Onions - Quantity, Quality and The Dreaded Thrips
24th May 2001
The large onions for the over 250 gram classes as well as those for the under 250 grams are all being grown in one polytunnel which has three beds in it. The beds are all two feet high and constructed from concrete blocks, though ideal for working on at this level, the blocks do have a tendency to soak up moisture from the bed so, initially, I do have to be very careful regarding the watering to make sure that the beds are always uniformly moist.
The quantity of onions for the over eight ounce class has been reduced so that in one bed, only just over 18 inches in width, I have one row along the centre whilst the centre bed which is slightly wider has two rows in domino fashion. I have a total of thirty onions this time and my thinking is that if I can't get a decent set of three onions for my collection from thirty, then the probability is that I wouldn"t have three from three hundred. At the end of the day it's the care and attention that you give your plants that gets the red cards not just sheer quantity.
Derek Raw proved that over and over again by winning the large onion class at Harrogate on many occasions with superb quality onions. When I visited Derricks garden two years ago I was quite amazed to see that he only grew thirty onions in a relatively small area and each onion was given plenty of tender loving care. Indeed he could easily have had five sets of six onions from the bed, there wasn"t a single bad onion among them.
My way of keeping the beds moist is by using the Eva flow seeping hoses that are positioned on top of the soil and underneath the black and white polythene sheet that covers the soil. I am often asked how often and for how long should these pipes be switched on. This is a very difficult question to answer in simple terms as no season is the same and over the past two years the Summers have been particularly dull and cold. Last year I'm sure that my hoses were only switched on three times during the early part of the season and after that just the occasional watering around each plant where the surface of the soil is exposed to the light and tends to dry out.
If you are in doubt regarding the watering, pull up an area of polythene and actually feel the soil, squeeze it in you hand to see if any moisture comes out of it. If there is then it is too wet, if it breaks down in your hand into fine particles it's too dry. Ideally one should aim for the squeezed soil to remain in a ball when you open your hand and no drops of water seeping through. After the end of June I tend not water at all as the onions roots are well down into the bed and no evaporation is taking place because of the polythene over the soil.
Now is the time to keep a really sharp lookout for the dreaded Thrips which can totally destroy your chances of showing any decent onions as they just suck away at the plants strength and weaken it. It is no use just walking casually along the path hoping to see them on the foliage, if you do that then the next thing you will notice is that your plant start to take on greyish or silvery appearance. This is a sure sign that you have missed the boat and the silvering (sometimes thought by some to be a sign of disease) is the result of the tiny pest munching away at the young foliage. You must have a regular proper look, use a magnifying glass if you have to, right into the heart of the growing onion foliage. Carefully spread them apart and if on your crop, you'll see them crawling around in the centre of the plant.
Control is not easy once the pest has taken hold so be vigilant from the start and spray with a range of pesticides including Polysect or Tumble bug. At Bangor University where my leeks and onions are grown for the Chelsea flower show, pest and disease control is undertaken by the University staff as there is such a diversity of crops being grown there. I have never seen any thrip damage on either my leeks or onions as they now control Thrips by using Nemolt and Dynamec which are commercial products and unavailable for amateur use.
Over the past two seasons I have had some great results with cauliflowers that have been grown in the polytunnel where the large onions grow. The idea is to plant out the caulis after the onions have been removed, one cauliflower plant per onion station will therefore give me some thirty plants. As the onions are usually harvested from the middle of July these plants have to be regularly potted up, sometimes into 6 inch pots until the station to receive them is ready.
I shall sow four kinds this year with the majority of them being planted out in a field that I use which is actually over a thousand feet above sea level. The varieties are all from my own seed catalogue; Beauty, Mexico (a new introduction fore this year) and Aviso, again a new introduction and one that did so well for me when I grew it for Chelsea in pots; the fourth will be Memphis and timed for late August early September. Broadcast sow the seed on some fine compost and cover over to twice their size with fine vermiculite and keep uniformly moist until germinated. Once through I will then prick them out into plantpak 40s from which they will be further potted up into 3 inch pots.