The Gardeners Almanac

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My Back Garden

Potato Onions


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Week Numbers

Allium rust

Compost

Containers

Crop Rotation

Downy mildew

Fertiliser

Frost Zones

Fungicide

Hardening Off

Heating

Lime

Planting out

Pricking out

Seed Sowing

Slugs & Snails

Soil pH

Soil preparation

Ventilation

Watering

Weeding

White rot

 

Overview:

The origin of the Potato Onion (Allium cepa var. aggregatum) is uncertain, the earliest clear records of their use was in France in the 12th century.

They were once a popular crop but fell out of favour in the early twentieth century.


It is thought that large scale production was virtually impossible because of the lack of a mechanical harvesting machine that could deal wth the harvesting of the bulb clusters.

Alternative method /s of harvesting were usually 'labour intensive' thus making the harvesting process quite expensive.

This resulted in the growing procedure of growing / harvesting crops becoming more suited to a garden or on a smallholding / allotment.


Attributes:

The Potato Onion also known as Nesting Onions or Multiplier Onions, is a member of the onion family that is reproduced primarily by division of bulbs, rather than by sowing seed.

They have similar attributes to Garlic rather than Standard Onions (Allium cepa), and are closely related toShallots, which are also Allium cepa var. aggregatum.


When comparing Potato Onions with Shallots the exact dividing line between the two is difficult to determine.

For example; Shallots have a relatively mild flavour but do not store well, whereas Potato Onions have a sharper flavour and store better.

Potato Onions are usually larger, divide into fewer bulbs, and are more commonly grown by replanting the bulbs rather than by starting from seed.

Potato Onion bulbs

Bulbs range from 12mm-50mm (½"-2") in diameter, although some newer varieties produce larger bulbs.


The bulbs can be replanted indefinitely making them more manageable than biennial onions which must be grown from seed each year.

Note: Potato Onion bulbs should not be confused with sets* that are often used to start biennial onions.

*Sets are actually small onions that have been propagated from seed.


It is best not to plant in the same bed for too long, particularly if any of the plants have been subjected to disease

Subject to the type of disease this can mean that crops from the Allium family should not be planted in this area for up to ten years.

However; on most occasions it is quite safe to grow in the same bed for upto three or four years and then relocate.


Potato onions are classified as hardy plants having originated in cold climates, where they are known to have survived in climates as cold as minus 20°F (-29°C)*

*If a thick mulch is applied they may tolerate temperatures colder than this.


Potato Onions like many plants have photoperiod* dependencies.

*Photoperiodism is the response of plants to the relative lengths of light and dark periods within a 24-hour cycle.

Light duration in the UK is longest from March 21 to September 21, (Week 12 - Week 38)


Plant development processes that are affected by photoperiod, include: Vegetative growth, bulb formation, and the number and size of the bulbs to name a few.


Depending on their response to daylength, plants are classified as either long-day plants (LDP), short-day plants (SDP), or day-neutral plants (DNP).

Potato Onions grown in the UK are classified as long-day plants (LDP)

Long-day plants (LDP) are those in which crop response occurs when daylength is in excess of a critical length while the response of short-day plants occurs when daylength is below a certain critical length.


Potato Onions are tolerant of a wide range of soil conditions,but will do best in humus rich, well drained soil with a pH of 6.5 - 7.

As mentioned above Potato Onions have similarities to Onion (Allium cepa) Shallots (Allium cepa var. aggregatum) and Garlic (Allium sativum) and as such cultivation shares many of the attributes of these species.

For example they can be planted out in the spring or autumn like many Onion (Allium cepa)or Garlic (Allium sativum)varieties.


Autumn planting can give a head start and may mature as much as a month earlier.

As with garlic an autumn planting will subject the bulbs to a cold period (vernalization) that will break the bulb dormancy which in turn can lead to better bulb production.


If your growing area is prone to waterlogging during the winter months you would be better advised to go for a Spring planting.


If winter temperatures have been relatively mild plants may run to seed and flower.

Having said that, this can be useful if you want to produce seed.

In the event that plants produce flower stalks these should be removed as they tend to reduce bulb size.


Pests & Diseases:

Potato Onions are pretty much pest free, although on occasions they can be affected by Onion Fly

Slugs and Snails can sometimes attack plants, generally this is after planting out when the leaves of seedlings are more succulent.

Bulbs can be cosmetically damaged during storage but usually survive to produce a normal yield.


Potato Onions are vulnerable to the same diseases as other members of the allium family.

For example: Allium Rust, Downey Mildew and White Rot.


Cultivation:

Onions potted up
Container Growing:

Potato Onions are well suited to growing in containers.

A 200mm (8") diameter pot will grow a single plant nearly as good as growing it in the ground, providing it is well watered.


Seeds:

Potato onion seeds* have a short storage life, particularly if stored at room temperature.

If stored below 40° F (4 C), they retain good germination results for at least three years.

*All varieties are hybrids, so they will not grow true from seed.


Week 2 - 4:

Tray of seedlings

Potato Onion seedlings

Sow seed in either celltrays / pots / punnets or standard trays filled with evenly firmed soil-less seed compost.

Sow the seed thinly, or singly, over the compost, and cover 5mm (3/16”) deep, with similar compost, water, then cover the tray with a sheet of glass and paper.

Germinate at a minimum temperature of 16°C (60°F)


As soon as they have germinated, normally in about seven to ten days, take covers off, and allow plenty light to the seedlings.

Grow on in Greenhouse at this temperature, ventilate as much as possible (depending upon the weather) giving maximum light at all times.

An alternative to the above is to collect seed and sow them in the same manner around Week 44.


Week 6-8:

Seedling at the 'crooked' stage

Potted up Seedling

As soon as the seedlings reach the crook stage, and the second leaf appears, prick them out into 70mm (3”) pots of any proprietary soil-less potting compost.

Grow on, giving them plenty of light, but being careful not to force them with too much heat, and too much water, 8°-10°C (45°-50°F) will do.

Watering the seedlings largely depends upon the weather, one should allow allow the pots to nearly dry out before watering!

Note: It is advisable to spray seedlings, from the second leaf stage with a fungicide at regular intervals to deter damping off.


Week 13:

Gradually harden off seedlings prior to planting out.


Raking in Fertiliser

Fertiliser raked in

 Fertilser applied

Fertliser dressing
Week 15:

Before planting out, dress the bed/s with a general fertilizer, applied at 60gms (2 oz) per sq. metre.

Lime should also be added at this time if the pH requires adjusting.

A pH reading of 6.75 to 7.25 is recommended.


 

Seedlings planted out

Week 17-19:

Set plants 150-200mm (6"-8") apart in rows 500mm (18")

Firm in the plant around the root ball.

Do not water in, unless it is absolutely necessary, let the plant find its own moisture.



Bulbs (Spring Planting)

Green shoot emerging

Signs of Growth

 Bulbs set up in 24 cell tray

Bulbs set out in cell trays

Week 11:

To get off to an earlier start set up your bulbs in trays / pots.



Week 16 - 18:

Plant out bulbs 150-200mm (6"-8") apart in rows 500mm (18")

Keep them well weeded as they don’t compete well with pernicious weeds.


Week 24:

From mid June onwards apply at 2-3 week intervals, Nitrate of Soda at a rate of 15-20gms (½ - ¾ oz) per sq metre.

Never feed during dry conditions, always ensure that the soil is quite moist, or it is about to rain before feeding.


Closer o#look at plants in bed

A closer look

 Onions growing in the bed

Plants growing in bed
Week 32:

Apply a 15-20gms (½ - ¾ oz) per sq metre dressing of Sulphate of Potash to assist the ripening process.


Circa Week 36:

As onion leaves yellow and wither, lift the bulbs and lay them out on the surface of the bed with the roots facing into the sun to dry off.

Dry off under cover if the conditions become wet.

Remove all loose outer skins and remnants of foliage before storing bulbs.


Bulbs (Autumn Planting)

Overview:

Autumn planting can give a head start and may mature as much as a month earlier.

As with garlic an autumn planting will subject the bulbs to a cold period (vernalization) that will break the bulb dormancy which in turn can lead to better bulb production.

If your growing area is prone to waterlogging during the winter months you would be better advised to go for a Spring planting.


If winter temperatures have been relatively mild you may find that your plants may run to seed and flower.

Having said that, this can be useful if you want to produce seed.

In the event that plants produce flower stalks and you do not want to save seeds, then it is advisable to remove them as they tend to reduce bulb size.


Week 39:

Prepare bed prior to planting out.

Do not over feed the bed as this may encourage lush top growth during the winter months.