[Spr0ut]

I know enough not to overwater or take them outdoors too soon, but I still get this yellowing off of leaves and eventually they keel over.

I even left one indoors in a south facing window to see if I was taking them out into the cold greenhouse too soon, but even that one is going the same way.

Maybe they run out of nutrients in the compost quickly? Not sure what I'm doing wrong.

[BabbyAnn]

Cucurbits like sustained warmth and good strong sunlight from the moment they germinate.  Looking at the photo, it is possible the nutrients in the compost are lacking rather than run out, but I suspect the plant has also got checked (maybe a cold evening?) at which point it started to struggle.  I often find they don't recover too well after so perhaps sow some more now and take advantage of the milder conditions

[JayG]

Yup - usual 'suspects' would be too much stress caused by temperatures too low or too high (or a see-saw combination of both,) overwatering, or even underwatering (less likely because you'd probably know if you'd let them dry out.)

If the nutrients were low the plants would more likely be pale and spindly rather than dying at the first true leaf stage, although they would eventually give up if they weren't fed.

Herbicide contamination of the compost is possible, although if you have grown other plants successfully in it that's pretty unlikely, especially using bought compost.

[Spr0ut]

Thank you - I think it's a re-sow and maybe start quite a few things a bit later in the year next year.