Preparation Pointers

Classification

Seed potatoes can only be grown in field which have been tested and shown to be free of PCN eelworm (see Pests & Diseases), and which have not grown potatoes for many years. Details of classification schemes are complex and vary from one country to another.

Initially, seed potatoes start off as a completely disease-free stock (e.g. as laboratory grown microplants) and go through a sequential scheme, where every year of field growth leads to a slightly lower grade. Government inspectors look at growing crops systematically and judge any visual symptoms of disorders against set criteria, with back-up laboratory assistance, as required. Depending on the results of their findings, they will award the grade applied for, downgrade of fail each crop.

Classified Seed Potatoes

Classified Basic Seed Potatoes are the highest grade of seed potatoes available. There are 3 EU grades of Basic seed potatoes, which we in the UK call

• EEC1 UK equivalents are VTSC1 and VTSC2
• EEC2 UK equivalents are Super Elites 1, 2 & 3 and Elites 1, 2 & 3
• EEC3 UK equivalents are AA
• Scotland only produces Basic seed potatoes

Classification does not end with the field inspections! Once the crops have been harvested, they must be graded to set standards, depending on the grade attained after field inspection. Before each consignment of seed potatoes can leave the farm of growth, they must have a plant passport label, issued by the relevant authorities and they must be inspected by a governmental inspector. Only when they meet all the criteria are they classified as seed potatoes.

Check the label on your seed potatoes – it should be an EU Plant Passport, showing the classification number, grade, size, certifying authority and many other details. Be very suspicious if labels are not present or if the label does not contain all the legally required information, which can be found in Seed Potato Plant Passport Information.

Certified Seed Potatoes

Certified seed potatoes are lower grade than Basic seed potatoes and therefore do not have such a high health status. They are however still inspected by plant health officials and are certified to a specific grade. The process is the same as that carried out for classified seed potatoes, but certified seed potatoes are lower grade.

Certified seed potatoes cannot be produced in Scotland

CC is a lower grade than those detailed above and does not qualify for EU grading. It should only be used in very limited circumstances.

Using Unclassified Potatoes for Seed

Using unclassified potatoes for seed can lead to great disappointment and is a false economy. Table potatoes and garden grown potatoes planted as seed potatoes carry more bacterial spores and fungal pests than high grade classified seed potatoes. In addition, non-classified potatoes can carry PCN eelworm (see Pests & Diseases), or can be treated with sprout suppressant. Despite all your effort, care and attention, crop yields will be poorer than expected and even worse, the crop might not grow!

Chitting

This process allows strong, green chits (sprouts) to develop on the tuber before planting. Dormant or just sprouted seed potatoes can be set up in trays with the sprout end (rose end) facing upwards. Place in a frost-free, light area e.g. the windowsill of an unheated room, as the warmer the room the faster the process. It is generally agreed that chitting is recommended for First Early varieties as it gives the tubers a faster start and the crops will reach a useable size slightly earlier.

If apical dominance is encouraged during chitting, by removing all side shoots and just leaving the rose end sprouts, the growing plant will produce fewer, but larger tubers (ideal for producing a crop of baking potatoes). If all sprouting shoots are left on the tuber, the growing plant will produce numerous, but smaller tubers (ideal for First Earlies and Salads).

The full situation is however more complex and the technical description – “premature ageing” – gives an indication of this. Chitting does speed things up in the beginning, but it also speeds up the onset of senescence (the natural dying-off of foliage), therefore potential yields can be reduced. This is not a problem with First Early varieties, as these crops are lifted as soon as they are ready. This also applies to some Second Early and Salad varieties. Chitting Maincrop varieties is not really essential, as these varieties have a much longer growing season therefore it’s not necessary to “hurry them up”, although if you do want to chit these varieties it won’t do them any harm, other than to perhaps reduce the yield slightly, and it can be used to manipulate crops i.e King Edward naturally produces many tubers, but by chitting the seed potatoes and encouraging apical dominance, the crop will produce potatoes which are ideal for baking. 

Late Cropping Seed Potatoes

Also called “Christmas-Cropping”, these are seed potatoes that have been held in specialised cool storage. They are normally available during late June / July, and should be planted immediately. The seed potatoes will be ready in October, but shouldn’t be harvested at that time. Instead the haulms (foliage) should be removed and the potatoes should be left in the ground (or container) and protected from frost. They can then be harvested at a later date (hence the name “Christmas-Cropping”). For more information look in Late Cropping (or "Christmas Cropping") Potatoes.

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