Dorset,
United Kingdom 
Hi bit of trouble with sweet pea seedlings again! I have done all the right things, planted them in seed compost, put them in special sweet pea deep trays, BUT they have come up all tall and straggly. I believe you are supposed to nip the growing tips off when they have 3 or 4 sets of leaves, but I have done that at 2 sets to try and stop them reaching for the sky! They are placed in a North facing windowsill, so they get light but not direct sunlight. So where am I going wrong, am I giving them too much water do you think, although they are not standing in water. Any ideas gratefully received.
22 Mar, 2014
Answers
They should never be allowed to grow indoors on a windowsill. That is the worst thing you can do.
They should be grown after germination as cold as possible in a cold frame, from seeds sown in October if possible. They do not need pinching out until February at the earliest if they have grown straggly, like mine did this year due to the very mild winter.
If you cannot sow seeds in October, the next best time is early February, again, getting them into a cold frame as soon as possible after they have germinated. February sown sweet peas can be pinched out as soon as you have two pairs of leaves.
Sweet peas should normally be planted now into the garden to get the best blooms... they will stand -5'c of frost no problem. Mine are all planted.
Yours will be OK provided you get them outside. Plant them when the laterals that should form are about 3" long.
22 Mar, 2014
If you decide to do this make sure you harden them off well they will be soft and fleshy at the moment,
personally its too cold here to plant out, they would need protection.
23 Mar, 2014
When I've planted them out this early they suffer from cold winds and go brown round the edges and look very unhappy. Perhaps you could put them in a cold frame or under some sort of cover to protect them from cold winds unless your garden is sheltered? A north window isn't light enough as the others say.Have you got a lighter unheated windowsill in the house?
23 Mar, 2014
increase the amount of light that slows the length of the plant. How tall are they now? They are a tall plant.
22 Mar, 2014