While from time to time, I have had rogue potatoes from the supermarket sprout in my compost bins and produce a bumper crop, more often, unless certified seed potatoes are used, the crop will be poor and slow to grow due to the use of a chemical called chlorpropham, applied to supermarket-grade potatoes to inhibit sprouting. On top of this, using these leftover spuds runs the risk of introducing diseases into the garden.
The term ‘seed potato’ is a little misleading, since they are actually small potato tubers. Potato plants do produce fruit that contain seed, but they cross pollinate and won’t grow true to type if the seed is used. Certified seed potatoes are guaranteed to grow true to type and be free of viruses and diseases.
Once you have your certified seed potatoes the next step is to chit (sprout) the tubers by placing them in a cool, dry, well-lit area (out of direct sunlight). An egg carton is an ideal size to hold the tubers and keep the end that has a cluster of eyes (called the rose end) facing upwards. Once the sprouts reach 25mm or so in length, which normally takes between four and six weeks, the tubers are ready to be hardened off and planted out.
Site the bed for potatoes in as much sun as possible and dig in plenty of manure and compost well before planting time to give a deep, rich, well-drained soil. Avoid growing potatoes in the same patch for at least two years, and remember that tomatoes, peppers and aubergines belong to the same family.
The next step is to dig 20cm deep trenches across the plot, spaced a generous 60cm apart. Incorporate a specialised potato fertiliser or other high-potassium fertiliser into the bottom of the trenches and cover with a layer of soil. Avoid any high-nitrogen fertilisers as these will produce leafy growth at the expense of tubers, and don’t add lime as this causes rough scaly patches on the skin of the potato called scab. Plant the tubers around 30cm apart, rose end up, 15cm or so deep, taking care not to damage the sprouts when covering the potatoes with soil.
Once there is 20cm or so of foliage growing above ground, mound up soil around the plants (taken from the space between the rows) until only around 5cm of foliage is left exposed. This will have to be done again every couple of weeks. Slope the sides of the mounds and firm them down gently so that the soil doesn’t wash away in heavy rain. Mounding or earthing up protects the foliage from frost early in the season, encourages more tubers to form along the stems, stops light from turning the top tubers green and poisonous, and makes harvesting easier. For the best crop, potatoes need to be watered well throughout the growing season. The soil should be kept moist, but not waterlogged.
Main-crop varieties of potato such as ‘Agria’ and ‘Rua’, which take 100 to 130 days to mature, aren’t ready to harvest until the foliage dies down completely. This is not the case with early crop varieties, which will be ready to harvest around the three-month mark, even if they haven’t produced flowers. If in doubt, feel gently around the plants to see how big the potatoes are. Small new potatoes can be harvested earlier. Main-crop varieties should be left in the ground for two to four weeks after they are ready for harvest to toughen up the skin for storage. Early crop varieties have thin skins, which make them unsuitable for storage. An advantage of early crop potatoes is that they suffer less from the blights that affect main-crop varieties when the weather becomes warm and humid later in the season.
A potato is born - Grant Dawe explains why certified seed potatoes produce a top crop.
Growing a good potato crop is not simply a matter of planting a table potato from a bag bought at a supermarket, or a leftover from the previous year’s crop. These are not certified seed potatoes. Certified seed potatoes are purpose-grown to maximise the likelihood of healthy, high-yielding crops and minimise the opportunity for disease. There are more than a hundred diseases that attack potatoes. An uncertified potato may have been infected with viruses spread from plant to plant by aphids, or the potato taken from a table bag may have been sprayed by a sprout inhibitor to stop it from growing. Both these factors increase the risk of getting no crop or at the very least of reducing the possible yield. Unhealthy potatoes can spread diseases in the soil, affecting other crops. Such effects would not be noticed until the following season when the garden produces less healthy vegetables. Seed potatoes are grown from healthy tissue of previous crops to ensure they are true to their variety (also called cultivar). Some varieties are better suited to certain uses than others: ‘Cliff Kidney’ and ‘Liseta’ are good for boiling as a new potato (especially for Christmas dinner); ‘Karaka’ and ‘Allura’ are good for roasting, boiling, mashing and wedges; ‘Desiree’ and ‘Rua’ (old favourites) are still good for all purposes. It is not always possible to tell the variety of a potato by looking at it. All bags of certified seed potatoes are clearly labelled. The seed-potato crop is tested for diseases at every step, including field production. Farmers’ crops must pass two assessments before they are allowed to be harvested for sale as seed potatoes. Certified seed potatoes also tend to produce cleaner, more attractive and uniformly sized potatoes, which are likely to store well. It is a good idea to buy fresh seed potatoes each season because older seed potatoes might produce plants with numerous stems that sprout quickly, but die early, resulting in smaller potatoes. It is best to plant potatoes in a new place each year to further reduce the risk of disease spread.