What is the purpose of the liner?

You could. But the cotton fabric will erode and eventually fall apart. Some people use burlap to line containers. It would probably last longer and it's natural and usually available by the yard at fabric or craft stores.

You could also use weed barrier fabric. But it may be constructed of something that is not as natural. Also you might have to buy a huge roll of it just to get the piece you need.

I'm with floral - I'd wonder for the need of any kind of liner.

I'm guessing that the soil might wash out through the drainage gaps? But you're saying the gaps are narrow enough for keeping the soil in?

They are narrow enough that they wouldn't bother me!! But then I often don't bother the even larger drainage holes in most containers either :-)

If you have concerns, just get some wire window screening. I keep some on hand for just that purpose - covering drainage holes. Except I use the plastic version.....I have no qualms about using plastic with any containers.

If you prefer something that IS biodegradable, use burlap or cheesecloth.

Perfect...burlap or cheesecloth. But I'll try it without liner.

This is my first planter, so I don't know how soil behaves. I mean I'm guessing the finer particles on the bottom will get washed out in the beginning? And then after a couple waterings, the drained water will be clear? Is that right?

It’s a beautifully built trough. My main concern would be the wood itself. Is it treated in some way against rotting? Especially the end grain on those corner braces. I have many containers in the garden and window boxes on my house but none have liners or anything over the drainage holes. It’s never been a problem.

Thanks! I built it myself using untreated cedar.

Thanks! I appreciate it.