I planted the potato (Maris ..something..) around the end of February here on the coast (weather like Los Angeles) in a pot, then when the plant was about a foot tall I added a second bottomless pot and added more soil. Picture shows it on the city balcony. Thanks
Great. Its just that it started to fade virtually overnight: I left it over a 2 day weekend and came back to find it had lost its green gloss. OK, I'll give it some time and hope to find afterwards some potatoes to harvest. Thanks
Sorry but the three most popular Maris varieties (Bard 1st Early, Peer 2nd Early, Piper Maincrop) all have life cycles longer than 9-10 weeks.
It is likely to be the case that there is a nutritional shortage in the plants, especially since they are pot-grown, since it is very easy with potatoes and their high water requirements, to flush out nutrients.
The ideal feed at this time, I'd say, would be comfrey or tomato feed and a little Epsom salts in the next watering.