High Tec Propagator

[jeremyf]

I was thinking myself very clever having built a thermostatically controlled/ heated propagator with an auto vent to live in the greenhouse. Its about 3 ft x 2 ft, fully insulated etc

It sort of works but the soil warming cable (37watt) struggles at cold night temps in the greenhouse and it drops to about 10 degrees but in the day it can get to over 30 and thats with shading. BUT, my tom seedlings are colapsing - sort of going soft just below the leaves. Any ideas - too hot/ too cold/ not enough ventilation?
Was thinking of a thermostatically controlled extractor fan but would be hard to insulate at night?

jeremy

[jeremyf]

[betula]

Just a thought have you checked for Pests and Diseases.?

Difficult to say if to hot to cold.

I always throw a net over the house in very hot weather for shade and lots of ventilation

Your night time temp could be the problem.If they were my plants I would be looking at increasing the night time temp.

[Barnowl]

You may well have done this already...

I lined mine with silver foil to spread out the heat, which I then covered with about an inch or so of sand. It acts as a heat sink and helps maintain temperature overnight. Suspect one should cover the sand with black plastic to be thorough.

Also you should ensure there is a good insulating material under the cable such as expanded polystyrene or that roof insulation stuff that has silver foil on one side.

[jeremyf]

I raided some skips for most of the stuff I used bar heated cable and auto opener- I found some of that building insulation 30mm foam with silver backing and put that around the sides. The cable sits on some foam/underlay for laminate floors. I could fill it with sand but makes it very heavy to move around. The top is glass with some bubble wrapped glued onto the glass. The heat can only really escape through any gaps where the lid sits. Is 10 degrees overnight really going to cause any stress to the plants?

[Barnowl]

I think if it's only dropping down to 10 deg it's more likely to be a ventilation issue.

If you don't have vents in the lid,  I would uncover the seedlings during the day even if it is a bit cool.

Standing by to be corrected.....

[PAULW]

You should only have the lid on until the seeds "hatch", once the seedlings appear take it off if you dont the seedling become leggy because they are looking for light even though the cover is clear plastic, they are quite happy with just bottom heat and if it is forecast to be frosty you could always cover with a fleece.

[jeremyf]

So as soon as the seeds have hatched for a few days, take them out into an unheated greenhouse with a bit of bottom heat at night? I have tried to avoid using just bottom heat as most of the heat just rises and is wasted? Would they need shading from direct sun. The problem with covering with fleece is I'm never there - I wanted to make an automatic growing thing - is there a total solution?

I find this growing thing really hard!!

[Barnowl]

I think I'd first take the lid off day for a few days before moving them out. I don't think chillies and tomatoes seedlings would have a problem with sun provided the soil hasn't dried out.  In fact I think they'd appreciate it. Mine are about 2" high on a south facing windowsill in direct sunlight .

[tim]

Jeremy -

1. No - there is no affordable foolproof system to my knowledge.
2. An auto-vent will never compensate for sunshine heat. A fan would be fun but a bit OTT?
3. Auto shading might be better?
4. A vent needs a through supply of air from somewhere.
5. Problem comes when you have seedlings AND ungerminated seeds together. The seeds are far more tolerant. Seedlings should always be ventilated during the day, especially if the 'bed' is kept moist. I do not always take seedlings out if they are things like Peppers.
6. Even with 50W, you are lucky to maintain 15-20C propagating temperatures.
7. Plants collapsing? Might it be that they are drying out in the heat? I have capillary matting on the base which I keep moist. Yes, I know, I'm always here!
8. What is your greenhouse 'bottom heat'? Can you not make a 'cold frame' to contain the warmth? ( http://www.allotments4all.co.uk/smf/index.php/topic,40817.0.html )

[Robert_Brenchley]

30 may be too hot, especially with such a wide diurnal variation. is there any way you can rig a thermostat to cut it off at, say, 20? Don't make it too complex.

[tim]

I read it as overheating in the sunshine - which it will do with or without heat?

[Robert_Brenchley]

It may be a question of finding the right balance between heating and ventilation.

[tim]

A challenge, even living on the spot!

[jeremyf]

Thanks for your replies.
I think I may have a change of plan then - Germinate in the house (17 to 21 degrees) and as soon as the seedlings appear, put them in the heated propagator in the greenhouse - keep the min temp 13ish and get as much sun as possible but keeping the temp below 30 - I'll fit a thermostatically controlled fan to blow through the autovent accross the seedlings to keep it below 30 on hot days? There is an autovent on the greenhouse roof and the propagator lid BTW.
I'll sow seeds in pots so can move just the ones that have germinated.
After a few weeks when the plants are well established, put them on the greenhouse bench with 50w of bottom heat during the night. Capillary matting above the heating cable? (may put some gravel to steady the pots above the cable?) or will this stop the capillary mat working?

Is this going to be ok?