I' an experienced veg grower but haven't encountered this before. I planted some mangetout seeds undercover a couple of weeks ago (I don't plant veg seeds straight into the ground because the slugs would have them practically before they appear) and they quickly produced strong healthy plants. Since then almost all of them have developed a tough 'pinched' area at the point the stem emerges from the compost. They still look healthy but gradually keel over and die because clearly water isn't getting from the roots to the rest of the plant.
They have been kept well watered but have been subject so some fairly extreme diurnal temperature changes in the greenhouse. Could that be the cause? If not, why is this happening?
Are you sure slugs haven't nibbled around the bottom of the stems?
They are in exacly the same compost as other veg seedlings and they haven't been affected. So unless they are particularly pea-loving slugs, no - I don't think it's that. They don't look bitten.
Calendula, if the problem area is at soil level might it be cutworms or similar? Have you had cutworm problems before?
Are you saying the seedlings are not developing in the correct way ? could they not be a problem with the seeds genetics - are they F1's.
If the temperature in the greenhouse is getting very high, some seedlings will do this, particularly plants which don't need high temperatures like peas. Make sure you have all of the vents and door open during the day - an unvented greenhouse can get extermely hot (over 50C with ease), even at this time of the year.
marshmello - the seeds are proprietary F1s and develop fine. They then (as I said) develop this tough 'pinch' at the base of the stem and start to wilt.
Bob - I think that must be the problem. Even with window and door open the thermometer has been showing over 30 degrees some days and down to near freezing at night. I will sow some more but leave them outside.
I sowed mine directly into the earth early April. Packet said March - May so they must be pretty hardy.