## Propagation 2010

You may recall last year the 'discussion' I had with MDS chairman Ron on 'armchair vs traditional' propagation. ( propagation 2009). Well, here is an update!

I have partially moved out of the living room to an indoor greenhouse and therefore needed to make 'hotbed'. I thought there may be interest in how I got on!

OK a hot bed - a bit of wire in a bit of sand QED.

It has been slightly more complex - a few questions needed answering:

1. How hot??

2. Heat mat, propagator or cable & diy

3. How big?

4. How heavy?

5. Electric costs?

6. How much?

1. **How hot?**. Since there is no heating, and recently the temperature has been down to 0deg C, advice from the MDS gurus was to aim for about 60 deg F i.e. 15 deg C.

Another aspect was - how to minimise wasted heat? You all probably know about U values to calculate heat transmission - so a few sums! U value 20mm wood is: 6.67W/m^{2} / deg C. So for an area of 1m^{2} and a temperature difference of 15 deg C we get a transmission of 100W - quite a lot! However people suggest that expanded polystyrene sheet (EPS) is used as well. 25mm EPS has a U value of 1.6, so doing the maths we get a composite U value of 1.28, this gives a heat transmission of 19W - an 81% reduction! 2" EPS gets down to 11W.

2.** Heat mat, propagator or cable & diy?** **Propagators** tend to be quite small - £77 for one sized 30" x 16", 50W. Looking at **Heat Mats,** one sized 16" x 48" costs £90, but have the advantage that they can be rolled up when not required. **Soil Warming Cable** was flexible in dimensions for the bed, but of certain lengths - a 20' soil warming cable and thermostat costs £46 - this will cover 8 sq ft.. In the end I went for the cable and thermostat.

3. **How big**?

I have used black seed trays in previous years - these are about 14.5" by 9". So in the end I made a box 58.5" long by 19" wide - sufficient to set up 8 trays. This is approx 7.7sq ft, so should be OK with the soil warming cable. I made the box 6.75" deep - I had sufficient 1" thick boards, also it needs 1" depth for expanded polystyrene sheet, 2" + 2" sand plus depth for the tray.

4. **How heavy? ** Given I am making it myself - how strong does the box need to be? How heavy is 7.7 sq ft x 4" of sand? A couple adverts for sand gave two different values! One said for 2" you need 10 bags @ 20kg, the other said for 2" you need 2 x 25kg bags. Jungle maths provided the answer - a cubic metre of sand from a builders yard is approx 1000kg ( i.e. a ton) so 2" is 2/39.3 *1000kg for 1m^{2} = 50kg QED, so I need approx 4 25kg bags for 4", total weight = 200kg, not too heavy.

5 **Electric costs?** The 20' soil warming cable is 55W, so @ 9.45p / kWhr, the hot bed will cost me approx 10p per day!.

6. **How much?** Total cost, given I already had the wood is shown below - don't forget the polythene sheet under the sand - I used Damp Proof Membrane from the builders yard.

Wood | £0 |

Sand | £11 |

DPM | £5 |

EPS | £6 |

Cable & Thermostat | £46 |

total | £68 |

Below are some pictures from Ron showing how tubers in hot bed should flourish! I shall soon see!

On the right you can see the bed with my tubers set up. No shoots yet!