[Amsterdam]

Hi I'm a total beginner when it comes to gardening I've about 6 propagtion trays with seedlings well and truly up including runner beans tomatoes, red cabbage also sunflowers and pumpkins. Now they are in heaven at the moment as they are in my sun room with a rad under neath them. I just acquired a plastic greenhouse/tent thing I'm sure there is a correct name for it but my concern is that my little seedlings have been spoilt with heat in the sun room will i kill them by moving them outside.

I really want my sun room back! How do I make the most of my greenhouse.

[Liparis]

I would suspect you certainly have spoilt your seedlings.
Never germinate seeds above a radiator, they dry out fast and need to be watered more, this can lead to great fluctuations between wet & dry. OK, perhaps you got away with that this time but bare it in mind for future. Apart from that, the heat can be too much, excessive heat (above 15c - 17c) can make your seedlings leggy, making them very susceptable to stem-rot on transplanting.
Your problem now is to get them from warm temperatures to relatively cold temperatures, some nights are as low as 2c - 3c. What you need to do is acclimatise by using intermediate heats, but it sounds like you don't have that. Temperatures should be dropped by two or three degrees over a period of a week or two. The basic problem here is sowing too early. Your Tomatoes I'm afraid are really destined to continue sharing your sunroom, or preferably a cooler room on a windowsill until they can go into your unheated poly-greenhouse in late May. The other stuff you might need to take the chance and get it into the greenhouse as soon as you can, luckily temps, especially night temps (which are the important ones) are to rise this week, so you might get off with it.
Cabbage, sunflower, Pumpkins and runner beans would have been happy enough being sown directly in the unheated polyhouse, then acclimatised to outside later on. All the time you have saved by sowing early is going to be lost now, because at best, your plants will receive a check in growth trying to drop their temps.
Move your trays out into the garden in the morning, then move them back indoors in the late evening, before dark, do this over a period of a week to ten days, then in the evenings move them from the garden to the polyhouse, Make a frame of some sort so that at nighttime you can throw hessian sacking or old blankets over the seed trays to insulate them from the cold at night. This should go a long way to helping them.
I would suspect that by the time your cabbages are ready for the garden in late April - early May, they might not make terrific plants, you may have pushed them on far too quickly. The ideal planting stage for Brassica is five leaves maximum, any more than that for transplanting in the garden and the stems will be woody and your resulting cabbage won't heart properly.
Get another crop going in your polyhouse now. Or better still in a seedbed in the garden.
Bill.
PS. When moving your seedlings into the sunroom at night, use the coolest part, on the floor somewhere, not above your radiator.

[Amsterdam]

Hi Bill

Thank you so much for your words of wisdom going to try and gradually get them outside. thanks again.

[Yorky]

I sowed some seedlings in a lean-to greenhouse on a heat mat and the seedling stems are weak but have foliage which is too heavy for them.

I've read the post above about too much heat and perhaps that's the problem. I bought two 96 cell Quick Pot Propagation Trays and sowed a selection of seeds, some of which are tomatoes, chillies and peppers so I set the thermostat at approx. 18 degrees c.

How could I have avoided this problem as I attempted to sow tomatoes etc. in seeds trays with lids in the lean-to and they didn't germinate?

[Liparis]

As soon as the seedlings break the surface, remove the seed trays from the mat. If all haven't broken the surface, they will soon follow, even off the heat mat. Bright filtered light is what they then need. air temp no lower han 50f (10c), but no higher than 70f.
Most seeds will germinate in a temp of 60f (12c) or 50f without a heat mat. Sow in trays, cover the tray with a sheet of glass and cover the glass with newspaper. Turn the glass morning and evening to get rid of condensation and prevent it dripping back onto the compost. As soon as seedlings start germinating, remove the glass and newspaper, but put the glass back on over night, as long as the seedlings aren't touching the glass. Remove glass again first thing in the morning. Lower the air temperature a couple of derees per day, you've started your hardening-off process. As soon as the first true leaf begins to appear, prick out into thumb-pots and repot as soon as roots reach the sides of the pots. Don't leave pricking out the seedlings too long or you will "draw" (lanky with no stem strength) the seedlings.
Bill.