PUBLISHED: 08:11 28 March 2014 | UPDATED: 08:51 28 March 2014
Brian Callaghan puts the case for growing potatoes
Green potatoes, harvest indexes
A harvest index is tool commonly used by agronomists to calculate the efficiency of growing a particular crop in a particular set of circumstances. In its simplest form, the useable part of the plant is measured against the waste part and the result is expressed as a percentage figure. So, a crop returning a harvest index of 20% is one in which 80% of the input of labour, light, water and nutrients are wasted, while one which has a harvest index of 90% is one in which only one tenth of the resources are waste.
Out of context, harvest index figures prove very little as many other factors must be taken into account when choosing what, or what not, to grow. Cost and availability of resources, demand and therefore market price are likely to be equally as important. However, the value of figures in the table below is in illustrating the relative returns when comparing the growing of three staple crops.
In a wealthy country where high waste levels are often tolerated, crops with a lower harvest index are grown so long as the price remains high enough for producers. Poorer countries and more demanding situations, such as occurred in the UK during World War II, force the emphasis of production to move from the luxury crops to those which provide our basic needs more efficiently. Given its high return on resource consumption there is an argument for claiming that potatoes are truly a ‘green’ vegetable.
Potatoes will grow on most sites but prefer reasonable drainage and plenty of light. They aren’t a UK native and don’t like frosts, especially late ones which will kill off any exposed foliage. Although this rarely ruins the crop, the time needed to re-grow foliage will shorten the effective growing season and thus reduce the overall yield, so avoid growing at the bottom of slopes, or in front of impermeable barriers where the heavier, cold air will collect and intensify damage.
The widespread popularity of potatoes is partly due to them managing to grow well on soils which result in poor yields for many other crops. Heavier soils should be double dug in autumn leaving clods unbroken to be worked on by winter frosts and thaws which will cause them to break down into a fine tilth in spring. A few weeks before planting, apply some form of organic matter – well-rotted manure or garden compost – to the surface to improve the soil structure.
Certified seed potatoes are produced on regulated sites in areas free of the main pests and diseases of potatoes. Although it means parting with some cash, purchasing certified tubers helps ensure an acceptable return on all the effort spent on preparing the ground, cultivating and harvesting the final crop.
It’s possible to obtain a crop of sorts using tubers saved from the previous year’s crop, but balancing the risk of failure against the costs involved, it’s rarely worthwhile.
Chitting is the pre-sprouting of potatoes – producing one or more sprouts on seed tubers before planting to achieve a number of advantages. Early potatoes can be started into growth before soil conditions are suitable for planting and so give a higher yielding crop.
Chitted maincrops may also benefit, especially where the growing season is often effectively shortened by blight. Chitting can also help reduce losses through tuber disease which can be spotted and removed before planting.
Place the tubers with the rose end (the one with the greatest number of eyes) uppermost in a tray (large egg trays work well) and store them in a light frost-free place until the shoots are 25-50mm long, when they should be planted out, taking care not to break the shoots.
Earlies, seconds and maincrops
Earlies on average mature within 110 days of planting. Although they produce relatively few tubers, they have a higher value because they’re available earlier in the year and are grown for taste rather than quantity. Aim to plant earlies towards the end of March when the weather and soil conditions are suitable.
Second earlies mature within 120 days of planting on average, and if plantings are timed correctly, should provide for the gap between earlies and maincrops. These should be planted mid-April.
Maincrops are planted immediately after the second earlies and they should take 140 days to mature. The relatively low prices of shop-bought maincrop potatoes dissuade some from growing this group, but all home-grown produce leaves quality control in the hands of the grower, minimises food miles and eliminates those fun trips to the supermarket.
Bear in mind that most spacings for vegetables derive from commercial situations where it’s imperative that the grower maximises the yield from the growing area available. For the most part, I follow the spacings here but wouldn’t hesitate to alter them where the situation demanded such as reducing the figures to squeeze another row or increasing the figures to allow for cultivation machinery.
Excavate a hole which allows the tubers to be planted with 50mm of soil covering the tops, drop them in rose end uppermost and then earth up immediately. Earthing up, or drawing up soil from between the rows to further cover the tubers, fulfils a number of functions. First, it marks the lines of planted tubers thus allowing the safe use of a hoe for weed control between the rows. Second, potato tubers are underground stems and when exposed to light, they begin to photosynthesise like other plant stems to aid plant growth. As the tubers develop, they expand in the soil and force themselves towards the surface and light. Unfortunately for us, this also builds up toxins in the surface of the tubers which are poisonous to humans. Covering tubers with soil prevents light reaching the stems.
Third, earthing-up prevents late frost damage to the soft tops’ early stages of growth, and as a bonus, the regular disturbance of the soil reduces weed problems as the crops develop.
Harvesting and storage
Early potatoes can be harvested as soon as the flowers begin to appear, usually around June. Lift one plant, and if the tubers are of a suitable size, lift the rest too.
Maincrops are grown for size and maximum yield, so should be left until September when the tops begin to die back. Leaving them in the ground longer can result in the unwanted attention of hungry slugs and it’s also often difficult to dry the tubers sufficiently before storage.
It’s estimated that up to 20% of potatoes are damaged during harvesting due to poor handling. Apart from the direct reduction of the value of the crop, much of the damage is internal and won’t become obvious until the tubers have begun to rot, possibly causing other healthy tubers to rot as well. A little care at this stage can avoid wasting all the previous effort, so ideally, choose a dry day and gently ease a fork into the soil from the side of the row and bring the tubers to the surface. Work back down the row leaving the tubers on the surface for a few hours to dry out. This will reduce the likelihood of them rotting in storage and the short exposure to sunlight helps toughen the skins slightly, again reducing the risk of physical damage during handling.
Collect up the healthy tubers, place them in a paper sack and store them in a cool (not frosty), dry place. Check the stored tubers frequently as a few will always begin to deteriorate. Removing these early prevents the disease spreading to the rest.
Still not convinced
Despite the lack of encouragement, I haven’t given up hope of turning the obstinate offspring into a potato lover. My latest strategy has been to present my son with the sort of information shown above indicating the nutritional benefits of the spud. He needs carbohydrates for energy, there is plenty of dietary fibre but hardly any sodium in potatoes and I can grow as many as he can eat, so for a large part of the year, he can have them fresh when their vitamin C content is highest. His response is that he’s fit, rarely ill, and that I ought to be grateful that he doesn’t like chips. Someday he will accept the benefit of my wisdom.
This article is from the March 2006 issue of Country Smallholding magazine.
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