I would like to know the very best potting mix for planting the iris in pots for outside not in a greenhouse. Most all potting soils out of the bag are so dense that the roots completely rot through the winter months.
I can get most anything for creating a custom mix of soil. I have come close with a mix of 2 gallon pots of steer manure 2 gallon pots of Perlite and 2 gallon pots of bark dust. But they do not grow to potential in the pot. When the roots go through the base of the pot into the soil they increase in height two fold of the ones that have not rooted into the soil. Any ideas. I am using three gallon black plastic nursery pots.

If steer manure is not aged very much or at all, it will burn the roots/inhibit root growth. Even if it is aged it is heavy. I would make it only 10-15% of my mix. What kind of bark dust are you using? Hopefully it is pine or fir. If it is fresh from deciduous trees it can also burn the roots. If you are using pine or fir bark it is very acidic, you should add dolomite lime, gypsum or something to bring up the ph/balance the ph. I have not done your math, but you want the perlite to be anywhere from 40-50% of the mix. It needs lots of drainage, because it will be out in the rain a lot.

Your mix is very similar to my grow mix, which is perlite, ground bark, peat, worm castings & dolomite lime. I used ground bark because it does not compact as much as bark dust. (For seeds I do use bark dust, but I also add lots of perlite and some turface to keep it from compacting.)

In conclusion 10-15% composted manure, 40-50% perlite and the rest bark, and add slow release source of lime. If you are going to stick with the bark dust I would make the perlite 50% of the mix.

Good Luck

Looking at the cultivation of Siberian Iris in the beginning of the iris database (The Irises Database) you will find that Sibs tend to like slightly acidic soil, so I would leave out the lime. Aged manure and/or compost is fine. See also Mt. Pleasant's care guide: http://mtpleasantiris.com/cult...

There are many iris types that do not grow as tall and hardy in a pot as they do in the ground. Sibs tend to like to be left undisturbed in an area where they can develop into a large clump. As such you will probably not get lots of stalks and blooms in a 3 gallon pot. They would probably do well in a VERY large planter of pot for a few years if they get good enough drainage.

for the link Leslie!

Thank you for the info I will try that out