I use the inners out of toilet rolls for slightly bigger things like beans, not sure if they would be big enough for your tree seedlings though....
Stand up a bunch of toilet roll inners in a seed tray, fill them with
compost, plant seed, top up, water. You can plant thw whole thing, but as someone else mentioned, it is best to take the top of the 'pot' off before planting - it can act as a wick to draw moisture up and out of the soil