Why do my brassica seedlings develop long stalks as they mature?

Hi Gerry, usually they are root bound when they become stalky. They have possibly been kept in their punnet for too long before planting out.

When planted they will quickly go to seed. Alternatively it could be the variety, are they right for the season they are being sown/planted or the time of year. Plant them out sooner and choose seedlings that are fresh looking.