|Most commercial potting compost is suitable for growing in Air-Pot containers as long as the component size is small enough. If there are large lumps these may not fill the cones in the wall and roots will not grow into the cones as intended. Air pruning is achieved when roots get caught in the earth filled cones and unable to turn around they are forced to halt at the air hole. A more branched root system develops with many fine roots, rather than fewer coarse roots which can become pot bound as they circle around. Commercial potting compost is usually finely sieved and good to use straight from the bag. If using homemade garden compost it will certainly be best to pass it through a riddle or sieve to remove sticks and to break up lumps. Previously I have hurled the garden compost through a large flat piece of mesh sitting over a wheelbarrow. This works well but involves a lot of shaking, riddling and tipping aside of coarse stuff and sieving a whole heap is fairly heavy going.
A great labour saving device to make this job easier is a rotary compost sieve or ‘trommel’. I see motor driven trommels can be bought for £450 or there are YouTube videos showing cobbled together hand cranked versions using a flimsy mesh cylinder and bicycle wheel rims. My homemade version uses stiff metal mesh panels within a wooden hexagonal frame mounted on a cement mixer. Even freshly dug wet compost can be fed into the open end of the hexagon as it turns and most of the material quickly falls out through the mesh. Lumps can tumble around further breaking up and eventually the stubborn sticks, mango stones and avocado pips can be tipped out. If you do not have a handy cement mixer I am sure a simple frame with a turning crank could be devised. I used this 25mm sturdy mesh because I had a sheet to hand, the length of the trommel was determined by the 66cm width of the sheet but seeing how the compost falls out I see it is longer than necessary so I may shorten it to around 50cm. The 25mm spacing of the mesh allows even wet compost to pass without clogging, for really finely graded compost for filling small pots I may re-sieve some compost once it is drier through a finer mesh inserted in the drum.
I have quite a few trees in pots, mostly just leafless sticks at this time of year. Two cherry trees in Large Air-Pot containers have established well after they came supplied in a very small pot or with just a tiny plastic wrapped rootball. They are ready to move on to the Extra Large size with plenty of that sieved garden compost plus a generous amount of blood, fish and bonemeal. I’m hoping for a decent bowlful of cherries this year.
For the first time I will be planting a large bare rooted tree in an Air-Pot container, this is an experiment and the three metre tall James Grieve apple tree may not survive as losses from planting bare rooted trees are always high. But I know some commercial potted tree suppliers routinely establish bare rooted field grown trees in Air-Pot containers before selling on to customers. The superior root system developed after a season or so in the Air-Pot containers means almost all these trees will survive after planting out. I will try to keep this old Scottish apple variety in the pot espaliered against a wall for a season or two, and know that it should grow well once it finds a permanent home in the ground.
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