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Seed Potato Sizing 12 April 2024

Regarding the size of the seed potatoes, this is an area which is easily misunderstood. Seed potatoes are generally sized 35x60mm. This means that the smallest seed potatoes in the sample will not fall through a 35mm square. If they balance on the square (at any angle) then they comply. The same sample must all pass through a 60mm square (at any angle). We have enclosed some photos of this procedure, which hopefully illustrates the point clearly (please note this uses 35x55mm size as an example, but it’s the same principle).
We always do a thorough quality inspection before accepting seed potatoes from our growers; therefore we are confident that the seed potatoes conform to this size specification.

For the record, accurate sizing is a legal requirement of seed potato certification and the standards are very strict.
Scottish seed potatoes are often sold as size 35 x 55mm. This means the tuber must not pass through a 35 x 35mm square but be able to pass through a 55 x 55mm square.

There is a common view amongst gardeners (particularly very experienced gardeners) that seed potatoes should be “golf ball” or “hens’ egg” size. Historically, some seed potato crops would have been grown as “dual-purpose” stocks, which means that after certification, the largest potatoes in the seed crop would have been consumed and the smallest potatoes would have been used as seed potatoes. This explains where the “hen’s egg” size originated. These days, seed potato production is very specialised and dual-purpose crops are a thing of the past. As a result, when ISJ agree contracts with our suppliers, we must market all the seed potatoes produced to maximise the return to the supplier, otherwise we will not have any growers prepared to plant specialised varieties. Hence, we purchase and supply 35x60mm seed potatoes.

Large seed potatoes have more “eyes”, (and more sprouts) therefore they produce more potatoes. For a First Early variety such as Arran Pilot, or a salad-type variety such as Charlotte, this is exactly what you want– lots of tasty potatoes, straight from the garden! If gardeners would like to see this in action, they should plant their seed potatoes by size – smallest to largest (or visa versa) - and then check the number of potatoes per plant as they harvest.

Small seed potatoes weigh less, so a pack contains more seed potatoes, which means gardeners should be able to plant a larger area. In addition, small seed potatoes have fewer “eyes”, therefore they produce fewer potatoes. What’s positive about that, you might think? Well, it means the crop will produce more, large potatoes and with a modern varieties such as Cara and Maris Piper, which can produce huge yields, this is ideal – a big crop of large, tasty potatoes to store through the winter. Commercial growers pay a premium for small potatoes, especially 25x35mm (these really are small!), for these reasons.

  1. Example of sizing square
2. 35 x 55mm tuber will not pass through a 35mm sizing square
3. 35 x 55mm tuber will be able to pass through a 55mm square in any direction.
The principle is the same for 35x60mm seed potatoes, except the tuber must be able to pass through a 60mm x 60mm square
 Summary of Legislation and Procedures Surrounding Seed Potato Production & Supply
  • All seed potatoes are fully traceable back to the original pathogen-tested, disease-free micro-plants. Without getting too technical, this is vital to ensure that, should there ever be an outbreak of disease, stock movement can be traced.
  • All seed potato crops from Scotland are inspected by SGRPID (Scottish Government Rural Payments and Inspections Directorate) several times during the growing period and then prior to despatch from farm.
  • All seed potatoes are issued with a plant passport prior to movement from the farm of growth and provided they meet the final inspection tolerances. Movement from farm is illegal until all inspections have been completed and a valid plant passport has been issued.
  • ISJ are licensed to replicate seed potato plant passports for the horticultural sector and are regularly audited by SASA inspectorate (Science & Advice for Scottish Agriculture) to ensure we comply with their conditions and meet the standards.
  • ISJ replicated plant passports, which are the labels of the seed potato packs, show all the legal information required to ensure that stock is traceable. If your customer still has the labels, we can double check the details and will be able to trace the stock supplied back to the original field of growth. The Crop No. is a 6-digit number which allows us to trace the stock.
  • The plant passport must also show the size of the seed potatoes. In most cases, ISJ supply size 35 x 60mm, although this can vary depending on the variety. We’ll explain this later.
  • As an aside, ISJ are also ISO 9001 accredited – this is an international quality management certification, which demands we are regularly audited to ensure we comply with rigorous standards. This is not obligatory for seed potato providers, but we do it to give our customers confidence that we operate accurate and robust prepacking systems.

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