Propagating with heat

When you sow seeds and put them in a propagator with heat, how long do they need the heat for? I imagine that you at least wait until they need pricking out, but then do they still need heat, or is a frost-free greenhouse sufficient.
I am thinking of half-hardy annuals ie. Morning Glory, Busy Lizzy, Lobelia etc...

 

to keep them growing . as they really need, you need to heat the greenhouse

 

you must keep them heated, at least 13c(55f) to keep them growing I presume your propagtor is set at around 21c give or take so the seedlings of half hardy annuals would not take kindly to a such a drop in temperature of an unheated greenhouse . This is rather early for ipoemea and impatiens (busy Lizzy)so they could become stunted or even rot off with really cool temperatures and cold damp compost

 

Thanks for the replies.
I have just put up a wallgarden (a small 6ft x 2.5ft greenhouse and so this is my first experience of greenhouse gardening).
I haven't sown any seeds yet, I was just wondering. I did though, buy some seedlings from a Garden Centre and some Geranium plugs, which I have put into the greenhouse.
I bought an electric heater, but when I thought about the running costs, it seems it might be cheaper to not bother with seeds!
I have set the heater to frost-guard, but now I fear I might have been too hasty in my eagerness!

 

geraniums need to be warmer than frost free, they will do ok in the house tho if you can keep em warm and well lit for a month

 

I suppose the question is; How economic is it to sow seeds when only a tray each of a few kinds are wanted? It seems to me that it could easily cost more than a hundred pounds to heat a greenhouse from Feb until Apr, therefore it would be cheaper to buy the grown plants later from a nursery...

 

yes with modern methods of production no one can really make it economic, although i don t think it would cost hundreds of pounds to heat, but you can potter and experiment with different varieties

 

I'm not sure how you have worked that out! I have a 10ft x 6ft Greenhouse with a 3KW heater on in there keeping a min temp of 10c, I cover the seedlings up with fleece overnight just as an added precaution, its cost me less than 20p per day over the last week although it has been mild but I can tell you this time next week how much it will be in colder weater as we seem to be in the grip of a cold snap again, don't forget get the bubble wrap it makes a lot of difference

Helen

 

Hopefully my calculations are wrong! (as usual) but I based it on my electricity bill. This shows I'm paying 6.96p per unit. I thought a unit was 1 KwHour, therefore 3Kw = 20.88p per hour.
Multiplying this (on a worse case of 12 hours per day) for 84 days comes to �£210!

I know the thermostat will switch in and out, but I just wanted a worse case cost.

Please tell me I'm way off! and how I should work it out. My wife is not too imperessed with my calculations, when all she wants is a couple of tomato plants in there!

 

Yes you have worked it out correctly but the heater is not on 24hrs a day as you say, I suppose you could look at it as the worse case senario, I think you need to sit down & work out exactly what you want in your garden at the end of it because if its only a few bedding & tomato plants its not worth it, you would be better off buying them in during May & using your greenhouse to harden them off, why not sow a few hardy annuals during March they don't need heat.

 

Jerry
It`s fairly easy to figure what heat you`ll need.
I work in sq ft and btu`s because i`m old

Surface Area x Temp difference / insulation value (A x Td / R)
Example:
A = 350sq ft (total surface area, excl floor)
Td = 18F (32F outside,assume 50F required inside)
R = 0.87 (single 3mm glass)
= 7241 btu/hr
1kw = 3413 btu, therefore 2.12kw/hr of heat needed to hold 50F temp inside greenhouse.

Note that windchill will drop the outside temp by a lot and using bubblewrap will add around 0.6 to the R-value.
This would equate to 1.25kw needed instead of 2.12kw as in the above example.

Reduce the surface area and/or increase the insulation value to cut heating costs

 

Thanks for the help all.
I have moved the seedlings and most of the Geraniums into my study and placed them on the patio door cill.
I have also just sowed a packet of sweet peas which I have put into the propagator on a low setting. The remainder of the Geraniums have gone in there with them.
I also potted up some Dahlia tubers which have gone into the greenhouse. The heater is set to frost-free.
As for sowing more tender stuff, I think I will wait till it warms up a bit.

There's certainly more to this than I imagined to begin with!

 

Think its expensive for you to heat your greenhouse???...what about that little greenhouse in cornwall.....Eden project!!!

 

The Eden project is very energy efficient,it uses a cliff face as thermal mass

Being a dome the larger it is... the cheaper it is to heat and cool unlike square glass boxes

 

Does it also have a micro-climate being in that quarry? I love your hexagonal shape. I fancied that sort of shape, but having to buy rather than make, it was out of my price range. :(