what will happen if you plant seeds early/late??

As a newbie, I would like to find out what will happen if you plant seeds outside the suggested planting months from the instruction on the back of the seeds packets?

Let say if I plant the tomatoes seeds in the greenhouse now, what will happen? Will they germinate/grow/even die?

Just would like to learn the basics.

Also can I use a permanent marker to mark the different coloured tulips bulbs?

any thoughts/comments will be much appreciated.

Many thanks

 

Whilst temperature and wet can affect sed growing, light is the big problem. Whilst some seeds do OK in the low light levels during the winter, a lot do not and become leggy and fail. You may get away with it sowing now, but not much later. You can of course invest in special greenhouse lights