Could you use the small coir block in the bottom of planters, or would it not hold when watering ? Mrs Bouquet
It will expand if put in dry and would push the compost up and out of the top. If you are looking for a cheap alternative to the Swell gel type products, there are a variety of things you can use and the internet is full of ideas, but a lot of them won’t rot away, so that is something to consider if you want to empty the compost out onto a flower bed.
I have seen broken up Oasis flower arranging foam or cut up pieces of an old coir hanging basket liner used to good effect. They don’t rot but are fairly easy to sieve out of old compost.
It also helps a lot if you mulch the top of the container. 1 bag of small decorative bark chips would do a lot of pots.