With the benefit of this protection, the potato plants grow much more rapidly than if they were in the open air.
|First Earlies "Annabelle" and "Juliette", covers temporarily removed|
A couple of days ago I judged that these first ones were ready for earthing-up (known in the USA and some other places as "hilling").
The stems of this vigorous "Annabelle" plant were already standing up a few inches above the soil surface, and it is this that makes earthing-up desirable.
The actual tubers (potatoes) that you harvest from a potato plant grow on things called "stolons", which come out from the base of the plant, underneath the surface of the soil. It makes sense to ensure that they have a good depth of soil in which to develop, and earthing-up has the effect of stretching the stems, which theoretically allows for the formation of more stolons. You could achieve this by planting the seed-tuber quite deeply, but if the emerging shoots have a long way to travel before reaching the light they may become weakened. Because of this, when growing potatoes in containers I usually earth-up in two stages, though this is not strictly necessary.
Here is the same plant, after another couple of inches of nice rich home-made compost have been added to the pot. The leaves are still exposed. but no stem is showing now.
This pot has two "Annabelle" plants in it, and they are clearly ready for the treatment too.
Here they are afterwards, nearly buried again:
If like me you grow your potatoes under plastic or glass, don't forget to check them frequently to make sure their soil / compost is still moist. Potatoes hate dry soil, and if they don't get enough water they will inevitably produce a poor crop. Growing them in compost rather than soil helps with moisture-retention. The presence in the compost of plentiful organic matter also helps to prevent the tubers getting the disease called Scab.