Mammoth Onions are best sown from Late December to Late February. A tray 30cms X 30cms will hold approx.100 seeds. Using John Innes sowing compost sow the seeds and cover with the same compost. Germination should take approximately 2 weeks at a temperature of 12deg C. Keep the seeds moist but never over water at this stage. We have found that a higher germination temperature will in many cases damage the seeds, thus giving poor results.
After germination and when the seedlings are at the ‘crook’ stage, transplant the young seedlings into stronger compost. At this stage individual pots may be used. Lift the seedlings gently as they are very easily damaged. Water only when the seedlings require it. Botrytis at this stage can be devastating so ventilate on all possible occasions but still keep the temperature at 12/13deg C.
Transfer the young plants out of the greenhouse to a cold frame during mid to Late March. This ensures the plants are well hardened off prior to planting outdoors from Late April. Planting times will vary according to the area and conditions at that time. Onions grown for exhibition will benefit from being covered by cloches at this time.
The smaller onions both red and white varieties are grown using a similar method. They can however be sown later.
The Onion Bed
Choose an open site with good drainage. For exhibition onions, trench 50cm deep. Fork up the bottom of each trench if solid. Into every four square yards of the bed work the following:-
- 4 Forkfuls of pea, bean or tomato haulms.
- 1 garden barrow of well rotted farmyard manure
- An alternative to the traditional separate fertilsers added at this time is to use a good general fertiliser such as GroMore, Blood Fish and Bone or Vitax Q4. These can be added just prior to planting and work just as well. It is important however to add the manure during the winter months.
First scatter the haulms at the bottom of the trench. Fill the trench with soil and manure in layers until all the soil is returned to the trench. It is essential that the greater proportion of the manure should be near the top of the bed, some within 10cms of the surface. This will enable the roots to come into contact with the manure during the early stages of growth. This work is best done in the late autumn or early winter when reasonably dry. The bed can then be left rough over the winter period.
In March work the top into a fine tilth adding the following to an area of 4sq metres.
- 55gms Superphosphate
- 28gms Hydrated Lime or 500gms Calcified Seaweed (This amount can vary according to soil type and whether the onions are to be grown for exhibition or kitchen, exhibition ground tends to be acidic quicker than general ground, hence the need for Lime or Calcified Seaweed. On ground for general vegetables this need only be done every third year on exhibition ground every year.)
There is no need to firm the ground unless it is very light. When conditions allow, and this will depend on several factors, plant out the now hardened off onion plants. The usual time for planting is Mid April to Mid May. An advantage is gained by the use of cloches both before and after planting. If cloches are placed over the ground prior to planting the area will warm and there is less chance of a check in growth. Care should be taken after planting not to leave covers over for too long. We have found that 3-4 weeks is sufficient.
Onions can be grown on the same ground for many years; our own onion bed is now 140 years old and has grown onions continuously for this time. However a strict health routine must be followed if you require using the same ground again and again. If any onions are suspect always remove not only the plant but also a small amount of soil from where the onion was growing. Hopefully this will remove any unwanted bacteria in the soil. The ground will also benefit from a watering of Jeyes Fluid after the crop has been harvested; this again will kill any unwanted bacteria or fungi.
If the onions are required for general kitchen use or it has not been possible to prepare the ground in the autumn we suggest the following. When it is possible work into the ground one barrowful of farmyard manure to 4sq metres. Prior to planting add a good general fertiliser. Calcified Seaweed can be added at this time. During the growing season never allow the onion bed to become dry, especially during June and July when the onions are growing well.
The planting distance for onions can vary according to the result required. For exhibition we suggest a planting distance of no nearer than 30cm X 30cm, for kitchen use this may be reduced to 20cm X 20cm.
Feeding onions on established beds should not be necessary. Feeding on ground, which has been cultivated for a number of years, can be harmful. If feeding is required it must be done in the early season. Over feeding can result in onions with thick necks and poor keeping qualities. Water on all occasions when onions are dry.
Onions will store better if harvested with a little growth left in the plant. For exhibition lift the onions 10-14 days prior to the show to give the bulb time to dry. Cut the root and top off, leaving enough top to tie down for the show bench. Remove any dead or broken skin and leave to dry in a warm dry area. A greenhouse is ideal for this purpose. Exhibitors will after 7 days be able to turn the onion top over for good presentation. For tying down we have found that rubber bands work very well, unless specified to be as grown by the show schedule. As the neck of the onion shrinks so does the rubber band, this avoids re-tying.
For kitchen use the onions, once dry, can be stored in a cool but frost free shed or garage.
The above method has been followed ever since the first William Robinson grew the large onions in the early 1900’s. It is a method often copied but that is because it works. There is no secret to the growing of large vegetables only good cultivation and patience.