Artificial Lighting, Soil Warming Cable and Watering System
3rd Mar 1999
It always amazes me at this time of year how plants seem to respond to the ever increasing day length with some onion and leek plants, that only a few weeks ago, seemed to be very small seedlings now developing into tall strong plants. Greenhouse space is always at a premium for, but at this time of year, it's probably at it"s worst as all the onions and leeks are now in their biggest pots and taking up a lot of bench space. However be very careful about moving them outdoors just yet as we can still have some hard frost that could destroy all the hard work that you have put into them.
The artificial lights that the onions have been growing under for sixteen hours every day, right from the day they popped their heads through the compost, will now be reduced to 12 hours until planting out time. Last year they were planted on the 27th of March straight from my greenhouse into the polytunnel soil. I must however make it quite clear that I was only able to do this because the soil had already been warmed up using a soil warming cable.
Soil Warming Cable
This will be my task this coming weekend, to bury the soil warming cable into the beds at a depth of around 7 to 8 inches and approximately 9 inches apart. It's important for the cable to be at this depth in order to be just below the depth of the plant pot so that the roots will benefit from the rising warmth. Laying the soil cable is not an easy task and an extra pair of hands at this point is very helpful as you need to form a Vee in the soil with a spade, whilst at the same time, laying down the cable before the soil falls back in.
A good tip before you start laying the cable is to completely unwind it and lay it over the bed making sure that no part of the cable crosses itself. Switch the cable on and leave it warm up for about five minutes, this will make it very pliable and a lot easier to handle in the soil. The cables that I have used in the past came from LBS polythene, and to be effective, they need to be laid as follows. A 12 metre length of cable will heat an area of between 15 and 24 square feet with a wattage of 150 and costs £31.45. A 24 metre length of cable will heat an area of between 30 and 48 square feet with a wattage of 300 and costs £42.51. A 48 metre length will heat an area of between 50 and 100 square feet with a wattage of 600 and costs £ 89.31. Of course these cables have no thermostat on them so the cost is increased considerably if you want to have a soil thermostat incorporated.
I have found that switching the cable on about four days prior to planting will warm up the soil sufficiently to allow me to plant the onions towards the end of this month. Prior to heating the soil the beds will obviously have been thoroughly prepared with the nutrients having been applied in accordance with the soil analyses report. Empty pots, identical to the ones that the onions are growing in, will then be buried to the rim in straight rows at eighteen inches apart and the same distance between the rows. Every pot is then filled to the brim with a maximum dilution rate of Armillatox which is allowed to soak slowly through the holes in the pots and into the surrounding soil.
This year I'm reverting to my old watering system underneath the black and white polythene which is the Eva flow system but using the misty spray type in preference to the side spray which is much more concentrated. The mist spray gives you soft irrigation from apertures made in four lines each facing in different directions which when covered over with the black and white polythene sheet should moisten the whole bed with less risk of the apertures being blocked by the actual polythene sheet. This pipe is green in colour and will give you a water volume of 0.2 litres per minute per mtr at a water pressure of 4 pounds per square inch. If you increase the water pressure to 10 PSI you will have a water volume of 0.6 ltr per min per metre. The EVAFLOW watering system is also available from LBS Polythene at £42.75 for 100metre roll or 48 pence per metre for smaller quantities.
The other beneficial advantage of using the above system is that inside the length of the seeping hose is a specially built in filter system that ensures that the fine pores will not clog up with soil or dirt. Once the polythene is in place on top of the pipes the heating cables can then be switched on and the polythene will actually help to conserve the heat inside the soil. Check the soil temperature on a regular basis using a soil thermometer and when at 60°F, you can plant the onions which I shall cover later on. I have found over the years that if the root system of the plants are kept warm all the time then they will put up with a really low temperature above ground, over the last few years, I haven't had any that have gone to seed on me as a result of a check to their growth.
LBS are based at Standroyd Mill, Cottontree, Nr Colne, Lancashire BB8 7BW or telephone them on 01282 873 311