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WHEN TO SOW SWEETCORN

Sow sweetcorn in pots either inside your house, conservatory or in a heated greenhouse during. the fourth week of April This is by far the most common and successful method for sowing sweetcorn in cooler climates such as the UK. This will produce developed seedlings at the earliest stage possible, crucial in the cool climates.

Sweetcorn seeds

It is also possible to sow seed directly into the ground with cloche protection during the fourth week of April although this method is definitely not as reliable.

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 cooler climates although it is something to bear in mind.

HOW TO SOW SWEETCORN INDOORS

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. Preferably not on a windowsill at this stage.

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 is 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.

HOW TO SOW SWEETCORN OUTDOORS

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 third week of May

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 than 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.

WHERE TO GROW SWEETCORN

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 Sparrow (extra tender sweet), Earlibird (supersweet) and Northern Xtra Sweet (supersweet).

HOW TO PLANT SWEETCORN

To prepare the soil, dig the area so that the soil is well broken up. 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.

The seedlings can be planted outside in or when they are 20cm to 30cm (8in to 12in) high if that is later.

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) and work it into the surrounding soil.

Now 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.

Sometimes, our readers ask specific questions that are not covered in the main article above. Our new
Sweetcorn comment / question and answer page
lists those comments, questions and answers. At the end of that page there is also a form for you to submit any new question or comment you have.