By John Curtis
What type should I grow – earlies or maincrops?
If you don’t have much growing space, give maincrops a miss and grow earlies.
Earlies have a waxy texture making them good for boiling and in salads. You harvest and eat them when they are expensive in the shops and, unlike maincrop potatoes, they rarely suffer from scab, blight or holes in the tubers from slugs or wireworm. Since you harvest then early, in late June or early July, you might be able to grow another crop (eg carrots, sweetcorn or squash) afterwards, for harvesting later in the same year. Their yield per square metre is lower than for maincrops, especially if you pick them small like the Jersey new potatoes you see in shops, but if you sacrifice the ‘small’ appeal and leave them in the soil for a few weeks longer, they get fairly big and the yield per square metre increases greatly. There are first earlies, and second earlies – plant and harvest the latter a bit later.
Maincrops have a floury texture – good for baking, roasting, mashing or chips. They are planted later than earlies and are harvested much later, typically late August for eating fairly soon after, since these won’t store well – or late September for storing, which gives them time to get bigger and develop the thick skins required for good storage. Since they are harvested late, it’s unlikely that you’ll be able to follow them with a crop to harvest in the same year, but you can plant crops afterwards in autumn such as over-wintering onions, garlic or broad beans for harvesting the following year.
Where to buy seed potatoes and how to choose?
‘Seed potatoes’ look nothing like seeds but look exactly like small potatoes, because they are in fact just potatoes. You could save some of your own potatoes and plant them the following year, but this isn’t normally done since there’s a chance that they may be diseased, even though they may look OK. You can buy seed potatoes from garden centres and sometimes from hardware shops and DIY stores. Some come pre-packaged in a bag – fine if that’s the amount you want to buy, and the size and quality are OK.
If you can, buy from somewhere where you can pick and choose each individual seed potato, then you can choose the right amount, size and quality. Often this is cheaper since the shop has bulk-bought without so much packaging, and they pass the saving on to their customers.
The usual advice is to select ones that are the size of hens’ eggs, and I’ll stick to that description since I can’t think of anything in the plant-based world that’s that size right now. Presumably if they’re bigger they don’t give a better yield but since they’re sold by weight they are less value for money and presumably if they’re smaller they don’t yield so well, but that’s just a hunch. Some people cut big seed potatoes in half since each half will still grow, whereas cautious people like me think there’s more chance that they may rot this way, so stick to uncut smaller seed potatoes.
For a wider range of varieties, including heritage varieties, try a potato fair if there’s one near you (search for one online) or try mail order or internet ordering.
Before planting them out – chit them
To get them off to an early start, and to increase the yield, around four to six weeks before you plant them out you normally ‘chit’ the seed potatoes. Chitting means allowing them to sprout before they are planted out in the soil. You might think that an earlier start can instead be achieved by planting them out unchitted, earlier than usual, but this doesn’t necessarily help – they won’t grow while the soil is very cold.
Back to chitting you spread them out in a tray in a – single layer and leave them somewhere light but not sunny, and somewhere that’s not too cold, eg on a sunless windowsill indoors. Over the course of a few weeks, the eyes of the potatoes start to produce short and sturdy green sprouts. It’s best to orient the seed potatoes so that the bits which have most of the eyes are upright. If it’s too warm or too dark, the sprouts grow much more quickly and are lighter in colour and lack sturdiness so snap off easily – not good. It’s generally said that chitting is more important for earlies than for maincrop varieties. Personally I think that for maincrops, chitting and planting out reasonably early if the soil isn’t too cold is strongly recommended, since if blight strikes, which kills the top growth, the yield will be higher than it would for unchitted and later-planted potatoes. Also, they can be harvested slightly earlier, with less damage from slugs and wireworm. Early harvest gives you a wider choice of follow-on green manures that can be planted to protect the soil over winter. For instance, winter tares might just be possible, which are best sown before mid-September.
Where to plant
Potatoes aren’t too fussy about soil type, but there is some sensitivity to pH. If the soil is alkaline, serious scab is more likely. Before planting, don’t add lime or anything else that’s alkaline such as wood ash from a wood-burning stove or a bonfire. An open sunny position gives the best yield, although they still grow and yield reasonably in partial shade. If it’s too dark, the foliage will be weak as the stems grow longer, searching out for lighter conditions.
Adding compost to the soil
The best time in your crop rotation to add compost is just before planting potatoes. It really is surprising how much greater the yield can be if you add a lot. Compost also helps to hold water in the soil, and potatoes yield better if they are kept moist. Some people dig compost in before planting, some leave it on the surface, and some only add it to the soil in the vicinity of the planting hole. Traditional advice is to add compost the previous autumn. I’m not convinced that this is such a good idea since some of the nutrients will be washed away during heavy winter rain. I always add home-made compost just before I plant the potato seeds.
A note on ‘earthing up’
Earthing up potatoes (drawing by Amanda Rofe)
Potatoes are usually earthed up after planting which means that, after the plants emerge, soil is drawn up into a ridge to cover the foliage, and this is repeated after the plants grow further. Earthing up helps to exclude light from the tubers to stop them from turning green, is said to increase yields, and protects the young plant from frost. If you don’t earth up or use other frost protection measures, a heavy frost after they’ve emerged kills the foliage. The plant survives and new shoots appear, but development is delayed. A downside to earthing up is that it makes watering difficult – the water tends to run off down the ridge slope rather than soaking in.
In rows, earthed up Traditional advice is to grow in rows. Space between plants within a row is 12″/30cm for first or second earlies with rows separated by 24″/60cm – or 15″/37.5cm for maincrops with rows separated by 30″/75cm. These rows can also be in beds if that’s how you like to grow things, and the rows can be arranged either lengthways or widthways. The wide separation between rows provides sufficient soil to allow earthing up. Earthing up can be hard work if you have compacted soil, most likely if your soil tends towards clay and hasn’t had huge amounts of compost added over the years. If, when earthing up time arrives, the soil surface has dried out it can set very hard, seriously hindering earthing up. Digging the soil before planting helps here, but if you prefer no-dig there are other options which follow.
Under an impenetrable light-proof mulch, not earthed up Holes are made in the mulch – black polythene is a common choice through which the seed potatoes are planted, and a plant grows through each hole. The tubers can’t be earthed up but the mulch prevents light from reaching them, preventing them from turning green. To protect the young foliage against late frosts, try covering it with straw or fleece if a frost is predicted. The mulch helps to conserve soil moisture by reducing evaporation, and it stops weeds from growing, except for those that find light through the planting hole. This method is good if you’re trying to grow on unprepared land with grass and weeds, which get killed off by lack of light. Instead of polythene, some people use sheets of cardboard weighted down by compost. Dave of Darlington recommended growing them under a mulch www.veganorganic.net/no-dig-potato-growing
Plant them in a hollow, then earth them level This is the method that I use. When I dig a hole for each potato, I push the removed soil to between the planting holes, then after planting the seed potato I only partly fill the hole, leaving a hollow. Rather than earthing up, as the plants emerge, I gradually spread the soil until it’s level. This makes watering easier since it doesn’t run off, and allows the plants to be equally spaced within rows and between rows but since the potato seed is set deeper, it’s probably not a good idea if your soil gets waterlogged as this might cause the potatoes to rot.
First earlies are planted late March to early April, second earlies a few weeks later, and maincrops in April. If you live in a warm spot in the south you can plant a week or so earlier, or if you’re in a cold spot in the north, try planting a week or so later.
When planting, if the soil is dry, consider watering it fairly deep, ie in the planting holes or trenches before planting the seed potatoes, unless heavy and prolonged rain is expected.
The base of the seed potato is normally planted 4-6″/10-15cm below soil level, then cover as described above.
I’ve found that yields are substantially higher in rainy years, which tells me that I don’t water enough in dry years. They really do need a lot of water, and it’s important to get the water down deep. If it’s dry, watering very heavily infrequently (say every week or two) is much better than watering lightly and frequently – the latter doesn’t get the water down deep enough. The most water is needed when the tubers are bulking up, ie getting bigger quickly.
Weeding and intercropping
Weeding is only necessary when the plants are small. They soon fill out and smother out light, preventing most weeds from growing. Because of the smothering and earthing up, people don’t usually intercrop, although something very fast-growing might be worth trying, like salad crops such as rocket or lettuce.
First earlies are ready to harvest in late June to early July, second earlies a bit later. You can try carefully scooping out the biggest spuds early, leaving the smaller ones so that they get bigger. This is only easy if the soil is crumbly.
Maincrops are ready to harvest from mid-August for eating immediately (you shouldn’t store these since they won’t have developed the thick skins needed for storage, but from late August onwards they should be OK to store). If the foliage is still green, harvesting maincrops later improves the yield, but you’re more likely to get slug and wireworm damage.
Try to avoid a rainy day if you can, otherwise lots of soil will stick to the potatoes. Dig them out with a fork. Put the fork in deep a reasonable distance from the plant then swivel, otherwise you might pierce them. Try to remove all the potatoes, no matter how small. Even small ones will grow the following year, which are difficult to remove in the following crop. If there was blight, it stores in the tubers over winter and the blight spores will spread from the foliage which emerges.
Dry them well, brushing off excess soil as they dry, then store them in paper or cloth bags. Avoid plastic bags otherwise they can’t breathe, which increases the likelihood of rotting. Exclude light, otherwise they’ll turn green, and store somewhere cool but frost-free. If it isn’t cool, they will sprout much earlier.
Potato blight is very common from mid-summer onwards. Blight spores blow in the wind over long distances so crop rotation doesn’t help. The spores only take hold when it’s warm, it’s humid, and the foliage is wet – all three have to happen at the same time. You can’t stop the foliage from getting wet from the rain, but try not to wet the foliage when watering. Blight attacks the foliage, which turns brown and dies off quickly. Spores fall on to the soil and might get washed down into the tubers which they then attack, causing them to rot either in the ground or after harvesting when in storage. Earthing up well results in a bigger barrier of soil which prevents this.
Earlies are rarely affected by blight since they are mature or nearly so by the time blight strikes. Maincrops bear the brunt of blight. Some people remove the foliage when blight strikes, others leave it to die off on its own. The spores don’t live for long in the soil so, for potatoes that you intend to store, it’s a good idea to leave them in the soil for a couple of weeks after foliage die-off before harvesting, by which time the blight spores won’t be able to attack the potatoes. Some sources say you shouldn’t compost blighted foliage or tubers. More recent advice seems to be that you can, so long as any tubers are cut up small so that they compost quickly, since blight can only survive the winter on a live tuber. A few maincrop varieties are completely immune to blight – the Sarpo family is an example.
Scab The surfaces of the potatoes look scabby. It’s only skin deep and doesn’t affect the eating quality so it’s best to ignore it. Common scab is the most common type, and keeping the soil moist reduces its likelihood, as does avoiding planting in alkaline soil.
Holes in the tubers Underground slugs and wireworm are the usual culprits, and can give you a serious amount of work and loss cutting them out when you’re preparing to cook them. The later you harvest, the worse it gets, so earlies are rarely affected much. The usual harvest time for maincrops is late September, but harvest them in late August if you have slug and wireworm problems. It’s said that wireworm damage is worse when land has recently been converted from grass (eg a lawn) to growing potatoes so, if that’s what you’ve done, hopefully the problem will decline with time.
The above three troubles are the most common of those which aren’t helped by crop rotation. There are lots of other problems that can occur which can normally be avoided by using crop rotation, and buying in potato seed from a good source rather than saving your own.
There isn’t the space and I don’t have the knowledge to list varieties here so an internet search is your best bet.
This article appeared in Growing Green International magazine Num 34 (Winter 2014/15), p28, but compared to the original article has been updated and expanded.