My potato tops are 700mm high and they have only been in the ground since the middle of August. Why have they grown so tall in such a short space of time?
While the size of your potato tops may appear to be excessive, this should not cause you any undue concern.
There are a few possibilities which may have caused this. In some seasons where air and soil temperatures increase very quickly in spring (as with this spring), there is a tendency for potato growth to be extremely vigorous. It is also possible that your soil is quite rich, especially in nitrogen, which leads to a lot of vegetative growth. Potatoes come in a wide selection of varieties, each varying in size, shape, flavour and timing when they crop and some can have more vigorous top growth than others.
In the end, what matters most is the quality of potatoes beneath the soil surface. Try to refrain from digging amongst the roots to see what is happening down below. Such root disturbance is detrimental to the formation of healthy potatoes.
Keep mounding or earthing up every fortnight by building up the soil around the bottom of the plant. You can even cover some of the new shoots and lower leaves as this will help towards a better crop.