When to Sow Sweetcorn in the UK

Quick Facts

Factor	Value
Soil pH	5.5 - 6.8
Sunlight	6-6 hours direct sun
Germination temp	10°C
Optimal growing temp	16-24°C
Minimum survivable temp	12°C
Time to harvest	80-95 days
Soil texture	RHS: "fertile soil"; no specific drainage requirement is stated by either source.
Drainage	RHS: "full sun" -- cobs are unlikely to ripen without it. Clemson: "full sun for optimum productivity".
Edible parts	cobs (kernels)

At a Glance

Regional Planting Calendar

Rows marked derived are interpolated between regions our sources do state, because no source we consulted gives a figure for that region specifically. They are deliberately wider to reflect that uncertainty.

Is May Too Late to Sow Sweetcorn?

No -- if anything, late May is closer to the safer end of the window than the risky one. Sweetcorn is frost-tender at every stage and needs soil that's genuinely warmed (RHS's own outdoor-sowing advice is late May to early June, once all frost risk has passed), so a cautious late-May sowing is doing the right thing, not falling behind. What "too late" really means here is running out of the roughly 80-95 days the crop needs before autumn's shorter, cooler days slow it down -- for most of the UK that cutoff sits well into June, not May.

Growth Stages

Soil & Growing Conditions

Water: RHS: water during dry spells while young, and during "prolonged dry spells, especially when flowering starts and when the cobs are starting to swell". Clemson: "at least 1 inch of water (rainfall or irrigation) per week".

Soil pH range

5.5 6.8

Span reflects genuine variation between the cited authorities, not a single agreed figure.

Harvest & Storage

When to pick: RHS uses the kernel liquid test: "If a watery liquid comes out, it's not yet ripe, if the liquid is creamy, the cob is ready, but if it's paste-like then it's over-mature". Clemson uses appearance: the "husk is still green and the silks are dry brown. The kernels should be full size and at milk stage".

Storage

Clemson: keeps "about five days" at "close to 32 °F" in a moist environment. RHS states no storage method -- sweetcorn is best eaten fresh.

Companion Planting

Can You Save Your Own Sweetcorn Seed?

For most gardeners, yes -- but the reason to think twice isn't legal, it's biological. University of Maryland Extension's guidance on patented seed restrictions is explicit that it applies to commercial growers who've signed a Technology Agreement with a seed company, not home gardeners. The real caution is simpler: most garden sweetcorn is an F1 hybrid, and F1 seed doesn't "breed true" -- plant the kernels you saved and next year's crop is likely to be far less uniform and sweet than what you grew. If you want to save seed reliably, choose an open-pollinated or heirloom variety instead of a hybrid.

Diseases

Pests

Pollination

Wind (abiotic pollination)

RHS: "Sweetcorn is a type of grass, so is wind pollinated". "To maximise pollination, plant in a block or group rather than a long row. That way, each plant is surrounded by several near neighbours, so the chances of successful pollination (resulting in a cob packed with kernels) is much greater, whichever direction the wind blows." "Poor pollination leads to sparsely filled cobs with missing kernels." Growers can also "tap the stems to loosen the pollen" once the tassels open. NOTE the exception: baby corn is sown "in rows not grids" precisely BECAUSE "the female flowers shouldn't be pollinated, otherwise the kernels will start to swell".

Sweetcorn, Plant to Cob

Planted in a block, not a row

Sweetcorn is wind-pollinated, so it is planted in a block or grid rather than a single line -- that arrangement gives pollen a far better chance of reaching every plant's silks than a long thin row would.

The tassel is the male flower

The feathery tassel at the top of the plant releases pollen, which falls onto the silks of the same and neighbouring plants. This is why block planting matters so much more here than for most vegetables.

Silk is the female flower

Each strand of silk connects to a single potential kernel inside the developing cob. Fresh, pale silk like this means pollination is still in progress; every strand needs a grain of pollen to land on it for that kernel to fill.

Silk browning signals ripeness approaching

As the cob fills out, the once-pale silk dries and browns. RHS's ripeness test still applies underneath: peel back a little husk and pierce a kernel -- watery liquid means not yet ready, creamy liquid means the cob can be picked.

The stage to eat at

Cook sweetcorn as soon as possible after picking -- the sugars that make it sweet begin converting to starch immediately after harvest, so the shortest possible gap between cutting and cooking gives the best flavour.

References

1. Clemson Cooperative Extension - Sweet Corn
2. Colorado State University, Food Source Information - Sweet Corn
3. FAO EcoCrop - Zea mays ssp. saccharata data sheet
4. Garden Organic - Companion or mixed planting
5. Global Biodiversity Information Facility (GBIF) - GBIF Backbone Taxonomy: Zea mays L.
6. Met Office - Why frost can still occur in late Spring | Met Office
7. NC State Extension Gardener Plant Toolbox - Zea mays (Corn, Indian corn, Maize)
8. Royal Horticultural Society - How to grow Sweetcorn | RHS Guide
9. University of California Statewide IPM Program (UC IPM) - Common Smut (Boil Smut) — Corn, Pest Management Guidelines
10. University of Maryland Extension - Growing Sweet Corn in a Home Garden

10 independent sources are cited for this crop. Where they disagree - soil pH is the clearest case - every figure is preserved rather than averaged into false precision.