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Sweetcorn developed thousands of years ago as a natural variant of maize which is a native of Central America, probably Mexico. It was brought to Europe by Columbus and subsequently spread worldwide to become one of the most important food crops for mankind.

Sweetcorn differs only in a single gene from maize which slows down the conversion of sugar to starch. This produces kernels with a high sugar content and pleasant texture in contrast to the starchy grains of maize. It now only exists in cultivation and as such could be called a man-made crop.

This vegetable was transformed for the commercial grower by the development of supersweet varieties in the mid 1980’s. These varieties possess sh2 genes which causes them to convert much less of their sugar to starch to produce kernels about 30% sweeter than the standard ones (known as normal sugar), but more importantly hold their sweetness for longer. The old varieties, once ripened, quickly converted their sugars into starch and had to be used immediately. Another type you may come across in catalogues is the ‘extra-tender sweet’ variety which is a sweeter and less chewy version of the supersweets.

Sweetcorn is one of the half-hardy vegetables and is better suited to the warmer eastern and southern parts of the country. But the development of new varieties has rendered the crop less susceptible to the vagaries of an Irish summer. That said it will always do better in a warm summer and select a warm sheltered site if possible. It is doubtful whether commercial production of this crop would be financially viable except in the most favourable parts of the country.

Most of the varieties available are hybrids. The supersweet and extra-tender varieties can be grown together but must be separated from the normal sugar varieties plus the whites and multi-coloured types. All varieties must be isolated from forage maize by at least 75m as the starchy character of maize is dominant to the sweet character of sweetcorn. If you’re growing sweetcorn in one of the colder areas of the country you’d be advised to stick with using just the early varieties such as Earlibird or Northern Xtra Sweet. 

Maize is grown commercially in Ireland for silage production and a lot of the crop is direct drilled through a strip of biodegradable clear plastic. The plastic increases the soil temperature by about 2-4°C which generates better growth especially for earlier sown crops. Maize requires a temperature of 10-12°C to germinate and doesn’t thrive at air temperatures of less than 10°C.

Unusually for a vegetable crop it's a member of the grass family (Poaceae) and as such is wind pollinated. For this reason sweetcorn is sown or planted in blocks to allow the pollen produced by the male tassels produced at the top of the plant to fertilise the female flowers known as cobs.

Sweetcorn can be either direct drilled or transplanted. It’s not a crop that takes to transplanting well from bare roots so always propagate the seed in pots or modules. Sow 2.5 cm deep in large modules or pots under glass in mid-April to early May and plant out when the seedlings are 10 cm tall, sometime in May. Don’t delay too long in getting the plants out – it only takes 3-4 weeks under glass to have a plant fit enough to plant out.

Plant in a block formation at a spacing of 45x45 cm or 45x40 cm for a higher direct drill the crop outdoors in May to the first week in June. In warmer parts of the country and if drilled under clear plastic an early crop could be sown in April. The earlier drillings can with advantage be covered with fleece until well established, particularly if the weather is on the cool side. If not covered by fleece or plastic crops probably shouldn’t be drilled until mid-May. For succession under plastic suggest direct drilling in mid-April, early May and mid-May.

Sweetcorn is quite a nitrogen demanding crop. Up to 70 g/m2 of sulphate of ammonia can be used split half at sowing or planting and the other half a month later. Sweetcorn can be harvested from August through to October. The silks which hang from the developing cobs turn brown shortly after pollination and to a dry dark brown when the cob is close to harvest about 30 days later. The final test of ripeness is to push a fingernail into one of the grains – if the liquid runs clear it's unripe; if it's milky it's ready to harvest. You will normally harvest 2 cobs per plant. The top cob matures first followed by the one further down the plant. The supersweet varieties will last satisfactorily in a fridge up to a week.


Normal sugar: Sundance F1
Supersweet: Northern Xtra Sweet F1, Earlibird F1, Mainstay F1, Seville F1
Extra-tender: Lark F1, Lapwing F1, Wagtail F1