The suggested advice is to gather pine needles in a tin lid, set fire to them and then mix this in with your compost in the seed tray. Alternatively line a seed tray with aluminium foil part fill with compost and sow the seeds. Add a layer of pine needles ad set fire to then water the ashes in. I have not tried either of these methods myself and wonder how you continue to water with aluminium in the seed tray.