In 2017, many areas of the US have experienced unusually wet and cool weather. As a consequence, a lot of people have reported seeing something unusual: potato berries. They usually look like small, green tomatoes growing on your potato plants. These are known variously as seed balls, potato apples, potato tomatoes, or simply as berries, which is the most accurate description, botanically speaking. Tomatoes and potatoes are both members of the same genus, Solanum, and they both produce berries. We eat the berries of tomatoes, but we don’t eat the berries of potatoes because they are somewhat toxic. Unlike tomatoes, potato berries are small, rarely much exceeding an inch in diameter. Most remain green at maturity, but some varieties turn purple.
If you were surprised to see berries on your potato plants, you are not alone. Potatoes are normally grown from seed tubers, so many people have never given much thought to the matter of actual seeds of the potato plant. When you grow a plant from a tuber, it is a clone. Every plant of a particular potato variety is, in a very real sense, part of the same original plant. But potatoes can also reproduce sexually. They flower and, if the flower is pollinated, later produce a berry that can contain several hundred true potato seeds. Unlike planting a potato from a tuber, a potato plant grown from seed is new and unique. It will have different characteristics than the variety that produced the seeds.
Even if you have grown potatoes for years, you might never have seen berries. For one thing, you might simply have never looked for them if you didn’t know about them. Even if you were observant, modern potato varieties tend not to flower or produce berries very easily. Many varieties never produce berries under normal growing conditions. Some are better than others in this regard. Many modern potato varieties have sterile pollen. Unless you are growing a fertile variety, none of your plants will produce berries. People tend to notice berries when they begin to grow multiple varieties, which can cross pollinate. If you have grown the potatoes or true potato seeds that we offer, you are much more likely to see berries on your plants than you would be with commercial varieties. Also, potatoes form berries most easily in mild and humid weather. When temperatures regularly climb above 80 degrees F, most varieties have a hard time forming berries. If you have had unusually mild or wet weather, recently started growing more than one variety or have a neighbor who is also growing potatoes, or if you are seeing increased pollinator activity, particularly bumblebees, you might find potato berries where you haven’t before.
It can be a little difficult to find information about potato berries and seeds because we call the tubers that we plant “seed potatoes.” There is a lot more information about seed potatoes than potato seeds, but search engines can’t really tell the difference between the two. We call potato seeds “true potato seeds” or TPS to better differentiate. They may also be called “botanical potato seeds.”
How Toxic Are The Berries?
Potatoes form toxic substances called glycoalkaloids. They are produced in small amounts in the tubers, but typically much higher amounts in the foliage and the berries. Berries have about 10 times the level of glycoalkaloids as the tubers of commercial varieties (Friedman 1992) although the levels in both berries and tubers vary significantly with variety and growing conditions. Potato tubers are considered safe when they contain less than 200 milligrams of glykoalkaloids per kilogram of tubers. Berries may range in glykoalkaloid content from as little as 177 mg/kg to 1350 mg/kg or more (Coxon 1981).
Some people begin to experience symptoms of toxicity at about 1 mg per kg of body weight (Ruprich 2009). Based upon those amounts, a sixty kilogram person could consume somewhere between 40 and 300 grams without developing symptoms, depending on the actual level of glykoalkaloids in the berries. An adult who eats a few potato berries might not have any symptoms or might spend an unpleasant few hours with vomiting and diarrhea. Of course, potato berries are much less tested than tubers, so you should definitely contact your doctor if you have eaten potato berries. The situation is likely to be more serious for children, since they can more easily accumulate a high dose for their body weight, so make sure that they understand that the berries are not edible.
Berries do not affect toxicity in the rest of the potato plant. The tubers are just as edible as always.
Should I Remove The Berries?
Maybe, but it isn’t a common practice. If you have young children and you are worried that they might eat the berries, that is probably the best reason.
Berry production may reduce tuber yield by diverting resources into sexual reproduction (Tekalign 2005). Tuber production and berry production both begin following flowering, so there is competition for the plant’s energy between the two. The difference is not likely to be so much that home gardeners will notice. Farmers do not normally perform reproductive pruning of potato crops, even with varieties that fruit heavily, since the cost of the work involved would exceed the value of the difference in yield. In general, the heavier the berry production in a variety, the greater the potential reduction in yield. If you are only growing a few plants, you could remove flowers before they form berries for a little boost. Most varieties produce so few berries that you won’t see any difference.
What Are They Good For?
As with many plants that we normally propagate from vegetative parts, like tree and small fruits, asparagus, rhubarb, sweet potatoes, and others, the seeds of potato plants don’t grow true. That means that when you grow a plant from those seeds, the tubers are not the same as what you started with. In fact, every plant grown from seed is an entirely new variety. Very few will be as good as what you started with when compared across a wide variety of traits, but many will be perfectly acceptable to home gardeners who like to be a little adventurous.
Will The Berries Lead to Weeds?
Maybe, but it is more likely that you will have problems with plants growing from tubers that you missed than from seeds. If you don’t collect the mature berries and allow them to fall on the ground, some will rot and release their seeds into the soil. If you live in a climate that doesn’t have long, deep freezes, those seeds can survive the winter and germinate. Normally, because the number of berries is small and the survival rate is low, volunteer seedlings are unusual. If you have a variety that produces lots of berries and you live in a mild climate, you could get lots of seedlings. This has happened here on occasion.
How Do You Get the Seeds Out of the Berries?
There are several ways to go about this. The simplest is to wait until the berries are fully ripe and very soft and just squeeze the seeds out. You can also use any method for saving tomato seeds. If you want very clean seeds that are suitable for long term storage, you can extract them with a blender and treat them with detergent to break down substances that inhibit germination.
How Do You Grow True Potato Seeds?
The process overall is similar to starting tomatoes from seed. For more information, see our growing guide for true potato seed.