My sweet pea's do the same thing every year and I think I must be the worst gardener alive....but somehow they send a bunch of fresh new shoots out and the plants begin to look wonderful...so don't worry they should come out fine.
Jan

Thanks so much for your reply. I was looking at them again this morning - and worrying. They seem even yellower, not growing much etc. A couple beds over I have shell and snap peas coming up that are dark green and look very healthy.

I've read over and over that sweet peas like it cool and that's what it has been, but maybe when the warmer weather strikes in June, they'll take off. Hope so!

My sweet peas are doing the same here in Zone 4. Interestingly I am still holding some of the seedlings in an unheated greenhouse and they are looking much better. The bunch I planted out and the bunch still in containers in the greenhouse were from the same seed stock and sown the same day. The outdoor planted seedlings(May2nd) are pale and stunted in comparison. All plants(indoors & out) have been pinched off about the same time and the ones out are not branching nearly as fast nor as much as the ones still in containers.
Perhaps the cool spring and fluctuating temps are the reason. It's certainly not too much water as we've had very little precipitation. I'm not sure myself and was thinking to side dress with some manure or some thing. I'm amazed at the growth of the ones in containers but afraid to transplant them out for fear of seeing poor results.
Good luck with yours & I sure hope they bounce back for you. BTW your plants look far more rigorous than mine right now. Keep us posted.

kanuk, my seedlings are not much better. They may have grown some but haven't been thriving and still look yellowish. I'm wondering now if I didn't harden them off enough. Maybe this is sunburn, or something of the sort. This is the first year I've grown sweet peas indoors from seed. I've done it a few years with vegetable seedlings, and I was just looking up information about how long to harden those off when I thought this might be the issue with the sweet peas.

Did you harden yours off before planting out? Have you done this before with different results?

I was so excited about these sweet peas getting a jump start, but it doesn't look like that's going to happen. Would love to figure out why.

I have a weird thing going on with my Old Spice. I have grown these sweet peas in the same bed for years now with great success, this year, I'm not so lucky. Some of the plants have yellowed at the base and the leaves and the flowers have taken on a mosaic yellow design. The flowers are striped and pinched and very small. This looks like something terrible and I am pulling all the affected plants and burning them. This means, unfortunately, that I will not be able to use this years seed for next year.

As to the planting and hardening off, I have found that by far the best method of growing these is to sow the seed in August/September outside, where they are to flower and let them overwinter. The resulting seedlings are usually strong well branched and ready to flower early in the season. That being said, I am in a zone 7, but it freezes quite regularly here in the winter and the seedlings couldnt care less! Best of luck. Kat

I too had this problem and thought it was: To much water, my zone, ext but when my bean plants started doing the same thing. WOW

I know what is was. You need to get some EPSON Salt this adds Magnesium and Sulfer and will fix the problem. Add in water or put strait around your plants: one tabespoon per foot plant height every two weeks. If they are yellow now! put 1/2 cup per gal of water and give to your plants.

Here is a link that might be useful: pioneerliving