Seed Potato Sizing
12 April 2024Regarding 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.
- 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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Andrew Tokely's Gardening Tips for April
04 April 2024The lighter evenings have finally arrived and after a wet March and wet start to April, hopefully the better weather will be with us all soon. Once the better weather arrives, the lighter evenings will give us all more time to spend in our flower and vegetable gardens. Happy Gardening!
1. As the weather improves and the soil dries out and gets warmer you can really get started on the Vegetable plot. This month you can make sowings of some of the smaller vegetable seeds like Carrots, Parsnips, Beetroot, Radish, Spring onions, Spinach, lettuce and salad leaves. You can also make further sowings of Peas & Broad Beans.
2. If you are planning to have a bumper harvest of Cucumbers, Courgettes, Marrows, Pumpkins or Squashes this year, then towards the end of the month is an ideal time to sow some under glass in a heated propagator at 21C (70F). I like to sow seeds into individual 9cm pots which are half filled with a good quality seed sowing compost. Once the seeds germinate and reach the top of the pots, I then carefully fill up the rest of the pot around the stems so the plants are sturdier and this will encourage further stem roots to grow. All these vegetables are of the same family so it is important when you sow them, always push the seeds in on their edge, rather than flat. This is so the water runs away from the sides of the seeds. This is important because if the seeds are sown flat, there is always a danger the seeds may get too wet and rot rather than germinate.
3. Once the soil dries out and is workable Early and second early potatoes can be planted, followed by main crop varieties towards end of month. Most Early varieties are ready to harvest in 11-12 weeks from planting as baby new potatoes.
4. Hopefully last autumn you left the old flower heads on your Hydrangeas. This was done to protect the young growth and buds during the cold frosty winter months, which is very important, if there is a winter cold snap. Towards the end of this month, it is the best time to cut off those old flower heads, and prune the plants into shape. I like to prune my Hydrangea down to 3-4 buds, as this helps to keep the plants at a manageable size and still rewards me every year with plenty of new flowers. Don’t be tempted to prune any lower than this otherwise you will get bushy plants but with a few or no flowers. Other plants to prune back at this time of year are the old growth of Penstemons and Hardy Fuchsias. These can be pruned back quite hard, down to about 15cm (6in) from the ground. This will encourage the new young shoots at the base of the plants to grow and replace the old woody growth you are cutting away.
5. If you are longing for a colourful garden this summer but gardening on a budget, then there is nothing easier or better value to grow than Hardy Annuals. There is a wide choice from Calendula, Cornflower ,Nigella, Godetia, Clarkia, larkspur, Poppy to name a few. These can be sown direct into the garden any time from this month up to the first week of June, as soon as the soil is dry and warm enough. These are easy to grow and will reward you with a glorious display this summer from very little time and effort as well as leave a few pounds in your pocket.
6. Towards the middle of this month, I will be sowing the last of my summer bedding in the propagator. This month is the time to sow African and French Marigolds, Cosmos, Phlox, Tagetes and Zinnias. These will quickly germinate and will be ready for pricking out into individual pots or trays in about 7-10 days’ time. If you haven’t got a propagator, you can still sow these flower seeds on a warm windowsill. Although sown later than a lot of summer bedding, they will still be ready for planting out at the end of May along with any other summer bedding plants.
7. If you are planning to grow some Sunflowers this year, either as cut flowers, to add height to borders or just some fun with the Children or grandchildren, then towards the middle of this month is a good time start. Sow the seeds into individual pots of seed sowing compost in the greenhouse or on a warm windowsill.
8. This month Perennials really put on a spurt of growth, so before they get too large, sprinkle some Blood, Fish & Bone fertiliser around the plants and work this into the soil. This will give the plants a boost, rewarding you with a bigger and better display this summer. As the plants grow add some twiggy sticks or a wire frame to support the plants as they grow. Putting these supports in early will allow the plants to grow through them, allowing there leaves to disguise them, still giving your borders that natural look.
9. Over the winter if your lawns have been sitting damp and, in the shade, or if your soil is not very well drained, then you may now have some patches of moss spoiling its appearance. This moss can be raked out (Scarified). You can either do this with a spring tined moss rake and a bit of elbow grease or use an electric Scarifier. Once scarified, mow the lawn again to tidy it up and leave for 1-2 weeks to recover, before applying a summer lawn feed.
10. As Summer and autumn Brassicas sown last month and the large onions sown in December under glass start to get larger, move to a cold frame to gradually harden off ready for planting out as soon as weather conditions improve. Gradually hardening off plants before planting out is always advisable, as this gradually acclimatises the plants to cooler outdoors conditions rather than shocking them, which would happen if planting straight from the glass house.
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