February 2010

(This photo shows the variety Sarpo Mira, a blight resistant late maincrop, interplanted with the companion plant Borage.)

I love growing potatoes as each time I harvest them I feel like I’m unearthing buried treasure (making it a real joy when our children help)! Carefully loosening the earth from around the plants and seeing how many tubers have grown from one little seed potato carefully planted weeks before is a real delight. However, the range and variety of potatoes available, as well as all the gardening terms used when talking about them, can seem quite bewildering to someone new to growing veg.

As a result of all the questions I’ve been asked recently about this magical crop, including  “what are earlies”, “how should I plant them?” “What’s blight” “what are floury/waxy potatoes” “what do you recommend”, the following is a guide to choosing and growing your own spuds.

Potato Groups

Potatoes are put into groups that describe how long it takes for them to reach maturity. Usually this is about:

Very early earlies        – 75 days (harvest around June)
Earlies (or 1st earlies)  – 90 days (harvest around July)
Second Earlies            – 110 days (harvest around August)
Early Maincrop            – 135 days (harvest around September)

Late Maincrop             – 160 days (harvest around October)

Earlies are often described as ‘new’ potatoes as they’re used when fresh rather than stored. They’re also planted closer together so are smaller and take up less space. They’re a great crop to grow if you’re nervous about growing spuds for the first time. Apart from harvesting them at a time when new potatoes are expensive in the shops, because they’re ready early to mid summer they usually avoid pests and diseases such as blight and potato cyst eelworm.

Maincrops are great for storage but can be used straight from the ground too. Every year more varieties of blight resistant varieties are introduced in an effort to combat this destructive fungus.

Cultivation

Potatoes are grown from what’s known as ‘seed potatoes’ readily available from garden centres, DIY stores and mail order catalogues from mid winter onwards. It’s not advised to save your own (or use supermarket potatoes usually destined for the table) as they may become diseased. If you have a choice when choosing potatoes, try and pick larger tubers for early crops and small/medium sized for maincrops.

There’s a lot of debate about lately on whether we should ‘chit’ potatoes prior to planting. Chitting means that you’re encouraging the potato to sprout before placing it in the ground in the hope of an earlier harvest. Potato tubers will eventually form on these sprouts.

Often gardeners will chit earlies but plant maincrops as they are. Chitting involves placing the seed potato (known as a tuber) in a container (like an egg box) and placing it in a dry, frost free room out of direct light so that it grows little shoots out of it’s ‘eyes’.

When the shoots are approximately 2.5cm long the potatoes can be sown outside. Don’t worry if they’re longer – just be careful not to snap the shoots off.

When can you plant them out? The soil where the tuber will rest should have reached a temperature of about 6oC for three consecutive days (usually March to April). Don’t attempt to plant them if the soil is too wet, sticky, dry or frozen. If you’re worried that the conditions don’t seem ideal and time is passing, try pegging down some clear plastic over your soil to help warm it up. The yields will be much higher in a long growing season.

Growing Methods

In general, the closer you plant the seed potatoes together, the smaller the harvested crop. As a general guide when sowing in rows, place

Earlies – 30cm apart, with 60cm between rows and 5cm deep

Maincrops – 37cm apart, with 75cm between rows and 10cm deep

They will grow in most soil types but prefer a sunny, frost-free area with a pH of 4.5 to 6. Potatoes can be a useful crop for breaking up soil as their roots are so deep. If you can, manure the soil the previous autumn as this crop is a greedy one. Once the plants are growing they will need watering – heavily and every week on dry soils.

Potato crops are not fully hardy. This means a heavy frost could kill them (as farmers have found to their cost this year in Ireland) and we have been known to have snow as late as May. If you can, cover the growing crops with horticultural fleece, newspaper or cloches to protect them if frost threatens.

Container planting choose a compact variety – early and second earlies are ideal. Plant 3 – 4 seed potatoes in compost in a large container (such as a dustbin or a heavy-duty plastic sack with drainage holes punched in the base) at a depth of about 15cm. When the foliage starts to grow (known as the haulm) add more compost to “earth up”. This encourages more side shoots to develop. Keep adding compost until it reaches the top of the container.

Planting in the soil there are a couple of popular methods used for planting outside. The first involves making V shaped ridges in the soil and planting the tubers in the bottom, using the soil on the ridge to ‘earth up’ the foliage (haulm) as it grows. The second method involves covering the soil with black plastic or weed membrane, making little X shaped cuts and planting the potatoes directly into the soil, making the slit larger as the plant grows.

The reason for earthing up or covering the soil with plastic is twofold. Firstly it prevents ‘greening’. As the plant grows the newly developing tubers get pushed upwards. If they’re exposed to sunlight they become green and produce a poisonous toxin called solanine. Secondly earthing up will give the tubers some protection against blight. If you choose to earth them up, do so in the morning – potatoes tend to droop as the day progresses making the job harder.

Diseases

There are over 100 potato diseases, which is enough to put anybody off growing them! In my experience the main ones to worry about are Blight and Eelworm – not to dismiss all the others but these are the nasties. Slugs also love potato plants so a daily patrol at dusk armed with a torch and an empty milk container half filled with salty hot water will keep them at bay.

Blight is at its worst in warm, humid conditions usually from July onwards. It’s a parasitic fungus that’s carried along in the wind currents. Beginners often find it a difficult disease to diagnose until it’s too late. Look out for brownish-black spots that appear on the leaves and stems. The undersides of leaves often have a white mould fringe around the spots. Tubers will have black marks on them.

Once it’s taken hold there’s nothing you can do other than remove the haulms (do not compost them) to about 5cm off the soil level and hope that the spores did not infect the tubers in the soil. Affected tubers cannot be stored. If they are infected the black marks can be cut out and the rest of the potato eaten but avoid giving them to pregnant women, the elderly or sick.

Avoid blight by buying resistant varieties, planting earlies and earthing up. Also dig up all tubers when harvesting and remove any infected plants from your site.

There are eight tips here for managing potato blight without chemicals.

Potato Cyst Eelworm This pest is very common in soil that has had potatoes grown on it previously. The plants are often stunted, with leaves that turn yellow and die. If you suspect eelworm Joy Larkcom in her great book Grow Your Own Vegetables suggests lifting the roots and plunging them in a bucket of water – the cysts will float on the surface. You may have to give up growing potatoes in that area for 10 years if infected!

To prevent Eelworm infestation follow a long crop rotation, grow resistant varieties, and only grow earlies (they mature before a major infection occurs) and only use certified seed.

Incidentally, Scab is caused by growing potatoes in soil that is too alkaline (high pH of 7 or more (they’re still safe to eat though).

Harvesting

You can start harvesting most varieties once they start to flower. With earlies just dig them up when you want them – preferably just before dinner! Maincrops too can be dug up as required but if you want to store them, and they’re healthy, they can be left in the ground until early autumn.

If you notice that your plants are starting to look diseased, don’t delay – cut back the haulms to ground level and leave the tubers in the ground for about two weeks before lifting which will help their skins to harden.

Varieties and Quantities

How do you decide what seed potatoes to buy and how many? There are over 500 varieties on offer all with different traits. Whether you grow waxy or floury potatoes is personal preference – what do you like to eat and cook?

Waxy potatoes are translucent and feel moist. They’re firmer and keep their shape, making them ideal for salads, chipping and boiling. Varieties include Coleen, Charlotte, Marie Peer, Aaron and Remarka.

Floury potatoes are drier and granular and are best where you want fluffy potatoes for roasting or mashing. Varieties include Estima, Records, Setanta, Maris Piper, Desiree, Golden Wonder and British Queens.

Other points to bear in mind include what variety best suits your area/location? For instance Epicure are a floury potato with more frost resistance than other varieties, making them an ideal choice for colder areas. The Sarpo range has been bred for their blight tolerance. Charlottes are a classic salad potato with good blight and scab resistance and Maris Piper have a good resistance to eelworm.

If you prefer to purchase your potatoes from the local garden centre, take a look through your gardening books or seed catalogues and jot down a few choices before you leave home. If your main consideration is culinary, check out www.lovepotatoes.co.uk/potato-varieties which has a comprehensive list of varieties and their best uses.

As a general guide to how many to buy and plant, potatoes are usually sold by weight. 1.5kg gives an average of 14 seed potatoes and requires and area of roughly 2.5sq metres to grow. The total yield will be an average of 10.5kg (depending upon variety and growing conditions).

Care

If your crop is healthy it’s much more likely to withstand attacks from disease and bad weather. A seaweed based foliar spray can be applied at least three times during your plant’s lifetime (including while they’re chitting) to give them a boost.

Copper sprays can be used against blight (the Irish organic standards for using copper based sprays are 8kg copper per hectare per season). Contact organic suppliers for more information.

If you can contain horseradish by planting it in a bucket at the end of the row (it can get invasive), it’s said to provide general protection to potato crops. Evidence is also showing that substances from the roots of Tagete patula (marigolds) can help to prevent microscopic eelworms and Borage is said to repel them.

So that’s it. Have a go! Good luck.