Microgreens

Seedlings topple at the soil line, with a cobwebby growth around the stems

Damping off. Dense sowing, still air and a permanently wet surface are exactly the conditions the fungi behind it need.

Fix: Sow thinner than feels right, water from below by standing the tray in a shallow dish, and get some air moving across the surface. Wash trays out between crops, since the fungi carry over.

Long pale stems that fall flat before the leaves open properly

Too long under cover, or not enough light once uncovered.

Fix: Take the cover off as soon as the shoots start lifting it, and put the tray straight into the brightest spot you have or under a light. A day of delay at that stage shows up across the whole tray.

Seed hulls still stuck on the leaves at harvest

The seedlings came up against no resistance, so the leaves pulled free before the coat had softened and split.

Fix: Weight the tray during the blackout period, with a second tray on top, so the shoots have something to push against. Misting at uncovering helps the rest let go, and a rinse after cutting takes off whatever is left.