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SWEET  CORN  (Zea mays)

The latest research suggests that corn was domesticated in Mexico some 9000 years ago from a wild grass. A few thousand years later, it was introduced to South America. Small fossils of corn have been found at two mound sites on the Peru coast. The natives used the corn in several ways including popcorn and making flour from corn. By the late 1700’s it is known that sweet corn was being cultivated by the North American Indians. By the 1900’s, there were many open pollinated varieties of sweet corn in cultivation. Since then, there have been many breeding improvements to sweet corn to increase the sweetness, texture and ease of growing in colder climates.

With the latest varieties, sweet corn is now an easy crop to grow outside, even in Scotland, provided a few precautions are taken. However, it is important to choose an early or mid-season maturing variety, for the North of the UK. If you buy your own seed, you can ensure that you are getting the right type for your area. You can buy a whole packet of seed for the same as you pay for one plant of an unknown variety at the garden centre! I prefer the “extra tender and sweet (tendersweet)” varieties as they also have more tender kernel skins, as well as sweetness. Forget about growing ordinary maize unless you want to feed it to animals or make your own cornflakes!

There are four main different types of sweet corn generally available in the UK for culinary use, and they can come in early, mid-season or late varieties.  These are:-

Extra tender and Sweet  (tendersweet) or (Augmented supersweet). The sweetest type (up to 44% sugar) with softer kernels.  Once picked, the sugars do not change to starches as quickly as the older varieties of sweet corn. Also can be quick to mature and should be ready to harvest in September in the North of the UK, and this is the type that I would recommend.

The seed of this type of sweet corn, are shrunken in appearance and will need moist (not sodden) compost in order to swell and germinate. A soil temperature of more than 15-16C is required for germination and is best obtained in a propagator.

Isolation of this type is required from standard (Su) sweetcorn varieties as well as from baby and pop corn, or the kernels can become tough and starchy and lose the tenderness. Isolation also required from the Supersweets (Sh2) type of sweetcorn or the tenderness will be lost.

Suggested varieties of Extra Tender and Sweet (tendersweet)

Swift F1.  Very early, very sweet and tender.

Lark F1.    Mid season, very sweet and tender.

Mirai types. Very early, very sweet and tender.

Supersweets (Sh2) Type. A sweet type (up to 44% sugar) but more chewy than tendersweets. Once picked, the sugars do not change to starches as quickly as the older varieties of sweet corn and may store for up to 10 days. The most likely type to be found for sale in the supermarkets. They do not have the creamy textures of standard sweet corn.

Isolation of this type is required from standard (Su), sweet corn varieties as well as baby and pop corn, or the kernels will become tough and starchy in both types. All yellow Sh2 types can be grown together, but Sh2 derivatives such as whites and bi-colours will require isolation to prevent colour contamination.

The seed of this type of sweetcorn, are shrunken in appearance and will need moist compost in order to swell and germinate. A soil temperature of more than 15-16C is required for germination and is best obtained in a propagator.

Suggested varieties of Supersweets.

Extra early sweet.

Sugar enhanced (Se) Type.

An early improvement over traditional types with up to 2 times the sweetness of Su type and softer kernels, but now superceded by more modern varieties. Can retain sweetness for 2 to 4 days after picking.

Traditional open pollinated or Standard varieties. (Su) or Normal Sugary Types Can contain up to 6% sugar but are slow to mature and not really suited for the North of the UK. In addition, any sweetness will quickly turn to starch as soon as the cobs are picked or are left too long before harvesting. If there is cross-pollination with maize or pop corn, standard varieties will become tough and starchy.

Baby corn and Pop corn.

Baby corn is basically immature corn that has been especially bred to be picked early before the kernels have been filled out or develop their sweetness. It seems rather a waste of space to me. You will have to experiment to find the right time to harvest your baby corn. Do not grow this type beside the previous varieties, or they all will be affected.

Suggested variety of baby corn.

Mini Pop F1. Several mini cobs should be picked before the tassels appear.

Suggested varieties of Pop corn.

Red Strawberry. Produces 5 cm long cobs.

Cross-pollination of sweet corn.

Do not plant any of the different types of sweet corn too close together, or you can get cross-pollination of the cobs. This includes popcorn and ornamental varieties of corn. The supersweet cobs can become starchy or lose some of their sweetness, and the tendersweets will lose some of their tenderness as well. If you plant early and late varieties where the pollen is not mobile at the same time, you may avoid cross-pollination issues.

The published literature suggests that all sweetcorn crops of different types, as shown above, should be isolated from each other and from Maize(grown for animal feed and cornflakes), by at least 75 meters or 3 weeks pollination time.

This can obviously cause potential cross-pollination problems if you are growing your sweet corn on allotments, where other types may be close by. It is likely that provided you plant your sweet corn as far apart as possible from possible contamination, any pollen reaching your corn will have been sufficiently diluted to only affect a few kernels on your cobs. While the resulting cobs that have been subjected to cross-pollination will be perfectly edible, it can adversely affect the taste, sweetness, colour and texture of individual kernels, as shown in the photo to the left.

You can grow different varieties of sweetcorn close to each other provided they are from the same group as shown above. If they are different coloured varieties, the kernels could be variegated, but the taste and texture should not be affected.

Unfortunately, most sweetcorn packets of seed give very little information as to which type of sweetcorn it is. Try doing a web search or look at http://en.wikipedia.org/wiki/List_of_sweetcorn_varieties for information as to the type of sweetcorn.

Sow indoors in heat, at 20 C, on top of the compost to prevent the seed rotting, in individual cells in late April. Remember that there is no point in sowing the seed too early, or the plants will become too big before you can plant them out into their final growing positions, when danger of frost has passed. Move the plants to a cooler frost free location such as a cold frame to harden off before planting out in their final positions.

Plant out in June in a sunny, sheltered position, when all danger of frost is past in your area. In Northern areas, I would recommend continuing to provide cloche protection for the first month or so, to counter the effects of late frosts or cold winds.The ground should be well fertilized with organic matter, and the plants kept well watered, particularly while the tassels are growing, pollination is taking place and the cobs are filling.

Warning. On no account plant out these sub-tropical plants before the last expected date for frost in your area. This could be mid May for the mildest areas such as Southern coastal areas, and early June in the North of the UK. Even then, provide cloche or fleece protection for the first few weeks as cold nights and winds will cause plant damage. Gardeners are regularly caught out by late frosts occurring during the traditional cold period known as “The Ice Saints”, usually just before mid May. Remember, one night of a late frost will kill your sub-tropical plants!!

As they are wind pollinated, rather than by bees, remember to plant them out in blocks, say 4 plants x 4 plants, with the individual plants being about 35 cm apart. This enables the wind to better carry the pollen from the tops of the plants, the male part, down to the female “silks or tassels”.

Do not remove the “tillers” ( side shoots ) as they increase the area of leaf available to convert energy from the sun. Some of the “tillers” will form smaller cobs.

You can check if the sweet corn is ready to pick when the female tassels shrivel and turn dark brown. Then  gently pull back the protective green sheath and squeeze the kernels at the end of the cob with your finger nail. They are ready when a thick cream oozes out, rather than a milky liquid.

Prepare the cobs for cooking by removing the outer green husks and silks as indicated in the photos. Cook to eat, or blanch for the freezer, within 12 hours or as soon as possible after picking, before the sugars start to turn to starch. This is why home grown cobs are always better than shop bought ones which have been probably travelling for quite a few days or even weeks.  See recipes.

Uses of sweet corn. Eating freshly cooked, freezing or making popcorn.

Pests and diseases. Few problems are apparent in the North of the UK. In the Americas, there are quite a few insect pests that are not present in the UK. In other areas of the UK, smut balls, large galls on the stalks and cobs can appear in hot dry weather. These should be cut off and burnt as they will release black spores, and the plants burnt at the end of the season. Remember to rotate the sweet corn in subsequent years.

Squirrels, badgers, rats and mice are commonly reported to be eating the cobs and destroying the crop. Apart from getting rid of the squirrels, try taking the base off a 500 ml used plastic pop or water bottle, and place it over the growing cob.