Bottom heat for show onions

[Protea]

Desperate to try growing some whoppa onions for show this year, so have ordered 24 seedlings from Medwyns which should arive mid-Feb. In his catalogue it says on reciept they must be put in a greenhous with a min temp of 10 degrees C, plus some additional botom heat (oo-err ).

Just been testing out my contraption for creating bottom heat in my greenhouse without electricity. I've filled 2 large deep baking trays with damp sand, put them on a metal stand thing in the greenhouse and put my paraffin heater underneath (as suggested by Nick the Grief).

Had it running all last night with an additional (new) paraffin heater heating the rest of the house to increase the overnight temp. The thermometer in the sand read 20 degrees C this morning when i checked it, which seems quite warm but the sand didn't feel particularly warm to the touch - what temp should i be aiming at? I can't move the heater any closer to the trays for safety, and tere's only about 30cm between them anyway.

The lowest temp in the greenhouse overnight was 11 degrees, which is fine although it wasn't particularly cold last night, i don't know if it will stay above 10 degrees on a chilly night?

Need to get this sorted and tested before my seedlings arrive in a couple of weeks! Any suggestions appreciated!

[Geordie]

I think 20 degrees is to warm I would aim for 12 - 14. I realise you are in a greenhouse but I sit mine in an electric propergator without the lid, all be it in the house. The ones in the greenhouse just rely on a paraffin heater without bottom heat.
If you are trying to grow 'whoppa' onions for the show bench each stage is important, but some are more important than others. The issue of bottom heat against overall heat fro growth is not the most critical you will face! I find the stage of planting out / potting medium etc the most challanging.
Good luck anyway and I hope you do achieve your monster onion.

[Protea]

Thanks Geordie,

Hmmm, well the 2 heaters on at the same time increase the heat as a whole in the greenhouse sufficiently i'd say - maybe i need to move the trays up a bit to reduce the heat in the sand (the sand really didn't feel warm at all - eventhough the thermometer said 20 degrees)?

I've no electricity in the greenhouse unfortunately otherwise they would go in my propagator. Its currently in the house on a windowsill full to bursting with other seeds at the moment!

Anyone else any thoughts?

[Snadger]

You are heating the water in the sand and using the sand to store the heat. To work, the sand will need to be kept quite damp! The downside will be evaporation causing moist air,along with water vapour from the paraffin heaters, which at this time of year can lead to fungal problems!

A max/min thermometer set up now will give you an idea of likely temperature in sand until you get your plants?

[Protea]

You are heating the water in the sand and using the sand to store the heat. To work, the sand will need to be kept quite damp! The downside will be evaporation causing moist air,along with water vapour from the paraffin heaters, which at this time of year can lead to fungal problems!

A max/min thermometer set up now will give you an idea of likely temperature in sand until you get your plants?

Yup - the damp sand stores the heat. I have a max/min thermometer in the greenhouse to measure air temp, and my normal soil thermometer stuck in the sand - can you get soil max/min thermometers?

I've opened the louvre vents a bit more to aid air circulation to hopefully prevent any fungal probs! I might also douse them with chesunt compound to be on the safe side. Got my Protea seedlings in there, best germination rate i've ever had - don't want to kill them off by encouraging disease!!

Do you know what temp the sand should be?