Why Are My Potatoes So Small? (5 Common Causes & Solutions)


Are you getting big potato plants, but finding only small tubers when you dig them up?  There are lots of reasons this can happen, but no matter what the cause, the outcome is disappointing.

So, why are your potatoes so small?  Small potatoes can be caused by a lack of sunlight, improper watering, nutrient deficiency, high temperatures, or harvesting too early.  Some potato varieties will naturally grow smaller than others, and even the potatoes on one plant can vary in size.

Of course, it would be nice to know which of these causes is leading to a harvest of small potatoes.  That way, you can take the right steps to fix the problem.

In this article, we’ll talk about 5 common causes of small potatoes, along with some solutions you can use to address the issue.

Let’s get going.

Why Are My Potatoes So Small?

Even if your potato plants look healthy, many factors can cause small potato tubers to form, including:

  • Lack of Sunlight
  • Improper Watering
  • Nutrient Deficiencies
  • High Temperatures
  • Early Harvesting
small potatoes new potatoes
Most of these potatoes are pretty small – but why? Let’s take a closer look to find out.

Let’s start with a common reason that you get small potatoes, even with healthy-looking plants: a lack of sunlight.

Lack of Sunlight

According to the University of Maryland, potatoes grow best in a full-sun location.  Depending on whom you ask, full sun means 6 to 8 hours (or more) of direct sunlight per day.

Without enough sunlight, potato plants will not be able to grow properly.  A lack of sunlight means less photosynthesis and less energy produced by the plant.

If the vines and leaves (the parts above ground) do not produce enough energy, there won’t be enough starches to put into the tubers (the part below ground).  This will lead to small potatoes, or even worse, no potatoes at all!

In fact, I got to see this firsthand in this year’s garden.  In the spring, we planted potatoes in two different areas over the course of a week.

The first area gets plenty of sunlight throughout most of the day.  The second area is shaded by lots of tall trees, since it is near the tree line of some woods in the back yard.

Honestly, there wasn’t a lot of difference between how the plants looked in these two areas.  The plants from both areas looked tall and healthy, with nice green growth from thick shoots and plenty of leaves.

However, there was a big difference in the number of potatoes harvested from these two areas.

large and small potatoes
The top bowl has potatoes from the sunny area in the yard. The bottom bowl has potatoes from the shaded area (they are washed clean). Sunlight makes a big difference!

In the first (sunny) area, some of the larger potatoes were as big as my fist.  This area also produced more potatoes overall.

In the second (shaded) area, all but a few potatoes were smaller than my eye.  Many were only as big as marbles or peas!

The most frustrating part is that some of the plants in this shaded area produced no potatoes at all.  There were nice green plants with healthy shoots and roots, but there were no tubers to speak of!

The moral of the story is that you will get small potatoes if the plants don’t get enough sunlight.  This is true even if the plants get plenty of water and nutrients in good soil.

Put your potato plants in an area where they will get full sunlight during most of the day.  Avoid planting them near taller plants (such as tomatoes!) that will block their sunlight.

You might also need to trim some branches from nearby trees if you plant near the woods.  If you don’t have much space and need to plant close to the house, put your potato plants on the south-facing side of the house.

There is one more thing – in the second (shaded) area, the potatoes that were grown in containers did better than the ones grown in the ground.   I don’t know why this would be – it may have something to do with water levels.  Speaking of which…

Improper Watering

If you know that your potato plants got plenty of sunlight, the next place to look is their water supply.  Potatoes need moisture during the entire growing season, but especially when the tubers are forming and growing.

Without enough water, potato plants will dry out, and in severe cases, a lack of water means they won’t be able to absorb nutrients from the soil.  This will prevent the plants from growing and producing energy to store in the tubers.

If your potato plants were often wilted and the soil was dry during the growing season, a lack of water might be the cause of your small potatoes.

Potato plants need plenty of water to form big tubers before harvest.

Fortunately, there are some ways to prevent this problem, even if you can’t afford to spend all day watering your plants.

First, remember that sandy soil drains faster than other soil types.  If your soil is sandy, mix in some organic material (for example, compost made from leaves, grass, and vegetable scraps).

Adding compost will help the soil to retain more water, and it will add some nutrients at the same time.  It will also attract worms and beneficial bacteria to your garden.

Second, remember that the soil in raised beds and containers will often dry out faster than the ground, due to the higher elevation. This elevation encourages water to drain out faster.

Raised beds are great, but the faster drainage means that they might not be ideal for some plants that need more consistent watering and moisture, such as potatoes.

Third, a layer of mulch will also help to keep more water in the soil.  However, that isn’t the only benefit of using mulch.

According to the University of Illinois Extension:

“Mulch is usually beneficial in growing potatoes. After the potato plants have emerged, organic mulch can be applied to conserve moisture, help keep down weeds and cool the soil.”

https://web.extension.illinois.edu/veggies/potato.cfm

You can use materials such as wood chips, leaves, and grass clippings for mulching potato plants.  This is especially convenient when hilling potatoes.

Hilling simply means piling up soil around the base of the potato plant.  This prevents the tubers from getting exposed to sunlight, which causes them to turn green and produce solanine (a toxic substance that can make you sick).

You can learn more about why potatoes turn green in sunlight in my article here.

Remember that uneven watering can cause other problems for potatoes.  For example, the University of Florida Extension suggests that uneven watering can cause rough skin on potato tubers.

Lack of Nutrients

A lack of nutrients can also slow down the growth of tubers, leading to small potatoes.  The Old Farmer’s Almanac suggests using a balanced fertilizer (such as 10-10-10), which has a mix of nitrogen, phosphorus, and potassium (NPK, or the big three nutrients for plants).

Remember that soil pH plays a part in nutrition.  If the soil pH is too high or too low, some nutrients become less available to plants, as you can see in this chart from Research Gate.

The ideal soil pH for potatoes is 6.0 to 6.5 (slightly acidic, since a pH of 7.0 is neutral).

Even if the soil is fertile and the pH is perfect, a lack of nutrients can still occur due to overcrowding.  If you plant potatoes too close together, it will lead to competition for water and nutrients.

According to the University of Maryland Extension, you can plant potatoes as close together as 9 inches if you want small (new) potatoes.  On the other hand, if you want the largest tubers possible, then plant potatoes 20 inches apart.

You can learn more about the proper spacing and depth for potato plants in my article here.

High Temperatures

Extreme heat can still result in small potatoes, even if your plants get the right amounts of sunlight, water, and nutrients.  Potatoes are a cool weather crop, so high temperatures decrease both the number of tubers that form and size of the ones that do grow.

According to the University of Illinois Extension:

“Maximal tuber formation occurs at soil temperatures between 60° and 70°F. The tubers fail to form when the soil temperature reaches 80°F.”

https://web.extension.illinois.edu/veggies/potato.cfm

Of course, planting your potatoes in a sunny area will cause the soil to heat up faster and reach higher temperatures.  There are a couple of things you can do to keep them a bit cooler in the summer and encourage more tubers to form:

  • Use cold water to irrigate your plants and cool down the soil at the same time
  • Use shade cloth or row covers attached to stakes to give your potato plants some shade during the hottest part of the day (at midday, around noon)
potato plants
Potato tubers will not form if the soil is too warm.

Harvesting Too Early

Finally, harvesting your potatoes too early will also lead to small potatoes.  A hint that it is too early to harvest is that the potato skins will easily rub away.

If you harvest early, the plant will not have enough time to send energy to the tubers.  The resulting lack of energy will keep the potatoes small.

However, there are some cases when small potatoes are desirable.  According to the Old Farmer’s Almanac:

“New potatoes, which are potatoes that are purposefully harvested early for their smaller size and tender skin, will be ready for harvest 2 to 3 weeks after the plants stop flowering. New potatoes should not be cured and should be eaten within a few days of harvest, as they will not keep for much longer than that.”

https://www.almanac.com/plant/potatoes

If you want larger, more mature potatoes, you should wait 2 to 3 weeks after the plant’s foliage has died back to harvest.  This will give the plant enough time to send energy (starches) to the tubers.

potato vines fallen over
Once your potato vines turn yellow and fall over like these ones, it is time to start thinking about harvesting the tubers.

You can learn more about how and when to dig up potatoes in my article here.

Conclusion

Now you have a better idea of why you are getting small potatoes, and how to address the problem.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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