What sort of seeds are they?
Well done, Cler and welcome to the forum.
Sweet peas are hardy but will need hardening off gradually if you've started them in the warmth of indoors - there are a lot of threads on here giving sound advice about growing sweet peas - use the search facility in the top right corner I think you could treat your violas the same way.
Keep your tomatoes indoors! They can't go out for ages yet - keep potting them on as they grow and keep them in as much light as you can to avoid them growing too leggy in search of light. If they do get leggy you can plant them a bit deeper when you pot them on as the stem will grow some roots. There are also lots of threads about tomatoes - again use the search facility to find what you need
I must admit I've not started any of my seeds off yet - I'm going to start my toms this weekend as I only have little cold growhouses too.
Your lettuces might cope with being moved outside if you harden them off gradually, putting them outside in the sun for a little while then back indoors, increasing the time they spend outside each day until you think they can cope with it - it does depend on the variety - some are hardier than others.
This is a very unusual spring, and you might have to start again with some of your seeds if you run out of room for them indoors. Such are the joys of gardening - welcome to the club
I'm sure there'll be lots of advice and encouragement from others later on today ....
I have viola's marigolds tomato's and lettuce in mine, they are growing slowly but surely. I would suggest you try some and see how they do, split them between the greenhouse and the house so if you lose some you won't lose them all.
I just found a handy tip to paint milk cartons black and fill with water then they will help store some of the days heat and release it at night. You could use a thermometer to measure the maximium / minimum temperatures in your green house if its keeping above freezing it should be livable for your plants.
I have some very small tomatoes. I'm putting them outside in the greenhouse during the day. to give them a nice dose of sunlight. But I'm bringing them into the house during the night.
The temperature inside my cold greenhouse last night was -2.5. Even with a fleece covering that's too cold for little tomatoes.
Hi Cler,please don't put them out as the GH you mention are plastic and it is VERY cold out there. It is 1c in the SE but feels like -5c! I did what you are planning to do and lost plants a couple of years ago.
SO, please hold fire and wait a couple of weeks at least.As you sound like a new, keen gardener it would be aloss if you loose heart. Good luck
You could stand your greenhouse on fleece or bubble wrap and put some more bubble wrap around it. I am about to put up a similar sort of greenhouse and plan to do that. Blackest's tip about the milk cartons may well work better in a small g'house like that than in a larger one, providing they get some sun in the day, I'm thinking. I'm planning to be cautious about what goes in it, too.
The calendar says it's nearly April but it's still winter as far as plants are concerned. Temperatures well below freezing every night and not much more in the day. Don't do it, listen to Dove and jatnikapyar
Violas are hardy, they are used for winter planting. Sweet peas and lettuces are hardy, but if they've been used to being warm then get them used to outside slowly, just day time to start with.