If you are using a growbag it would be difficult to top it up with more compost, especially if its the flat pack variety in which you can grow tomato's in

Hi Jacky5 Welcome to GOY, If you planted your seed potato in the growbag lying flat,you do not need to top it up. If you put the grow bag on end before you planted your seed potato, you would have removed all but eight inches of compost from it and planted the potato seed 4" deep. You would have rolled the bag down to that level. When the leaves grow to around 4" above the compost, unroll the bag to just above the height of the leaves and fill the bag up with compost to cover them. Continue doing that until the bag is fully extended. Keep watering and wait for them to be ready. You should have punched holes in the bottom to allow excess water to escape.

The reason potatoes are "earthed up" is to ensure the tubers don't break the surface and go green (which makes them poisonous). If you are growing in bags and can eliminate all the light from getting into the growing spuds then you shouldn't need to earth them up!

Welcome to GoY and good luck with them.

Ian is right green potatoes are poisonous so you do need to earth them up unless they are growing in flat bags. Potatoes only produce potatoes above the seed potato. By earthing up we are burying the stems more deeply and that part then becomes part of the root and produces potatoes on all parts of the stem which we have covered up. Growing in flat bags, by cutting a slit for the leaves to poke through, the roots spread out sideways and because the bag is covering the potatoes we do not need to earth them up. Before you cut the slit in your growbag, if you,choose to grow them flat, give the bag a good shake to loosen up the compost inside. Good luck and we'll look forward to hearing how your crop gets on.