When to pick out seedlings from communal tray

Dear Toronto Master Gardeners, I have started several flower species indoors under grow lights. The tiny Heliotrope ‘Marine Lemoine strain’ seeds were sown in a flat tray. I searched online but cannot find any info for when to pick them out and transplant into individual pots. They were started March 11, ~3 ½ weeks ago. … Continue reading When to pick out seedlings from communal tray