at harvest time. Large numbers of aphids on the cob create cosmetic problems and add to the pack shed costs. Over-stripping of the cob sheaths also spoils the cob appearance and degrades shelf life. Western Corn Rootworm has recently been found in the UK however it has not become a widespread problem
Other, pests include rooks and game birds which pull up seeds and seedlings. deer and badgers can also be a problem.
Sweetcorn is very poor at competing with weeds and thus pre emergence herbicides are commonly used. In some situations further herbicide application may also be necessary, particularly with non-mulched crops.
Damping off
Damping off is a term referring to the infection of a seed by soil or seed borne fungi. Sweetcorn seed is most susceptible to these fungi if the seed coat is damaged in some way or if the seed sits for a prolonged period in cool damp soils. The seed treatments applied to most sweetcorn seed eliminate most of the risk but growers of untreated seed should take particular care.
Rust
Rust is a common occurrence on sweetcorn leaves; it does not affect the quality of the cob but affects the ability of the host plant to photosynthesise. Some varieties of sweetcorn are listed as rust resistant, however in our trials they do not appear to be resistant to rust it the UK.
Smut
Smut, Ustilago maydis is easily recognised as enlarged fungal swellings on the stem, developing cobs or tassels. The fungus overwinters in the soil and spores may be blown into a sweetcorn crop from a distance away. Smut is considered a delicacy in some areas of Central America!
Virus
Identification of different virus infections can be very difficult, however Barley Yellow Dwarf Virus has been identified in UK crops and some varieties appear to be more susceptible than others. The distinctive symptom of this disease is the discolouration of leaves from green to yellow.
Water management is required to produce optimum yields and quality ears if water stress is likely. Yield and quality can be significantly reduced if moisture stress occurs during pollination and kernel fill. Short periods of moisture stress in earlier in crop development will not usually affect yield.
The optimum temperature for sweetcorn growth is 23 – 30˚C, clearly in the UK this is not achievable through a whole season and in some years barely achievable at all. In practice this results in plant growth slowing down and consequently it is difficult to predict with any accuracy how long a particular variety will take to mature in the UK, it also means that the economics of sweetcorn production become less appealing in the more northerly counties. Problems can occur when night time temperatures are cold during pollination, this can result in irregular kernel formation and thus to unmarketable cobs. Sweetcorn plants will stop growing completely in temperatures below 10˚C
Sweetcorn varieties can be day-length sensitive which means that extensive trials are required to ensure that a variety is adapted to northern European conditions. A short day variety grown in long day conditions will tend to grow very tall but fail to flower.
Sweetcorn in the UK usually matures 25-30 days after silking. A cob is mature when the silks at the end of the cob become dry and brown and the kernels become plump, milky and tender. Sweetcorn has a short harvest period and harvesting on the day of optimum maturity is important for good quality and yield.
In the UK the crop has traditionally been hand harvested however there is a move toward machine harvesting due to the cost of labour.
Unless it is to be marketed immediately Sweetcorn should ideally be chilled after harvest to maximise it longevity, also any long flag leaves should be removed as these will draw moisture from the kernels. Quality of fresh market sweetcorn is judged by its fresh, uniform appearance, uniform and well filled rows, plumpness of kernels, milky kernel contents, and freedom from damage and defects. Cobs are also required to be filled with grain all the way to the end. This is referred to as ‘Tip Fill’. Most multiples will also require cobs to have a minimum number of grain rows around the cob, referred to as ‘Row Count’. 16 rows is the ‘magic’ number, although fewer rows are permitted on early cultivars.
Sh2 sweetcorn can be stored for up to 2 weeks if kept at low temperatures (0° – 1.5°C; 95-98% R.H) however care should be taken as freezing injury will be initiated at -0.5°C.
Sweetcorn is generally sold in the multiples un-sheathed in twin packs or cut into sections. Outside of supermarkets it is usually sold still wrapped. Sales of sweetcorn cobs continue to grow in the UK and in Europe although the quality of summer weather has large bearing on consumer purchasing.
