Hi Alan, light can definitely make a difference, I have planted the same seeds the same way, in the same compost, at the same time and had them germinate at different times. in different light. It also makes a difference to growth. I planted four achocha seeds at the same time, in the same compost, in pots in the same place, they all germinated and grew unifomly as seedlings. As I wasn't sure what conditions they like I planted two in the more shady lean-to greenhouse and two in the more sunny outside greenhouse. The achoka vines in the more shady greenhouse are now more than twice the height with much longer sideshoots, than the ones in the sunny greenhouse. It's not down to the potting compost - they all had the same.

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