Funny how memory works, but as Rick mentioned temperature I just remembered an additional method to prevent leggy seedlings in greenhouses that I learned about a long time ago from a commercial greenhouse owner. There is a method called negative DIF that is effective on many plants to prevent plant stretch. With this method the aim is to keep night temperatures higher than day temperatures. There is also a variant of this called "Cool Morning Pulse" in where you lower the temperature at dawn for a few hours.
For more details, I'd suggest to look here:
http://www.pthorticulture.com/... . There is also a small table that shows what plants responds best to this and a few other suggestion to control plant height as well. The problem that I can see is that it costs a lot to warm greenhouses and small greenhouses also often get warmer during the day than huge commercial ones, but keeping this in mind one at least has an additional tool to work with.
An interesting idea would of course be to implement this indoors as warming costs and excessive heat during the day would be less of a an issue. Could be interesting for those with a dedicated growing room.
