Answer: when it's a cucumber !
We've tried growing melons over the past few years, without much success. So this year, I was determined to get a good crop of them and researched the particular varieties that tend to do well in cooler climes.
So in March this year, I ordered some seeds online, of the variety 'Alvaro F1'. £3.50 for just five seeds.... It's a hybrid that's not available in seed packets at our local garden centres.
The seeds were duly sown and placed on the kitchen windowsill, and grew away happily until they were put out into the greenhouse and cold frame in early June. All cucurbit seeds, and the plants In the early stages of development, look pretty much the same whether they're melons, marrows, squashes, pumpkins or cucumbers.
However, in the middle of June, I began to suspect all was not as it should be, since the first female flowers were borne on small fruits which looked suspiciously like cucumbers, and not at all like the flowers on melons I'd grown before. But even then, I thought there was an outside chance they would still come true as melons after fertilisation.
Alas, it was not to be - here's the second of the fruits harvested, and it's definitely a cucumber, although of a variety I haven't seen before. The fruits are quite fat and lobed in a slight triangular fashion in cross section.
They're very tasty, but we've already six other 'Femspot' cucumber plants growing away in the greenhouse, so it looks like we're in for a glut this year.
It's far too late to try to sow melons again this year. So, we've been ripped off by an eBay seller, who sent us ten-a-penny cucumber seeds masquerading as very pricey melons. We can't even leave negative feedback for the seller on eBay because we're well past the time limit after purchase.
Next year, we'll buy Alvaro F1 seeds again, but this time from the Thompson & Morgan site.
Caveat emptor is a little difficult with seeds that take months to show their true colours.....