Q: For two years, towards the end of December, I have planted a few late potatoes. The tubers were sprouted and grew well until flowering then seemed to decline so I dug them up and found nothing other than roots – no new potatoes at all. The ground where they were growing produces good main crops. Could it be that potatoes need lengthening daylight hours?
A: This does seem a little unusual but there are several things which separately or in combination, could result in lack of tubers. The first and most likely is the variety you planted.
Potato cultivars are roughly grouped into early types such as ‘Cliffs Kidney’ and ‘Swift’, mid-season types such as ‘Ilam Hardy’, ‘Desiree’ and ‘Nadine’ and late or main crop types such as ‘Rua’ and ‘Agria’. The early ones are best planted around June-August for harvesting in December-February, the others around late October-November. Mid-season ones can then be harvested in mid season, while the late/main crop ones can stay in the ground for later harvesting and storing.
Actual times will vary, depending on climate and soil, but as a rule a cultivar planted at an unsuitable time of year will probably not produce good yields. Depending on the cultivar, it takes 90-140 days from planting until tubers are ready for harvest. If you potatoes flowered and/or started to die down much sooner, other factors to consider are disease and water.
Potatoes are susceptible to several diseases that affect the roots and tubers so you should avoid growing potatoes in the same soil year after year. Ideally, let two years elapse before planting potatoes in an area again to avoid disease build up in the soil.
Too much water can cause tuber rot but too little can hinder their development. During long, dry periods, it’s best to water potatoes deeply once or twice a week, rather than a little every day.