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HOW TO GROW
Sweet corn is a genetic mutation of maize which has been cultivated in Central America for thousands of years. The key difference between the two is that maize is generally more suitable for consumption when the kernels are hard whereas sweet corn is best eaten when the kernels are soft and the liquid inside is milky. Sweet corn, as the name implies is sweet when harvested at the correct stage of its growth.
Recent varieties of sweet corn have been bred to thrive in most parts of the UK and similar climates, so let this article guide you through the sowing, growing and harvesting of sweet corn the easy way.
SUPER SWEET SWEET CORN
Also known as SH2 corn. This type of sweetcorn should be grown well away from other varieties of sweetcorn if you want to retain the super-sweet taste. However they are the sweetest of all sweetcorn and retain that sweetness for the longest of all sweetcorn with the best keeping qualities if stored correctly. See our notes on how best to store sweetcorn here.
EXTRA TENDER SWEET CORN
Also known as Tendersweet and SE / SE+. These sweetcorn are not quite as sweet as the super sweet varieties but are most definitely sweeter compared to standard sweetcorn. They have a storage life of about 5 days if stored correctly. They can be grown next to the other two types without any problems. Their texture is more creamy and the skins more tender than super sweet varieties. Many people consider extra tender sweet corn to have a more authentic, full taste and easier to eat than the other two types.
STANDARD SWEET CORN
Also known as SU. These are the least sweet of the varieties being the original sweetcorn varieties. Their quality is not affected when grown with other types. Their major drawback is that their texture and sweetness deteriorates at an alarming rate when picked. Within an hour or so of picking a good proportion of their sugar will have converted to starch making them not so pleasant to eat. Cook this type within minutes of picking and you will have the very authentic taste of sweetcorn.
HOW QUICK TO MATURE
Another way to classify sweetcorn, especially if you want a long cropping season, is how quick they are to mature. If you sow two or three varieties with different times to maturity the harvesting period can be extended considerably. However, bear in mind that Supersweet varieties should not be grown anywhere near other Supersweet varieties. For this reason we would suggest only growing Extra Tender varieties in the same garden if you are growing more than one variety. See our page on sweetcorn varieties for much more information on maturity times.
VARIETIES OF SWEETCORN IN THE UK
The choice of sweetcorn varieties is bewildering so we have created a page with detailed reviews of some of the best available. Click here to go the now and choose the best variety for your growing conditions.
QUICK CALENDAR FOR GROWING SWEET CORN
Sow seeds under cloches - the fourth week of April
Sow seed in pots indoors - the third week of April
Sow seed outside - the second week of May
Plant out seed sown in pots indoors - the third week of May
Begin to harvest - the fourth week of July onwards
The best place to grow sweetcorn can be summed up as, in full sun, in free-draining, water retentive rich soil and protected from the strong winds. They also are best grown in blocks rather than straight lines. Each of these requirements is described in more detail below.
Sweetcorn will always produce better cobs when grown in full sun. They will tolerate some shade but of all the common vegetables grown in the UK they are the the most sun-loving. Recent breeding programs have produced sweetcorn varieties which are better suited to the UK climate so if your site has partial shade try a variety that will tolerate those conditions well. See our page on varieties of sweetcorn for full details. A couple of good varieties for partial shade are Earlibird (supersweet), Northern Xtra Sweet (supersweet) and Sparrow (extra tender sweet).
Sweetcorn is best sown indoors or in a warm greenhouse, this method will produce developed seedlings at the earliest stage possible, crucial in the cool UK climate. They can also be sown directly in the soil after all danger of frost has passed although this is only really suitable for warmer parts of the UK. A third method is to sow them directly outside but with the protection of cloches. Each method and recommended dates for your area are described below.
Whichever method you choose it is important to remember that sweetcorn seed will only germinate where the soil is at or above a certain temperature. Standard sweetcorn require a minimum temperature of 10°C / 50°F to germinate, supersweet and extra tender require a minimum of 15°C / 60°F. The very best temperature for germinating sweetcorn is 30°C / 85°F but that is very unlikely to be achievable for most amateur gardeners in the UK although it is something to bear in mind.
To prepare the soil for all three of the methods described below, dig the area so that the soil is well broken up into reasonably fine crumbs. Add lots of well-rotted organic matter to the soil if you have any and also add a handful of blood fish and bone fertiliser plus a handful of Growmore (nitrogen high fertiliser) every square metre - work them into the top soil.
Sweetcorn are wind pollinated and it is definitely best if they are planted in blocks at least four plants wide. Planting in single rows will make pollination a problem so allow the correct sized area for the number of seedlings you have. Allow 45cm / 18in between plants and 60cm / 2ft between rows. The block can be as large as you want but try and keep it as square as the space you have allows.
SOWING SWEETCORN INDOORS / HEATED GREENHOUSE
In our experience with the UK, this is by far the best method to start off sweetcorn seeds. Sweetcorn should only be planted in the open when all danger of frost has passed. With this in mind, sow seeds indoors around the beginning of the third week of April in the UK area.
Fill a 7cm / 3in pot with multi-purpose potting compost and place two seeds into the compost just below the soil surface. Water the compost, mark up the pots with plant markers and store them in a warm part of the house (see a couple of paragraphs above for correct temperatures) either in light or dark. If you have difficulty in getting sweetcorn seed to germinate concentrate on trying to raise the soil temperature and avoiding large fluctuations in the temperature. A windowsill may be good enough to grow seedlings on but the difference between day and night temperatures may well stop or delay germination.
The seedlings should take 7 to 10 days before appearing through the surface of the compost. Immediately this happens move the seedlings to a light area which is also cool. The seedlings will not stand any frost but as long as they are kept above 8°C / 47°F they will continue to grow. We keep our seedlings outside in the open only bringing them inside when cold weather threatens. This ensures that the seedlings are fully hardened off at an early stage. If you grow the seedlings on indoors, start to harden them off a week before planting outside.
The germination rate of sweetcorn seed varies wildly depending on where and how it is sown and also the temperature of the soil or compost. If you have two seedlings in one pot it is quite safe to gently prise them apart at planting time. Some articles say that sweetcorn hate root disturbance but this is not our experience. Indeed many garden centres sell medium sized pots full of sweetcorn seedlings and in the past we have separated them and planted them very successfully.
The seedlings can be planted outside in the third week of May. To do this simply dig a hole in the prepared soil which is slightly wider than the seedling pot and 5cm / 2in deeper. Add 5cm / 2in of multipurpose to the bottom of the hole (to gently ease the roots into growing into the soil) then place the seedling and as much of its surrounding compost as possible into the hole. fill the sides in with crumbly soil and gently firm down. Water the plant in well.
SOW SEED DIRECTLY OUTSIDE WITH CLOCHE PROTECTION
If you have sufficient cloches to cover the ground needed for sweetcorn then put them in place two weeks before the sowing time. With the soil prepared as described earlier above, sow the seed in the soil just below the surface. The best time to sow seed using this method is in the fourth week of April.
One of the disadvantages of sowing directly in the soil under cloches is that germination of the seeds is not guaranteed, indeed you can expect a failure rate of about 1 in 4 seeds sown using this method. With some F1 sweetcorn seeds being expensive it is a balance between sowing more seeds that you need, to allow for failure, compared to the overall cost.
We suggest sowing one seed every 20cm / 8in and sow with rows 30cm / 1ft apart. Don't thin the seedlings when they emerge unless the germination rate is very high. Thin them when they are about 20cm / 8in high leaving just the strongest growing ones.
SOW SEED OUTSIDE WITH NO PROTECTION
We would not recommend this method but if you wish to try, then only do so in warmer parts of the UK. Sow the seed in the second week of May exactly as described above but without the use of cloches.