the root of the question

The day started quietly, and I was able to get quite a long way further round the tree line, scooping up leaves and scraping slime off the Mypex fabric.  With my gaze largely limited to ground level, I had plenty of time to look at the pots and the lower parts of the trunks, and noticed how some of the crab apples were making adventitious roots from the first inch or two of stem.  They were all grafted varieties, and I made a mental note to try and remember to ask the manager about it in the morning.

Species crab apples can be apomictic, meaning that they set seed without sexual reproduction, which is genetically identical to the parent.  Again, I realised I didn't know how common this was among Malus, though I know (since checking on Wikipedia) that some other members of the rose family do it.  It matters to the nursery grower because if the plants reproduce sexually, and are in close enough proximity to related species to hybridize, then seed raised plants won't necessarily come true, and your only guarantee of making more of the same is vegetative propagation, which means grafting in the case of apples.  I'm sure I've been told that if you sow seeds of Malus transitoria you are pretty much guaranteed to get more M. transitoria, and not a cross between that and something else.  But if you were to grow a pip from a 'Cox' you would most definitely not get another 'Cox'.

The grafted apples were all named hybrids, hence the use of grafting.  The above ground roots were chubby little pinkish orange affairs, growing thickly on the first bit of the stem above the level of the compost.  I wondered why they were doing it, and whether it meant that the rootstock had been planted higher up in its container than it had originally developed in the open ground, and if so whether it should be buried deeper again on planting to cover the aerial roots.  The normal advice is to plant the tree to exactly the same depth as it was growing in the nursery.  Do not bury the trunk. Repeat, Do. Not. Bury. The. Trunk.

But I'm sure that in his lecture Tony Kirkham also warned us not to plant too shallow.  In fact, he said that the flare on the young tree's stem, the part just above ground level where the stem got fatter, should be left just above the soil surface, and that this would develop into the buttresses you see on mature trees.  Only some of the crab apples did not have any discernible flare, just the funny ruff of roots above the compost.  I must remember to ask.  It's so often the way, once you look closely at something, you realise that it's not nearly as neat and straightforward in practice as it sounds in theory.

Tony Kirkham himself admitted as much, since the first two or three metres of trunk of the ancient Giant Redwoods are indeed buried.  The trees are so old that since they started growing, the soil surface has slowly risen by that amount, burying the bottom portion of the trunk like a Roman pavement.