Large Concrete container drainage hole clogs

I have a number of these, typically with one hole (3/4"), which clog up after a couple years. I use non-metal window screening on the bottom of these pots to minimize soil loss. The soilless mix is amended with extra pumice & drains very well. I do not place anything extra at the bottom of the pot (fine gravel, pot shards, packing peanuts, etc.) in order to avoid a perched water table. The only thing I can imagine might contribute to this is worm castings. I don't add worms to the pots, but do find them in there occasionally. Hoping someone might have some thoughts on this, & possible solutions/ideas. Thanks in advance!

Katye---
I can think of only 2 solutions to your problem.

1--as hard as it may be--try to elevate the concrete container so it does not
sit flat on the ground or floor...wherever you are keeping it. Maybe some bricks
or chunky rocks to set the pots on top of. This will keep water flowing through the hole. If you keep it flat on the ground/floor--the soil will start clogging up the hole.
Some soil loss is inevitable through drainage holes but if the pot is elevated,
water may have a chance to drain through.

2--Worms could certainly enter the pot through the drainage hole.
Another reason to elevate the pot a few inches off the ground.

Nothing brilliant here--but these ideas may help Gita

Thanks much for your response, Gita. The pots all sit on ~1" rock bed. Each pot weighs somewhere in the neighborhood of 200+ pounds, so due to the weight, I'm not certain what else they could be set on that would support the weight. I suppose I will need to empty & then refill each one prior to Winter. I have no other ideas!

A few things to add. Clear the gravel directly under the drain hole and place 3 larger flat rocks to ensure the gravel does not touch the underside of the hole. Go to the thrift store and buy an old sieve. Turn the sieve upside down and cover it with a piece of landscape fabric on the outside so that the fabric extends beyond the sieve. Cover the hole with the sieve upside down and make sure the soil is placed ontop of the edges of the landscape fabric. This should buy you a few more years than you have had before redoing it.

growin--

I also had the same idea about a sieve.
I guess great minds think alike.....:o) Gita

One of these containers is quite small at the bottom so it wouldn't accomodate a sieve. I suppose I could craft one from wire & mesh. Worth a try. Also, these containers sit on beds of rocks; the rocks generally exceed 1" diameter -- think 50 cent pieces.
In any case I believe the clogging is a result of fine particle migration to the drainage hole. That's why the worms were implicated.

The other thing you could try is to make a cup-shaped mesh, spread perlite over it, cover it with landscape fabric. The fabric prevents the silt/fine particles from clogging and the perlite will allow free flow of water. Air is a wonderful way of ensuring this doesn't happen in the future so making sure that the underside is not touching rocks is good. We all forget that oxygen is good for roots too.