yamsIt's not often that I find myself wishing for a good hard frost but, this year, that's exactly the situation. It's all because of my yam crop and its (still) thriving, bushy, green foliage. I adore yams (not really yams, at all but, rather, a South American tuber known, variously, as oca, oka, Oxalis tuberosa, Oxalis yam, or Oxalis crenata).

I grow three different varieties: the pinkish-white, pinkish-red, and yellow strains. Quite frankly, I don't care what colours they come in because they're all so totally delicious, and they fill a roastable gap for those of us who don't live in kumara country.

I'm probably worrying unnecessarily about the yet-to-arrive frost because having the bushy tops die-off is more a signal that it's time to harvest, rather than a necessity. So, if you live in a frost-free zone, don't not-grow them.

I must admit, yams are a pretty mysterious crop to cultivate and, for years, I didn't get the hang of what they were up to under the ground. Then, I discovered that the tubers don't actually start putting on weight until daylight and dark hours are about even – that's the autumn equinox period. You need to get them into the ground as early in spring as possible, and hold back on the nitrogen (which only causes the inedible tops to grow stronger). They chomp up well on potassium (that's sea weed in organic-speak) and they like, worked into the soil, a good tipple of super (bone or fish meal, rock phosphate, or compost, for the organic-grower) and plenty of organic material (think 'compost'). Once they're established, they're happy to do their own thing although some people like to hoe them up a little as you would for spuds. I prefer to let mine ramble because they become their own mulch and weed suppressant.

Harvesting yams is a joy because they are so-o-o-o beautiful with their naturally shiny skins (which is good enough reason, alone, to grow several colour varieties). In the Andes of South America, the harvested tubers are left out in the sun for a period to shrink and become sweeter so, toward the end of winter, if you find some shrivelled specimens in a forgotten corner of your cupboard, don't hesitate to cook them (you may first have to knock off any shoots).

Yams for cultivation aren't always easy to find in garden shops but those bought from the supermarket will usually grow when planted. It pays to leave them in the sun to see if they develop shoots, though, just in case they've been treated to prevent germination. Even if you can grow kumara, give yams a go. Mysterious, delicious, and with a fabulous history, they make a talking point at any table.

• once the yam tops have been frosted, get in fast with harvesting – birds also adore yams and will gobble them up before you do if you're not careful.
• Shop-purchased yams aren't always easy to come by, and seldom come down in price. They're also easy-care so, if you're looking for pocket money, they make a good marketable crop.

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