It was tempting to make this post a joke and just leave it a blank page. That would, in fact, have communicated just about everything that it is useful to know about indeterminate potatoes for casual growers. Unfortunately, the proliferation of incorrect information about potato determinacy requires a little more work to correct. Ten years ago, almost nobody was talking about indeterminate potatoes. Over the years, interest has crept up to the point where I now receive many inquiries each year about which of the varieties that I offer, or which potato varieties in general, are indeterminate. Social media is loaded with questions from people asking if this or that variety is indeterminate. The indirect but more useful answer to those questions is: it probably doesn’t matter because it won’t result in a difference in the way tubers grow. The direct but potentially misleading answer is: if the variety has late maturity, it is indeterminate. We have click-bait websites to thank for this confusion. Want to know that one weird secret for wasting your time and money trying to achieve unrealistic yields? You’ve found it.
Most people are familiar with determinacy from their experience with tomatoes, where it is an important trait. Determinate tomatoes are generally compact and early, while indeterminate types continue to grow and flower over a long period of time. A potato plant with a determinate habit flowers at the end of each stem and then grows no larger. This is how it works in theory, anyway. Many potato varieties don’t flower much, but whether or not they actually flower makes no difference in habit. A plant with an indeterminate habit flowers on laterals, but not at the tip of the stem. The stem continues to grow and produce more laterals, so the plant is able to continue growing and flowering for longer. Determinate potatoes normally top out at about two to three feet tall. Indeterminate varieties can grow stems as much as seven feet long, eventually becoming sort of sprawling vines. While a determinate variety will produce a single crop of berries, ripening over a short period of time, an indeterminate plant can still flower and form new berries when you have already harvested ripe berries from the lower laterals.
You may deduce from that description that indeterminacy works the same way in tomatoes and potatoes. That’s right. Many people start there and then draw an additional, and incorrect, conclusion: that the underground structure will mirror the above ground structure and that the plant will continue to produce additional lateral stolons with more potatoes. Unfortunately, that isn’t the case. It is easy to imagine how that might be a great thing. Wouldn’t it be nice if potatoes went on growing more and more, if you just knew the secret for picking the right kind? This is great clickbait material and a tempting idea, but if you think about it in a little more depth, you might wonder where the extra energy to produce those tubers would come from. In an indeterminate tomato (or potato), the fruits ripen and drop from the plant in succession. Potato tubers, on the other hand, are not fruits; they are modified stem material and remain connected and actively growing until senescence of the plant. There is never a time when those tubers stop receiving sugars from the plant, so no opportunity for succession. Because of this, even if indeterminate potatoes did produce additional levels of tubers, tuber size and yield would be smaller, which is not something that most people would want. I have already covered this topic from a different angle, in much more detail, in my article about potato towers.
As you can see, determinacy is not really a useful concept with potatoes, unless you are specifically concerned with flowering and berrying or the structure of the foliage. When it comes to tubers, it makes little difference. The more useful and much more commonly used concept with potatoes is maturity. While the terms are similar in regard to the actual outcome, determinacy is rarely mentioned in the research literature and, in everyday discussion, it is overwhelmingly associated with a misunderstanding about how potatoes grow. Early and mid-season potatoes are generally determinate; they grow to a certain size, flower and begin forming tubers, and then die back uniformly. Late potatoes are generally indeterminate; they keep growing until killed by frost or with herbicide, as is often done at commercial scale. (And they will eventually senesce naturally, but you would have to plant very early or live in a frost free climate to see it.) Late potatoes can yield more than early or mid-season types and usually produce both more and larger tubers as well. They invest more of their initial energy in producing a larger plant. If the season is long enough, they can leverage that additional foliage to collect more energy to form more and larger tubers. This is purely a function of time though. A late variety in an appropriate climate has more time and more foliar surface area to collect energy. It is worth considering that, while a late variety can yield more, it will also take up more space. Because of that, late varieties are not necessarily the most efficient way to get more yield. If you could grow four mid-season plants in the same space as one sprawling, late variety, they will probably outproduce it.
If you see people using the terms “determinate” and “indeterminate” when discussing potato tubers, there is a good chance that you are reading nonsense. Perhaps they know a lot about tomatoes and haven’t grown as many potatoes or perhaps they simply accepted information found on the Internet uncritically, as we all do at times. Potatoes all form tubers in about the same way, determinate or indeterminate, early or late, whether you grow them in the ground or torture them in a tower.