Why Are My Potatoes So Small?

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Are small potatoes giving you grief?

I understand how disappointing it is to spend weeks tending to your potato crop only to discover at harvest that the tubers are much smaller than you expect. 

To figure out a solution to your tiny potato problem, I put together this troubleshooting guide. Inside, I go over why your potatoes may be undersized and explain how to fix the issue before planting your next crop.

There is nothing more satisfying than a bountiful harvest as a gardener, so let me help you get your potatoes up to size!

8 Reasons Your Potatoes Are So Small

First, let’s get an overview of why your potatoes are not growing to their full potential. The factors include:

  • Potato variety 
  • Age of seed potato
  • Plant spacing
  • Watering level
  • Nutrient/Soil pH conditions
  • Timing of planting/harvest
  • Sun and weather exposure
  • Pests or diseases

As you can see, there’s a wide variety of issues that can cause small potatoes. In this next section, I break down each reason and offer solutions to avoid the issue in the future.

1. What Variety Of Potato Was Planted?

The variety of potato you plant will have its own specific mature size, so selecting the right one is the first step to harvesting full-size potatoes.

The variety and length of time till harvest go hand-in-hand when striving to hit full potato maturity before collecting your crop. 

Choosing the correct season to plant a specific variety will also increase the chance your potatoes can reach maturity before the first freeze or the heat of summer.

Most potatoes take between 65-90 days for full growth, so if you find that you are pulling your potatoes too soon due to weather, that could easily be the reason they are smaller than expected. Adapt your planting season length to a potato variety that can mature during this time for optimal growth.

Like to plant your potatoes above ground in pots or bags? A study in Wisconsin called the Kenosha Potato Project has found that ‘Charlotte’ and ‘Calrose’ varieties grow the largest in container gardens, so look for them when choosing seed potatoes to up your crop yield.

2. Are You Using Old Potato Seeds?

Using old or non-certified seed potatoes can greatly reduce not only size at harvest but also the amount of tuber production per plant. 

Increase your chances of a successful crop by only buying certified disease-free potatoes for seed from reputable dealers.

If you are using potatoes from last season’s garden for seed, check over them very carefully and toss any that show bruising, softness, or disease. Using poor potatoes for seed is going to result in weak growth and more disease issues, which ends with small potatoes.

Never use supermarket potatoes for seed because most have a sprout inhibitor application before shipment to increase shelf life and will not grow well in your garden.

Make sure if you are cutting up potatoes for the seed that you allow them to cure for several days to seal off the flesh. If you bypass the curing process, the chances of the seed potatoes rotting in the ground before producing any foliage is much higher.

3. Distance And Depth Planted?

How you plant your potatoes very much affects the size and amount of tubers the plant will produce.

For best results, make a trench for your potato seeds eight inches down and six inches wide, so the roots will have plenty of loose soil in which to spread. Space the plants around 15 inches apart, and keep the trenches at least two feet apart. 

After you plant, you can then begin the filling, hilling, and mulching process as usual. Potatoes need space to grow to full size, which includes soil that is loose so tubers can expand freely. When you space plants too close together, they naturally retard root and tuber growth.

Container garden potatoes feel the most effect from plant crowding and will grow much smaller potatoes as a result. Change your bins to a much larger size, or reduce the amounts of plants per container. 

The last thing you need to remember is to always rotate your potato crops into a new section of the garden every year. Potatoes heavily deplete nutrients in the soil and attract pests and diseases that will weaken the chances a new crop can grow to full potential.

4. Too Much Or Too Little Water?

Water is another critical factor when you want larger potatoes. Along with well-draining soil, your potatoes will also need an average of one inch of water per week during initial growth and two inches per week after flowers begin blooming.

Tubers need water to gain bulk (size), and if they don’t receive enough at the roots, the foliage will suck up most of it, leaving you with tiny taters.

Mulching helps retain moisture in the soil, which allows tubers to grow larger during this crucial stage.

A great way to assure your potatoes are getting adequate water every day is to use a rain gauge in conjunction with a soil moisture meter with a long probe so you can get a good reading at the root level.

Don’t get overzealous when it comes to watering your potato plants. A lot of water is just as bad as too little and can quickly rot out your crop of potatoes. 

Lastly, make sure you stop watering your potato crops two weeks before harvest, so the skins can toughen up, which reduces damage as you dig them out.

5. Did Your Potatoes Get Enough Fertilizer?

A good fertilizer will aid in helping you grow larger potatoes. Potatoes consume an extreme amount of nutrients from the soil during tuber development, and without enough, the plant will naturally stunt tuber growth.

But don’t let quick-growing, thick foliage fool you into thinking your crop is doing great. You may actually be fertilizing wrong. Using a product with too much nitrogen will grow potatoes foliage vigorously but do nothing to help the tubers develop.

To get the largest-size potatoes possible, let’s look at the ideal fertilizer requirements.

6. When Did You Plant Your Potatoes? (Seasonality)

The timing of planting and also harvest can alter the size of your potatoes. To get full maturity out of your potato crop, you need to time these tasks perfectly for your region since they grow best in cooler temperatures.

In early spring, don’t try to plant seed potatoes until the soil reaches 50-degrees Farhenheit.

If in your region, the temperature quickly moves from cold to scorching heat, you may find that only planting potatoes in the fall is more productive.

SPRING PLANTING

For people living in the North, expect to plant in April, which is about six to eight weeks past the last frost. The ground should be workable, and excessive moisture from snow or rains should mostly dissipate by now.

If you plant the potatoes, then experience one or two frosty nights, cover emergent growth with mulch, or cover with plastic containers or a fabric. 

You want to form a protective barrier that will retain enough heat to keep plants from frost damage. Remove the barrier as soon as possible to allow sunlight to reach the foliage.

In Southern regions, most gardeners choose to grow potatoes as a winter crop and plant anywhere from September to February, whichever range of months that presents the coolest temperatures.

FALL PLANTING

In the fall, you can start potatoes around three months before the first frost, depending on the variety you choose. 

For full-size potatoes, it’s wise to give yourself a two-week buffer from expected frost dates to ensure your tubers can grow as much as possible before you prep for harvest.

For example, if your potato variety needs 65 days to mature and the frost date for your region is December 15, plant your seeds around the end of September instead of October 10.

Harvesting

Always allow your potatoes to grow until the plant’s natural death or as close as possible to this time. Pulling potatoes too early will result in smaller tubers.

While you may want a few small potatoes for grilling or soups, leave most in the ground to continue fattening up as much as possible.

Harvest potatoes two to three weeks after the upper foliage die back. Cut off brown foliage to prevent mildew from forming and wait 14 days before harvesting so that the skin can thicken.

If during this time, your region gets a lot of rainfall, you may need to pull the potatoes sooner, to avoid having the crop rot out in the moisture-laden ground.

7. Sun And Weather Conditions

If you want large mature potatoes, they need sunlight to help generate and store that energy within the tubers. Potatoes require full sun. The more shade they receive, the smaller the tubers will grow.

Potatoes require six hours of full sun each day for optimal growth. Keeping the foliage in full sun exposure while protecting the soil is extremely critical. Use lots of mulch and hill several-inch-deep barriers of earth, so there is plenty of shield from the sun’s warming rays.

Potatoes are a cool-weather crop because they will stop tuber production once the soil reaches a specific temperature. Air temperatures can be warm, but keeping soil temperature in check is imperative for healthy growth that leads to full-size spuds. 

This is especially helpful for those who use grow bags or containers for potato crops because you will need to take extra precautions to keep the soil inside them cool while keeping the foliage in full sunlight.

For above-ground potato gardening, using mulch and providing shade to the containers will help keep soil temperatures down. I like to drape shade cloth over my grow bags (not the foliage, just over the base) if the weather heats up. I also make sure there is always moisture inside the pots, which helps reduce temperature through the evaporation process.

Keep in mind that potato plants signal roots to stop tuber production when the soil gets over 75-degrees Farhenheit, even if they continue to sprout new foliage and flowers. Using a soil thermometer is a fantastic way to monitor soil heat levels quickly, so you can avoid crop damage that leads to small potatoes.

8. Pest Or Disease Infestation

Following good gardening protocol to avoid pests and diseases within your potato crop will help increase the size of your potatoes. Pests and diseases can cause small potatoes to grow by depleting a plant’s ability to absorb nutrients fully.

Avoid early and late blight issues by only using certified seed potatoes and keeping plant foliage as dry as possible. Water your potato garden early in the day so the leaves have time to dry in the sun.

Don’t overcrowd plants, which creates more shade and less airflow allowing moisture to remain on the foliage.

Immediately remove any dead, yellowing, or diseased leaves and tubers you see. Potato varieties such as Elba, Rosa, Allegany, and Sebago have excellent disease resistance.

In Summary

Every step of the potato growing process can make or break the size of your potatoes. 

Now that you have the information you need to grow potatoes each season successfully, you can avoid many common problems that reduce spud size and enjoy an abundant harvest of large potatoes!