Benefits of under soil heating and artificial lighting
6th Mar 2003
Extended Growing Season
When you have the benefit of under soil heating and artificial lighting it certainly extends your growing season considerably. This can happen to such an extent that some growers will have already planted their large onions under the protection of polythene during last month. Most of these however are growers that are chasing the record for the largest onion in the world and are therefore prepared to spend some serious money in trying to beat Mel Ednies current record of 15lbs 15½ ozs.
This means that they will have sown their seed early, some around mid October in an endeavour to optimise their chances of getting a truly giant onion. I have to say though that it' going to be an uphill class in the immediate future, but I do see the record being smashed within the next two or three years. No one has been within a pound of approaching Mel"s weight so it certainly isn't going to be easy. We must remember that when Mel won that record, not only was he a great grower he also had the best of facilities available from day coupled with changing his soil from the beds and getting some really good soil instead. On top that we had a marvellous warm spring that year coupled with a lovely hot summer with plenty of sunshine day after day. Mel lives in Anstruther on the East coast of Scotland close to the sea which gave him some cool breezes to pass through his tunnel, that very fact alone must have helped him get the maximum out of each and every onion in his bed. They were a sight for sore eyes as well, when I saw them that year, every onion was healthy with lovely straw coloured skins.
If you just want to grow the best quality onions then, with the competition being what it is today, you have to grow them under covers and yes, preferably with heating cables in the soil as well. The work must start now to prepare the second stage of your soil, During late Autumn you should have been preparing the beds properly by getting a soil analysis and based on that, incorporating plenty of organic matter. In my case the beds were really in need of some farm yard manure as none had been applied for many years. I was very fortunate therefore to get some really lovely stuff from an organic farmer, which was duly dug in layers.
Controlling the level of salts
The important aspect now is to make sure that the level of salts in the soil is as low as you can get it. In my case I have, in the past, removed the polytunnel cover and allow the Winter rains to get at the soil, cleaning it out whilst at the same time lowering it's conductivity reading. Other growers such as Jim Thompson from South Wales throw out the soil from the beds and leave it heaps to be weathered before throwing it back in again. John Branham grows his onions under a specially constructed structure that he is able to dismantle every year, again allowing the winter rains to do it's work.
Over the past few years I have reverted to keeping the beds thoroughly saturated by allowing a sprinkler to oscillate over the beds for a few hours at a time washing the salts well down into the sub soil. I like to have my heating cables buried once I have rotovated the beds, just in case the machine happens to get tangled in them. burying the cables is much easier done if you have some help with you. I use a spade pushed into the soil to a depth of nine inches or so and then moved back and forth to form a Vee. The cable is then placed at the bottom of this Vee prior to filling the trench back and connected to the power supply.
I have never used a heater in any of my polytunnels to keep the air temperature above freezing. Unless we have some really serious frost in March, I am happy that if the soil is warm, the roots will be warm and keep on actively growing the onions. From my experience, even though the air temperature may have dropped down to below zero, sometimes with the tips of the leaves being slightly scorched, it's never affected my onions. They have still grown on with none going to seed and very soon start to really romp away.
The lights in the greenhouse have now been reduced to twelve hours per day, generally supplementing the natural day length and very soon, around mid March, they will be totally switched off and the panels around the growing cabinet removed and stored away until next year.
Potting and Compost
After my disaster last year with the onions getting severely pot bound because I couldn't plant them in time, I have gone up slightly in pot size to just to accommodate for some extra growth. If you are going to be potting up any leeks or onions, the mixture that I have used for a few years now is as follows and both the leeks and the onions revel in it.
4 parts of Levington M3, 3 parts of soil and 1 part of Vermiculite, each part for me is a builders bucket and as the Vermiculite is an inert material with no food value at all this means that the mix is somewhat weaker and I therefore add 20 ml of Chempak potting base to 16 litres of compost. When mixing a small quantity of compost 1 part is a 2 litre pot.