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Tips for growing potatoes

If you want homegrown spuds for Christmas, you need to start now.

My Irish grandmother liked to grow spuds for the Christmas table. But it was always a race to get the seed potatoes into the ground in time, and then, of course, there was the waiting around for the crop to develop. She loved potatoes and green dresses; they were nostalgic symbols she clung to. As for the rest of Ireland’s history, her view was “Six of one and half a dozen of the other. That’s all there is to say on the matter.” But when it came to potatoes, she had a firm view: regardless of whether they were mashed, roasted or boiled, butter couldn’t melt on them fast enough. Best of all, her potatoes were always homegrown in Taranaki soil.

If you want a crop of spuds in time for Christmas, choose an early harvesting variety and buy the seed potatoes between late August and mid-September. Allow four to six weeks for them to sprout (or chit) as well as time for the crop to develop. Even the fastest-growing early varieties take at least 60 days to crop, but most need 90-120 days.

Season of October: The Potato Gatherers (1879), by Jules Bastien-Lepage. Image/National Gallery of Victoria

More than 50 varieties are grown in New Zealand. When deciding which to plant, consider flavour, colour and consistency as well as the crop’s maturation time. Remember that some are better suited than others to particular cooking methods. If you have space and want a more continuous yield, grow both early and late varieties.

If you buy certified seed potatoes, which are sold at garden centres, you’re assured of getting the correct variety and more disease resistance. If you want varieties that aren’t available in supermarkets, buy heritage seed potatoes, which vary greatly in shape and colour. They can be bought online or by mail order from such outlets as the Koanga Institute in Wairoa (koanga.org.nz). Heritage potatoes are less likely to be certified, but this is okay.

Potatoes can also be classified according to harvest times. Early varieties such as jersey benne, karaka and cliff kidney can be harvested once the flowers open, whereas main croppers, such as agria, rua, red rascal and moonlight, need to have completely died down before harvesting can occur, usually in early autumn.

An early to main cropper can be harvested once the flowers open fully or at any time up until the plants die down in early autumn. Second earlies are, as the name suggests, earlier than the main crop but not as early as the earlies. They are harvested in the same way as earlies and need the same growing space.

Potato plants. Photo/Thinkstock

Main croppers store well and are good value. They tend to need the most space because of their spread and longer growing time. Once the foliage has yellowed, cut it down to ground level and leave the potatoes in the ground for a couple of weeks to harden off before lifting and storing in a dry, dark place. Cool, airy conditions are best and it’s preferable to use sacking or paper rather than plastic.

Earlies are in the ground for a shorter time, so are less susceptible to disease; they’re also more suited to smaller gardens where crop turnover is important. Lift the spuds once they’re the size of a hen’s egg.

Sprouting or chitting can be done before spring and the last of the frosts. Lay out the potatoes in a light, dry place with most of the eyes facing up. Egg cartons are ideal for this. Label the cartons, then at planting time, label the garden rows, too, as different varieties need to be harvested at different stages of the growing cycle. Once seed potatoes have good strong shoots at least 2cm long, they are ready for planting.

Potatoes can be grown in garden beds or containers, such as sacks, bags or stacks of tyres. The traditional method is to plant them in garden trenches about 10cm deep and 50cm apart for earlies or 75cm apart for mains. The trenches should be lined with blood and bone, well-rotted compost or even a thin layer of lawn clippings.

Position the potatoes with the best shoots facing upwards (weak shoots may be removed, leaving the strongest three) and space the tubers along the trench (30cm apart for earlies or 40cm for mains). Handle them carefully, as the shoots break off easily. Sprinkle over a layer of soil to just cover the sprouts. As the plants grow, build up the soil level every couple of weeks until it’s about 15cm high, with a flat ridge over the trench. This is important, as water needs to run away from the trench. Potatoes do not thrive if drainage is poor.

If you’re using a container, the growing principles are similar. Place the seed potatoes on a soil base of at least 15cm, then build up the soil as the plants grow. If using tyres, start with one and add more as the soil level is increased. At harvest time, the top tyres can be lifted off without the worry of the potatoes’ tender skins being damaged by fork prongs.

Seed potatoes need light to encourage sprouting, but planted potatoes must be completely covered, because they turn green when exposed to sunlight. If eaten in quantity, green potatoes can be poisonous.

Consistent watering is vital as containers are prone to drying out. Alternating between watering droughts and binges can cause potatoes to split or the skin to crack.

Grandma grew her potatoes in compost and blood and bone, with a sprinkling of lime. It’s anyone’s guess whether it was that combination or her green Irish fingers that made them grow so well, but I’ll be keeping a foot in each camp and hoping for a Christmas harvest.

Send your questions to: goodtogrow@xwd.co.nz