Sweet peppers can ripen well – even in dreich old Scotland. Fifty years ago you would have been hard pressed to find people growing sweet peppers here, but they’ve now become nearly as popular as their cousins, chillis. And breeders have responded by producing hundreds of shapes, colours and sizes.

For best results, these Central American plants need as much heat and light as possible. I look back fondly to the summer of 1984, when my peppers wallowed luxuriously in the sun and rewarded me with the sweetest crop ever. But even now, looking at my New Ace pepper’s five large fruits and five small growing ones, I know they’ll acquit themselves well.

I may not achieve a night temperature of 20C followed by 30C during the day, but the greenhouse should keep tolerably warm. And, during the summer, it provides all the summer light the plants require, provided the glass is kept clean and there are no looming tomatoes nearby. In early September I’ll plug in my growlights to compensate for falling light levels.

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That’s because these tropical plants would naturally develop over a longer growing season than we’ve got, and although breeders have developed faster-growing varieties I believe you’ll get better results by choosing ones that produce smaller fruits. Just as bush tomatoes ripen much more quickly than beefsteaks, small peppers such as Joker reward you with an early dish of sweet, tasty peppers. These tiny peppers look like chillis but don’t burn the roof of your palate.

And there’s a fine choice of colours in these smaller specimens, with the orange fruited Mohawk or tiny Sweetonia Orange and the purple Tequila. And you get many more fruits than on a large plant.

We are naturally attracted to red fruits so people used to think larger and redder peppers would be much sweeter than those of smaller varieties, but another trial conducted by the RHS last year knocked that idea on the head.

The RHS selected sweet pepper varieties that had received their coveted Award of Garden Merit and, among other things, tested them for sweetness. They used the Brix scale, a system familiar to every home and commercial brewer and vintner as it measures the volume of soluble sugar in liquid.

For booze, Brix measures the potential strength of the liquid, but for peppers it assesses sweetness, flavour and nutritional value. On a scale of four to 12, the higher the score the better.

The test showed that size was no guide to quality. One of the larger varieties, Monanta, scored a reasonable seven, but much less than the smaller, top-scoring Hamik at 10. Hamik went on to break the rule on colour, producing orange fruits, not the expected red. And, to make matters worse, yellow-fruited Corno di Gaillo scored nine while Bianco got the dunce’s cap with six, despite being red.

Whether the final colour of a pepper is red, yellow, orange or brown, it will need ethylene as well as autumn sun to ripen and sweeten. Ethylene acts as a hormone in plants to stimulate the ripening process. As a fruit ripens, it emits ethylene which, in turn, stimulates ripening in nearby plants. When one tomato ripens, others quickly follow.

Fortunately, with all these luscious tomatoes, the greenhouse is awash with ethylene in August and September, so this fairly boosts peppers into ripening. Experiments have shown that high levels of ethylene can speed ripening of peppers by up to a week.

And ethylene may not be the only ripening trigger. I’ve sometimes seen a pepper damaged by an epicurean slug turn red more quickly than its neighbours. Does a threatened fruit rush to premature seed ripening?