Covering Your Onions and Tomato Seedlings
4th Apr 2002
There is still plenty of time to plant your large exhibition onions outside as the soil is rarely in good condition at this time of year and a check to their growth pattern being planted in cold soil could be fatal. However if you can construct a small framework with fairly strong timber covered over with some polythene, it's amazing how fast the plants will respond when planted up in soil that has warmed up. The other element to remember as well is that when the young plants are offered some form of protection from the elements there is no chance of the foliage been blown about and snapping off.
Constructing a Cover
If you are going to construct a cover, do make sure, even though it may only be of a temporary nature, that it is solid and unlikely to be blown down with the wind. I remember a friend of mine some years ago constructing a polythene sheet barrier all around his onion bed in order to prevent any wind damage. However he had only used some clout nails or plaster board nails to fix the polythene to the wooden uprights. The consequence was that after an evening of gales and strong winds the polythene became detached from the timber uprights and flapped around. The resulting damage was total devastation as the sheet continually scythed through the young plants. The result was that he only had a few plants left that had any leaves on them at all, a whole years work destroyed. If you are fixing polythene onto some wooden supports always use a batten on top of the polythene to really keep it firmly in position.
Even If you are growing in a polytunnel do make sure that the soil temperature is up to a minimum of 60°F prior to planting the onions. In my case I am fortunate enough to be able to have an electricity supply on hand and I can make use of soil warming cables in the raised beds. If you haven"t got any soil warming cables then you can help to increase the soil temperature by covering over the beds with some clear polythene. A lot of growers are mistaken in thinking that black polythene is the best material to cover the soil with as it heats up faster. It is true that anything black left in the sun will heat up quite quickly, however the problem is that it doesn't pass that heat through to the soil as efficiently as clear polythene does as it retains most of the heat within itself.
Soil Warming Cable
Once you are happy that the soil has risen to the minimum of 60°F you can then plant the onions, my soil is now at that temperature having had the soil warming cable switched on for the past ten days or so. I can therefore dispense with the clear polythene sheet and proceed to lay the black and white one directly on to the soil with the white side upwards. From my experience, once I have laid the polythene onto the soil the temperature within the bed will drop slightly but the cables will soon recover that loss. In polytunnels without heating cables though, that loss can be as much as five degrees hence the need to ensure that your soil temperature is initially up to 60°F as it can fall by as much as 5°F.
Growing for Showing
For those new to growing large onions for showing I shall explain the thinking behind the use of black and white polythene on the beds. It has a number of positive reasons for using it and over the years these reasons have almost certainly been a contributory factor to the much larger specimen that we now see on the show benches. The black and white polythene prevents the evaporation from the soil so there is no need for continuous watering, indeed excessive watering can definitely prevent your onions from achieving their optimum growth. Under this type of polythene last year, my onions were watered twice through the seeping hoses underneath and three times around the planting hole area.
The polythene also acts as a cooling mechanism because white repels the heat hence the need for maintaining a high temperature under the clear polythene first. This fact can be big bonus later on in the growing season as the Summer temperatures rise so the soils can get very hot restricting a good root run. The white side up also reflects more light on to the foliage and this can be extremely useful when planting out early when the light intensity of the sun is not just yet at it"s peak. One chore that I hate is weeding, to me it is just so time consuming when a lot of other work needs to be done. The polythene sheet blocks all the light from getting at the soil and the only weeds you will see is where the soil is actually exposed around the edges of the planting hole in the sheet.
For those of you who need to transplant some of your tomato seedlings, it's always a good tip to plant them as deep as you can initially. I always pot up into a three inch pot using Gro Bag compost and I just put in the bottom of the pot an inch or so of compost. The seedling is then carefully removed from the tray by holding it by the seedling leaf and suspending it in the pot at a position just above the rim. You can then add the compost all round the seedling gently tapping the pot on to the bench to settle the compost. The seedling leaf of the tomato plant should now be just above the level of the compost. This means that the stem of the seedling plant will now throw out adventitious roots, or extra roots from the seemingly hairy like stem which will help make your seedling a much stronger and sturdier plant when it comes to planting time.