Midlands Dahlia Society Tuber Propagation Hot Bed

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/m² / deg C. So for an area of 1m² 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 1m² = 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.