Why Are My Tomato Plants Stunted? (Plus What Causes It)

Are you frustrated by stunted tomato plants?  If so, you are probably wondering why they are so small, and what you can do about it.

So, why are your tomato plants stunted?  Tomato plants will grow stunted due to environmental problems, such as extreme cold, uneven watering, soil problems, or lack of light.  Certain pests and diseases will also cause tomato plants to grow stunted.

Of course, it is helpful to know exactly what is causing the problem so that you can treat it.

In this article, we’ll take a look at causes of stunted tomatoes and how to fix or prevent the problem.

Let’s get started.

Why Are My Tomato Plants Stunted?

If your tomato plants are stunted, the first thing to check is the environment.  This includes factors like:

• Temperature
• Watering
• Soil
• Light

It is also possible that damage from pests or diseases will cause your tomatoes to grow stunted.

Let’s start off by looking at temperature.

Temperature

A period of extreme temperatures can damage young tomato plants, even if they survive the ordeal.  Both cold and heat can cause stunted growth in a tomato plant.

You can see my illustration of ideal temperature ranges for tomato plants here.

Cold

Tomato plants cannot survive frost without protection.  This is true whether they are young transplants or established vines.

However, it doesn’t take freezing temperatures to damage tomato plants!  According to the University of Idaho Extension, tomatoes are susceptible to cold damage at temperatures below 45 degrees Fahrenheit (7 degrees Celsius).

If a young tomato seedling is damaged by cold, it may have trouble growing to full size later in the season.  This will reduce your yield from that plant (if you get any tomatoes at all!)

Your tomato plants are more susceptible to cold damage if you transplant them too early.  You should always wait until after the last frost date to transplant tomatoes outside.

You can find the last frost date in your area by entering your city and state or zip code in this tool from the Old Farmer’s Almanac.  (Remember to start your tomato seeds indoors 6 to 8 weeks before the last frost date.)

Sometimes, frost will come late in the spring and threaten your tomato plants.  To avoid this, use one of these methods to provide cold protection for your plants:

• Cloches – these covers protect young plants from cold and frost.  Most cloches are made from plastic, but you can find glass cloches as well.  You can also buy or build a cloche out of wire.  However, you will need to cover it with a blanket, a towel, or a plastic cover to protect a plant from the cold.  You can learn more about cloches in my article here.
• Row Covers – row covers are made from fabric that keeps plants warm while letting sunlight through.  A row cover is long enough to cover a whole row of plants.  However, you can also cut them into pieces that are the right size for a single plant.
• Greenhouses – a greenhouse retains heat from the sun to keep plants warm.  In cold climates, a greenhouse can give you an extra week at both the start and end of the growing season.
• Mulch – most people hear “mulch” and think of wood chips to decorate a landscape.  However, mulch also works to insulate soil and keep plants warmer when cold weather looms. You can also use grass clippings, fallen leaves, or cardboard as mulch.

You can learn more about how to protect tomato plants from cold and frost in my article here.

If your tomato plants have already been damaged by cold, there is not any way to reverse the damage.  Consider it a lesson learned for next year’s garden.

Heat

It is true that tomatoes are a warm-weather crop.  However, excessive heat and humidity in summer can prevent pollination and delay fruit set.

Extreme dry heat can cause a different problem.  In that case, a tomato plant will lose water through its leaves (transpiration) faster than it can absorb water through its roots.

This can happen even if you give the plant enough water.  If this goes on long enough, you will start to see slow and stunted growth.

Watering

Tomato plants need consistent watering to avoid problems such as blossom end rot.  An inconsistent watering schedule (too much or too little) can also cause stunted growth.

Over Watering

When you water tomato plants too much or too often, the soil remains wet for too long.  When this happens, the roots stay wet and cannot get enough air.

Eventually, this causes root rot in tomato plants.  After the damage is done, your plants will no longer be able to get water and nutrients from the soil.

So, over watering can actually lead to stunted growth.  You can learn more about over watering your plants in my article here.

Watering too often can also encourage shallow roots. This may end up causing exposed tomato plant roots on the soil surface.

Remember that over watering can spell the end for your plants.  Basically, you are giving them too much of a good thing.

Under Watering

Under watering can also lead to nutrient deficiencies that will stunt plant growth.  According to the Michigan State University Extension, plant nutrients need to be dissolved in water so that plants can absorb them through their roots.

Watering consistently will save your tomato plants and improve their stunted growth.  Don’t wait too long to make any necessary changes!

If your soil is sandy, you can add compost to help with water retention.  A layer of mulch will also help to retain water in soil by preventing evaporation.

Nutrient Deficiencies

If temperature and watering are not the cause of stunted growth, then soil is the next place to look.

The first step is to get a soil test, which will tell you about any issues.  You can learn more about how to do a soil test in my article here.

A soil test may reveal several potential problems:

• Soil pH Imbalance (the soil is too acidic or too basic)
• Lack of Nutrients (not enough of one or more nutrients in the soil)
• Excessive Nutrients (too much of one or more nutrients, which blocks uptake of another nutrient)

Let’s take them one at a time, starting with pH imbalance.

Soil pH Imbalance

Tomato plants like soil with a pH in the range of 6.0 to 6.5 (slightly acidic).

When soil pH is too low (acidic) or too high (basic), you will see symptoms of nutrient deficiencies in your plants (such as yellow leaves or stunted growth).

This is because soil pH has a huge effect on the availability of nutrients in soil, as you can see in this chart from Research Gate.

As the chart shows, phosphorus becomes less available when soil pH goes below 6.0.  Nitrogen, potassium, and magnesium also become less available as pH decreases.

There are several ways to increase soil pH, including:

• Lime – lime (calcium carbonate) raises soil pH.  It also adds calcium, so don’t use too much.  Otherwise, you will get a magnesium deficiency in your soil (more on this below).
• Dolomitic lime – dolomitic lime (calcium and magnesium carbonate) raises soil pH.  It also adds both calcium and magnesium to your soil.

You will also see nutrient deficiencies when soil pH is too high.  Adding elemental sulfur or sulfates (iron or aluminum) lowers soil pH.

Remember: don’t add any soil amendments until after you have done a soil test.  Otherwise, you are trying to solve a problem that might not even exist.

Lack of Nutrients in Soil

If the pH of your garden soil is in the proper range (6.0 to 6.5 for tomato plants), you may still find a lack of nutrients in the soil.

A deficiency of any nutrient in the soil will prevent a tomato plant from absorbing enough of that nutrient through its roots.

A good place to start is by adding compost to your soil each year. However, some nutrient deficiencies may persist.

To fix a deficiency of nitrogen, phosphorus, or potassium, you can use an NPK fertilizer.  I explain what the numbers on a bag of fertilizer mean in this article.

You can also use supplements that are meant to provide a dose of a specific nutrient:

• You can learn more about high-nitrogen fertilizers here.
• You can learn more about high-phosphorus fertilizers here.
• You can learn more about high-potassium fertilizers here.
• You can learn more about high-magnesium fertilizers here.
• You can learn more about high-iron fertilizers here.

Excessive Nutrients in Soil

On the other extreme, there is another way to end up with a nutrient deficiency in your tomato plants.  If your soil has too much of a nutrient, it could stop a plant from absorbing another nutrient.

For example, let’s say you add too much lime (calcium carbonate) to your garden.  Then, you will have excessive calcium in your soil.

Calcium and magnesium are chemically similar.  So, they “compete” with each other in the soil for uptake by a plant’s roots.

With too much calcium in soil, plants cannot absorb enough magnesium.  This leads to stunted growth, since magnesium is the central atom in a chlorophyll molecule.

No magnesium means no chlorophyll, which means no photosynthesis, which means no energy production and no growth!

Lack of Sunlight

Tomato plants grow best with direct sun, and a lack of sunlight will also cause stunted growth in tomato plants.  Without enough light, plants cannot create enough energy by photosynthesis.

Without enough energy, plants cannot grow to their full potential.  A symptom of a lack of sunlight in young plants is a “leggy” appearance.

This happens when the plants grow tall and spindly in an attempt to grow taller to reach limited light.

You can avoid this by using grow lights to start tomato plants indoors until transplant.  After you put them outside, make sure that no trees or tall plants block their sunlight.

Pests

Pests are another possible cause of stunted growth in tomato plants.

Some pests damage the roots of plants.  This prevents plants from absorbing water and nutrients from soil, causing stunted growth.

Other pests, such as aphids, will drain liquids from the stems and leaves of plants.  This will also take away precious water from plants, leading to stunted growth.

According to the University of Minnesota Extension, a large number of aphids feeding on a plant can cause stunted growth and curled or wilted leaves.

You can learn more about how to prevent aphids in my article here.

Diseases

There are several diseases that can cause stunted growth in tomato plants, including Cucumber Mosaic Virus and Tomato Spotted Wilt Virus.

Cucumber Mosaic Virus

According to the Colorado State University Extension:

“Cucumber mosaic virus causes tomato plants to yellow and become bushy and stunted.”

https://extension.colostate.edu/topic-areas/yard-garden/recognizing-tomato-problems-2-949/

The virus can be carried in tomato seeds, which is why it is important to buy seeds from a trusted source.  The disease can also be spread by aphids or by touching plants that have the disease.

Since it also affects cucumbers, it can spread to the tomatoes in your garden if they are nearby.

Unfortunately, there is no cure for Cucumber Mosaic Virus, so you will need to remove and destroy any plants that are infected.

Tomato Spotted Wilt Virus

According to the North Carolina Cooperative Extension:

“Tomato plants infected with TSWV may at first appear stunted and pale.”

https://pender.ces.ncsu.edu/2020/03/why-are-my-tomatoes-dying/

You may also see black or brown spots, lines, or circles on the leaves.  Eventually, the leaves may take on a bronze color.

Unlike most other diseases, Tomato Spotted Wilt Virus affects the top leaves of the plant first.  The virus often spreads by thrips, which are very small insects that are hard to prevent.

Tomato Spotted Wilt Virus often spells the end for a tomato plant within a week of infection.  There is no cure for Tomato Spotted Wilt Virus, so you will need to remove and destroy any plants that are infected.

To prevent the disease, choose tomato varieties that have resistance.  For example, Mountain Glory tomatoes from Harris Seeds have resistance to Tomato Spotted Wilt Virus.

Herbicides

Herbicides can also cause stunted growth in your tomato plants.  If you use compost or manure in your garden, then you might be exposing your plants to herbicides.

The worst part is, you might not even realize it!  Here is an example of how it can happen.

First, let’s say your neighbor hires a landscaping company to take care of his lawn.

Next, they come by to mow the lawn and spray with an herbicide to prevent weeds from growing.

Then, you ask the landscapers for the grass clippings to put in your compost pile.

Later, the grass (sprayed with herbicides) becomes compost and is added to your garden.

Finally, the herbicides on the grass prevent your tomatoes from growing (since herbicides do not discriminate!)

Herbicides can also survive in an animal’s digestive tract.  So, if a horse eats hay that was treated with an herbicide, then the manure you put in your garden may still contain some of it.

Transplant Damage or Transplant Shock

Transplant damage is another possible cause of stunted growth in tomato plants.  It is often overlooked because it happens early in the season and is forgotten when problems emerge later.

If you plant tomato seeds too close together, the roots of the emerging seedlings will get tangled up.

Later, when you pull them apart to transplant outside, their roots are more likely to get damaged.  This damage is not obvious right away, but it will stunt growth later.

Transplant shock can also occur if you fail to help your tomatoes adjust to the outdoors.  When you start seeds indoors, they are accustomed to controlled temperature, moisture, and light.

Once they go outside, all bets are off!  This harsh transition can stress the plants, unless you make it a little more gradual.

Giving plants a slow adjustment to the outdoors is called hardening off.  You can learn more about how to harden off plants in my article here.

Conclusion

Now you have some idea of why your tomato plants are stunted.  You also have some ideas about how to fix the problem, or at least how to prevent it in the future.

Bacterial wilt can also cause stunted growth in tomato plants – you can learn more about it in my article here.

You can also learn about other causes of curled leaves on tomato plants in my article here.

You can learn more about everything you need to grow tomatoes in my article here.

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