How to Grow Sweetcorn from Seed.

27 June 2020 | SimplySeed

The Basics.

Luckily, growing sweetcorn from seed is a straight forward affair requiring little technical knowledge. Probably the most important point is the time the seed is sown, and this is all to do with temperatures. In northern European climates, the seed can be started off under protection from mid April onwards with a view to planting out from the end of May up until the middle of June. Of course, planting out will all depend on how late the frosts are in your area because your sweetcorn crop will be lost if it is hit by a late frost. Secondly, planting out into open ground should really wait until outside temperatures are consistently reaching at least 16 degrees Celsius during the day. If it is colder than that then the crop could be at risk from fungal disease.

Sowing Sweet Corn.

To ensure an early crop, sow sweetcorn seeds - either indoors in a propagator or in a heated greenhouse – around about mid April. Using modular trays or 9cm pots, fill with a good quality compost, then sow two seeds into each unit covering them with a good inch of compost. Water well and if they are being germinated indoors - move to a warm, bright windowsill. If you are using a heated propagator or greenhouse then the temperature will need to be kept at approximately 18 – 21 degrees Celsius.

The sweetcorn seedlings should germinate after 10 – 12 days, and once they have fully emerged the weakest seedling from each pot should be removed and can potted separately. If they have been germinated in a propagator the lid can also be removed and the temperature turned down to 16 degrees Celsius. However they are being grown, do not allow the compost to dry out or to become waterlogged as this can increase the risk of fungal infection.

The young sweetcorn plants, can be planted into their final positions once all danger of frost has passed – around the middle of May, but remember that they will need to be hardened off for a week or so before hand. This can be achieved by either bringing them back under protection over-night or placing them under a cloche or poly-tunnel outside.

Planting Sweetcorn.

Unlike many of our common food crops sweet corn are wind pollinated so once they are hardened off - and ready for their final position - they will need to be planted into block in order to ensure good pollination as the male flowers at the top of the plant need the opportunity to shed their pollen on the female tassels below. The absolute minimum number of sweet corn plants you would need to grow for effective pollination is 6. These will need to be planted in the shape of a square For this shape there should be a gap of approximately 12 inches between each plant...Poor pollination will only result in a poor crop!

For effective pollination you would need to consider planting at least a dozen – preferably more. Each plant will produce one or two cobs, so work out how many cobs you're likely to need (you can freeze them) and provide enough space to achieve this. However, there are no rules about how large the block has to be as it can only be determined by how much corn you decide to grow, When creating larger blocks, plant 30cm apart with a 45cm space between rows.

Sweetcorn prefer a free-draining, and moisture retentive soil. If you can remember to, it is best to try and prepare the bed the previous autumn before planting the following spring. Dig it over to remove any perennial weeds and add plenty of well-rotted manure to the soil as you go.

Don't worry if soil preparation ends up being a ‘last minute’ exercise, sweetcorn beds can still be prepared in the spring. Again, remove any weeds and dig over the site with a spade, removing any particularly large stones.

Roughly level the ground and then work over the area with a rake to leave a fine finish. Then - to ensure your crop gets off to a flying start - spread some general fertiliser granules over the planting area and gently rake in to the surface. Try to have this done two or three weeks before planting or sowing. Once the plants are of a reasonable size you can also water using a liquid feed once a week to help accelerate growth.

Keep plants well watered and the soil weed free, but take care not to sever the surface growing roots of the sweet corn when using a hoe. In fact to protect the surface roots – as well as give the sweetcorn plants more stability - pile soil up around the stems using a trowel or a draw hoe.

When is Sweetcorn ready to harvest?

It is going to be difficult to know when sweetcorn is ready to pick, especially if you have never grown them before because the entire cob is hidden by layer after layer of thick, fibrous husk. The trouble is that you need to get the timing right – pick too early and you have just wasted an ear, pick too late and the corn has lost all of its delicious, sweet flavour.

The ripening of the cob will of course depend on the weather, however you can expect it to be ready for harvesting anytime from 17 to 25 days after the time the first silks have appeared - but you are going to have to keep a close eye on it. A good indicator for this time would be when the silks have turned very dark brown to black. Of course sweetcorn will mature more quickly in hot weather and slower in cool weather.

Sweetcorn are ready to harvest when the kernels are in the 'milk' stage, and there is a simple test that you can do to check this. When the fluid inside each kernel is still liquid and the skin of the kernel is still tender then the cob is ready for picking so puncture a kernel with your thumbnail to make sure. If a clear liquid appears, then the corn is immature. If the liquid is milky, then the corn is ready, and if no liquid appears then the corn is over-ripe.

Once the cobs have been picked they can turn very quickly as the sugar within the kernels will rapidly turn to starch. Even when stored properly they can be past their best within a couple of days, however there are steps you can take to make the most of this short harvesting period.

Try to harvest sweetcorn in the morning before you get a build up of field heat but if this is unavoidable submerge the cobs in cold water for a minute or so to let them cool down. Then – as soon as you can – they need to be refrigerated with a mind to being eaten over the next couple of days. There is an old wives tale saying that the pot should be boiling when the corn is picked and although that is a slight exaggeration it does convey the importance of speed when it comes to picking and eating corn at its best.