Potatoes and Yams are both part of the 'Tuber' group of vegetables, with tubers being swollen parts of the roots that function as storage organs for the plant.
WHEN TO PLANT: Early crops can be planted in August and September, main crops planted in October
SEED POTATOES: Potatoes are usually grown from 'seed potatoes', which are simply potatoes that have been saved from the last years crop for the purpose of re-planting. Usually smaller potatoes are kept as seed, because they have less eyes, meaning less main shoots will emerge from each, which makes things less crowded in the rows. About golf ball size is said to be good for seed potatoes. It is widely reccomended to use 'certified' seed potatoes, which are disease free. This is what is sold at garden centres. The reason for this is that potatoes are vunerable to several diseases, such as potato scab. If you are using potatoes that are saved from yours or a friends previous crop, be sure to choose ones that look healthy, and have come from a potato bed that is in a crop rotation system.
SPROUTING SEED POTATOES: It's a good idea to secure your supply of seed potatoes at potato harvest time, in late summer. But you can aquire them anytime up until planting time. One method of storing and sprouting ('chiting') seed potatoes is as follows: From the supermarket or greengrocer, get some grape or tomato trays (polystyrene or plastic), which are usually avaliable for free if you ask, and can be stacked on top of each other. Or if you can find a wooden equivalent that is good as well. Set the tubers (seed potatoes) in the trays and stack them in a light, frost free shed or other storage area. The key things you're going for is good ventilation and a bit of light. The potatoes will begin to sprout and can happily sit there sprouting until planting time.
PLANTING: As mentioned above, early crops can be planted in August and September, main crops planted in October. To plant, dig a trench that is 15cm to 30cm deep and however long you want. Potato trenches should be spaced about 40cm apart for early varieties and 50-60cm apart for main crop varieties. The purple 'maori potatoes' are quite small and can be grown in rows 30-40cm apart.
The trenches can then be lined in the bottoms with one or more of the following: compost, fresh cut comfrey leaves, pine needles, leaf mould, a bit of wood ash, grass clippings etc. Never use lime in your potato beds as they prefer the soil slightly acidic.
You then lay the seed potatoes, shoots pointing up, on top of this layer, and then cover them back over with the remaining soil, being careful to not damage the shoots. Don't pack the soil down. If the shoots are damaged, then the seed potato has to grow new ones from scratch.
EARTHING UP: When the shoots are 15cm or so high, they should be earthed up or mulched. To earth up, use a rake or similar tool to draw soil up from between the rows and mound it around the stems of the plants. To mulch, get some stuff like straw, dry grass clippings, sawdust, etc. and spread it around the plants to mound them up. Some gardeners like to use a mixture of the two methods, because using a mulch increases the height to which you can mound up, but using purely mulch and no soil often leads to too loose of a mounding layer, allowing light in and greening the potatoes. Whenever the stems get to be 15cm above the mound, earth them up again. The idea behind this is that when covered over, the stems of the potato plants send out roots, which grow potatoes on them. Thus, earthing up your potato plants is kind of like building a highrise of potatoes, making the most of the vertical space. Some people plant potatoes in an old car tyre, and keep adding car tyres to the stack whilst mulching the potatoes, creating a column of potatoes.
HARVESTING: For early varieties, harvest can begin when the first flowers open. Carefully use a garden fork to loosen up the mound. Or in the right kind of mound potatoes can be dug by hand. From here on in you can harvest at whatever rate you want to, keeping in mind that the more you harvest earlier on the less you'll have later in the season and for storage. Dig up only what you require for use that day. Some people prefer to lay their potatoes out somewhere in a ventilated area for 24 hours to loose their 'earthy' taste.
SEED YAMS: The concept of seed yams is similar to that of seed potatoes. Virtually any yam can be used as a 'seed yam'. Even very tiny ones. By the time spring rolls around, many yams will have developed a few sprouts. These are perfect to plant in your garden to get a fabulous crop of yams in autumn!
WHEN TO PLANT: Yams are planted in the spring to early summer. In Otago, yams can be planted anywhere between October and December.
PLANTING and CROP CARE: Yams are planted in a similar way to seed potatoes. Trenches or individual holes can be dug to a depth of around 15cm. Seed yams are placed in the bottom of the holes or trenches and then they are covered up with soil. After a couple of weeks, you should see the yam shoots come up through the surface and start growing leaves which look a bit like clover leaves (so don't mistake them for weeds). Similarly to potatoes, once the yam shoots reach above 15-20cm or so above the ground, a mulch layer can be applied to cover most of them up leaving just a few tips sticking out to continue growing. This will increase the harvest by encouraging the plants to grow more yams within the mulch layer.
HARVEST and STORAGE: In the early autumn, the plants will begin to die back. This is the time to harvest the yams. Harvest your yams by digging them out of the ground either by hand or with a garden fork. Store them in a cool, dark place until needed. Yams can be stored for several months. And don't forget to save a few good ones for next years crop!