Introduction

Sweetcorn (Zea mays) is an annual plant that can grow on average between 75cm - 1.7m (30in - 5.5ft). Each plant has an average spread of 45cm (18in). Each plant carries both male and female flowering parts - the cob being the female part and the tassels at the top of the plant being the male parts. Cobs come in all shapes and sizes. The small 6 - 8cm (2.5 - 3in) 'baby' cobs are used in stir fries and tiny 5cm (2in) red coloured jewel like types are used for making popcorn. The big ones come in lots of colours. The most common types for eating tend to be golden, yellow, white or bicoloured. The multi-coloured forms (reds and blues predominantly) tend to be used for decoration or speciality flour production.

There are main types of sweet corn for eating: Normal, Sugar Enhanced and Super Sweet. Each one has a different taste and tenderness. 'Normal' sweetcorn vary in sugar content, but the sugars very quickly convert to starch (flouryness) after harvest. 'Sugar Enhanced' sweetcorn also vary in sugar content, but have increased tenderness and the conversion from sugar to starch is slower than 'Normals'. 'Super Sweet' (sometimes referred to as 'Shrunken') are very sweet. The conversion of sugar is very slow and they therefore store reasonably well (approximately one week) without going 'floury'.

NOTE: Never allow cross pollination between types as this will result in tough, starchy kernels in all types.

Sweetcorn must have a long warm season to perform well. This means around 70 - 110 frost free days after planting and a temperature between 16C - 35C (61F - 95F). Sweetcorn are shallow rooted, so protection from extreme winds and water loss in the soil is very important when choosing the site for planting.

Cultivation

Sweetcorn seed does not germinate below 10°C (50°F); Super Sweet types a little higher. Start the plants off indoors in cells, pots or modules filled with moist compost. The compost should be moist, but definitely not wet. Wet compost (which can even be the case straight from the bag) will be cold and reduces the oxygen that the seeds need to germinate. It may even stop the seeds germinating at all. If the compost is too dry this will also cause problems. If this is the case, sprinkle a small, amount of warm, clean tap water over the compost and leave it to soak in thoroughly. When the compost has absorbed the water, take a small handful and squeeze it in your fist; if water drips out it is too wet, so leave it for a short while in a well ventilated spot to dry off. If it holds together when you open your hand, it's just about right. You can now fill your seed tray or pot with the moist compost and you don't have to water it again before you sow the seed. The seed can also be planted in situ if the soil is over 15°C (59°F).

Sow 2 or 3 seeds 2.5cm (1in) deep every 20 - 25cm (8 - 10 in) depending on the variety. Mini cob types can be planted 15cm (6in) apart. Thin the seedlings to the strongest one on germination.

Sweetcorn should be grown in blocks of 4 rows to ensure even pollination. Leave 60cm (24in) between the rows. Plant out young plants in blocks at the same spacing. Hoe lightly when weeding to avoid root disturbance and in exposed areas earth up the stems to 13cm (5in) to increase stability. Although sweetcorn can tolerate high temperatures and drought, this can lead to poor pollination (empty cobs). Therefore in hot dry weather and during flower and grain development extra watering may be required (22 litres per square metre - 5 gallons per square yard).

Harvesting

This is generally when the kernels are full and, when squeezed, produce a 'milky' coloured juice. If the juice is watery the cob is under ripe or if it's hard or doughy it has gone over. This ripeness is also indicated by the change in colour of the silky tassels coming out of the cob tops. This changes from light yellow to a rich dark brown. Try to pick and eat the same day as this will give the best flavour and sweetness.

Ornamental types are generally left on the plant until after a frost or two. The cobs can be picked after the outer leaves around the cob or husks have started to brown and dry. Make sure the cobs are fully mature - hard and glossy - before picking. After picking and removing the husks allow them to dry in a well ventilated place for a few weeks before use as popcorn or winter dried flower displays.

Pests and Diseases

Sweetcorn is generally pest free, but still keep an eye out for greenfly/aphids, mice and slugs. If they become a problem, use a recommended proprietary brand of insecticide or use traps as appropriate.