Onions - Soil Warming Cables and Polytunnels

6th Apr 2000

Soil Warming Cables

The soil warming cables in the onion beds were not switched on this year until about ten days ago and the soil temperature underneath the black and white polythene is now above 55F so the onions can be planted safely this coming week. To be honest, I am not in a desperate hurry to plant them too early as they are still growing away well in their pots and are now over three feet tall. I'm not too concerned about planting early because, if I do, they inevitably mature too early; onions are about ready to harvest when they are about 21 inch in circumference. Last year they were all ready around the third week in July which meant that I was struggling to stage quality onions from late August onwards.

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Polytunnels or other protective structures

Those of you who have no polytunnel or other protective structure to help the onions along during the initial cold period are much better off potting them on until the weather has really settled down. Anglesey is an island which sticks out into the Irish sea and we are very prone to strong winds. During my early growing days, when I had no protection at all under which to grow them, I used to be tempted to plant them outside as soon as we had a couple of nice warm days this month. Of course, what happened next was that the winds returned with a vengeance, flapping the leaves about until they were severely set back and took weeks to re-grow. If you are going to plant outside, the most important thing is to erect a wind barrier, using some strong posts to fence around the growing area and wrapping some polythene or even the new Enviro mesh around them to cut out the wind damage. If you do erect this sort of system, be very careful how you attach your polythene or Enviro mesh to the posts; it must be really secure. There is only one other thing that can be worse than the wind damage itself and that is a sheet of polythene becoming loose at one end of a pole and flapping around the onion bed, flattening everything within its turning circle.

Taking out of the greenouse

The onions were brought out of the greenhouse about ten days ago at the same time as I switched on the soil heating and were laid out on top of the polythene so that they could cool down a little whilst at the same time getting used to the normal day length without any artificial lights. The evening before planting, every pot is given a good watering so that the rootball within the pot is fully charged. I had previously prepared the ground well with my new Honda rotovator, the Eva flow pipes were laid, pots the same size as those in which the onions are growing in were also positioned in a straight line at 18" (450mm) apart and the whole growing area covered over with black and white polythene. All I need to do now is to neatly cut through the polythene where each pot is located, making a diagonal cut from just beyond the corner of the pot and past the other corner. The resulting 4 flaps can then be tucked underneath the sheet and the pots carefully withdrawn.

Prior to Planting

Prior to actually planting the onions, I scatter a small amount of slug pellets underneath the polythene as well as some Clorophos around the planting hole. I then hold the onion plant upside down and carefully tap the edge of the pot on the side of the raised bed to release the root ball. There should be plenty of white roots really wrapped around the soil and working their way up the side of the pot if they have really grown on well. Take time to study the plants" foliage in order to ensure that you position every onion in the planting hole in such a way that the foliage is all facing along the length of the row and not across it. This is particularly useful as the plants develop otherwise, as you walk along the path between the rows, there is every chance of damaging the protruding foliage.

I press the root ball lightly into the hole and then compress the surrounding soil around the root ball. I remove the polythene flaps from underneath the sheet and position them around the onion. This will help conserve moisture around the establishing root ball during the initial period as strong sunshine during April can dry out the surrounding soil quickly. Finally, I water around each plant with a pint of water per onion to make sure that the root ball and the surrounding soil become as one, allowing the roots to really get stuck into their new home.

Growing in Pots

When growing in the pots, the onions were encased within a system of plastic support clips and the whole system is planted with the onions which means that the foliage is still supported during their establishing period. Whether you continue to support them is very much dependant on how much time you have to spend on them. Some growers have special large circular rings attached to canes through which the foliage grows and is supported. I prefer to remove the canes as soon as the plant is really seen to be growing away, Take great care when doing this as the leaves may well want to bend down on to the soil. Take the weight of the leaf with your hands until the weight is taken by the soil to prevent the leaf from cracking or splitting away.

Clips

The important thing with onions is to make sure that the main stalk from which the foliage grows is kept perfectly erect so that any harvested onions for exhibition will all have good Shape, or more particularly, good Form. If you decide to remove all the clips, it pays to leave one cane fairly close to the onion and use a single plastic support clip to form a complete circle around the main stem, close to the top where the foliage emerges as this will prevent the plant from leaning over and growing out of shape.


The soil warming cables in the onion beds were not switched on this year until about ten days ago and the soil temperature underneath the black and white polythene is now above 55F so the onions can be planted safely this coming week.
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