Our Chief Horticulturist Guy Barter loves sweetcorn. Here he shares the science behind its sweetness and offers some growing tips
Sweetcorn is of ancient origin, and has been grown by indigenous American people for many thousands of years. However, more recently, plant breeders in North America have bred for specific genetic types to enhance sweetness and tenderness (using conventional techniques and not genetic engineering).
It's in the genes
The original sweetcorn has 'SU' genes which give good sweetness, but the cobs quickly lose sugars after harvest. Supersweet corn is, as the name suggests very sweet; and, crucially, retains this sweetness after harvest. Supersweet corn with 'SH2' genes has almost completely replaced the SU originals.
However, supersweet corn is rather chewy, and it's slightly trickier to grow – needing to be sown shallowly as it has limited food reserves in its seeds.
Alternatives that are both very sweet and very tender include include sugar-enhanced corns ('SE' genes) and extra tender sweet cultivars. The latter have ‘stacked’ genes combining SU, SE and SH2 genes. Breeders have not stopped there – new corns contain not only SU, SE and SH2 genes, but also modifier genes that affect flavour.
How to grow sweetcorn
Sweetcorn is sown direct in any fertile garden soil from mid-May, 2cm deep ideally under fleece or cloches, thinning to just one plant later. In the north and for early crops sow in cell-trays indoors from mid-April.
Allow at least 40cm (16ins) between plants and set out in blocks to facilitate wind-borne pollen transfer. Avoid growing supersweet cultivars with any other types, as poor quality cobs can result.
In small gardens buy only one packet of an early corn and make sowings in April, mid-May and early June for a succession of maturity in late summer. In larger gardens it is worth buying two packets, one early and one mid-season seed to spread maturity, but again sowing three times from April until June. Watering if weather dry at flowering time will help set good pods.
Recommended varieties
1. 'Mirai 003' – Early (suited to northern areas where later cultivars might fail to mature), SH2, SE and SU genes, very tender, easy to grow, RHS Award of Garden Merit (AGM). See photo.
2. ‘Sparrow’ – Mid season, SH2 and SE genes, very tender, AGM.
3. 'Seville' - Mid season supersweet, SU genes, long-lasting after harvest, AGM.
4. ‘Swift’ – early, (suited to northern areas where later cultivars might fail to mature), SH2 and SE genes, very tender, AGM.
5. ‘Northern Extra Sweet’ – Very early especially well suited to northern areas where later cultivars might fail to mature, SH2 genes so plenty sweet, AGM.
