pros: compostable, cheap.
cons: can’t separate each plant easily (have to rip box), dry out quickly or stay very wet (i.e. don’t drain well).
Bought, plastic modules in tray of 15
pros: keeps all of one seed type together nicely.
cons: made from flimsy plastic that scrunches at the bottom, degrades (but does not decompose) with light, difficult to take out one plant individually.
Tin can with bottom put back in over ring pull edge
pros: readily available, re-used and recyclable.
cons: I nearly cut myself trying to get the plant in/out, it went rapidly rusty and stained the patio wood.
Compostable food trays
pros: available with organic tomatoes etc. from (some) supermarkets, a good size for 2 – 6 seedlings, compostable.
cons: dry out quickly or stay very wet (i.e. don’t drain well).
Bought plastic trays or plastic trays from supermarket food
pros: lots of space for seedlings and a different tray can be used for each type
cons: thin plastic and therefore inevitably going to photo-decay, needs to be lined with newspaper to hold water better, trauma to plants when pricking out as roots are disturbed.
Plastic food pots (e.g. yoghurt pots) with holes punctured/ plastic pots from nursery bought plants
pros: re-used and may be recyclable, a good size for large seeds and small plants, minimal root disturbance when transplanted.
cons: plastic… see above.
Paper pots (see this post)
pros: recycled (from free newspapers), good size for one medium sized seed or two smaller ones, individual modules makes it easy to discard ungerminated seeds, can write on newspaper what the seed is, readily takes on water and drains well, can be put directly into the ground causing almost no root disturbance.
cons: mould sometimes developed on the paper which often obscured the writing of what the seed was, not good for bigger seeds (like runner beans) or seeds with deep roots because too small and not deep enough, generally need to be sat in water to ensure sufficient water.
For us, the best solution I think is a combination of paper posts for medium seeds, seed trays for small seeds and individual plastic pots for big seeds.
I would go for all natural instinctively, but thick plastic actually works quite well and lasts a long time. We have these plastic pots, so to use them is probably more sustainable than to recycle them and buy a wood/paper/other alternative when they are still serviceable. The paper pots though are great – can’t fault them.