I have just had to try to explain to someone why it is bad when a variegated plant produces a plain green shoot and, even worse, why the plant was variegated in the first place. It is not quite rocket science but, to understand, it is necessary to have a little botanical knowledge. Yet, like many garden writers, I often glibly order the green shoots to be removed without explaining why.
Leaves and stems are composed of layers of tissue. Leaves have more layers in the centre and fewer around the edge. When a mutation occurs that causes the outer layer of tissue to grow without chlorophyl, (the green pigment), the leaves have a white edge where there are no green cells (variegated mint above). But sometimes, the growing point bursts through the outer layer – like a finger pushing through a damaged glove – and the leaf or shoot grows without the outer layer of white tissue. The white tissue, because of its lack of carbohydrate-manufacturing chlorophyl, is a drain on the plant, having to be sustained but serving no purpose, so variegated plants grow less vigorously than all-green plants. If green shoots appear on variegated plants, because of their greater vigour they can ‘take over’. See the variegated euphorbia at top of page with two green shoots.
Remember that, in nature, these mutations would simply not survive because they are less vigorous and would be swamped by their less colourful comrades. They exist because humans value them and propagate them.
Similarly, if an all-white shoot appears it will eventually die because it cannot manufacture its own food. Less often the inner layer is white and the outer layer is green. In this case the leaf will have a green edge (all green cells) and a yellow, cream or grey central patch because the thickest part of the leaf has a white core overlaid with green cells.
So the danger with ‘reverted’ shoots is not that they kill off the variegated parts, as the suggestion that they will ‘take over’ suggests, but that they grow so much more quickly that they overwhelm the plant.
Variegation can be caused by other factors, such as genetic, when the plants can be propagated from seed (variegated lunaria among others) and virus.