9 Reasons Why Your Tomato Plant Has Stunted Growth

Although tomato plants are some of my favorite crops to grow in the garden, they present their fair share of challenges. During my ten-plus years of growing these popular plants, I’ve seen tiny seedlings that barely seem to grow and vibrant plants that suddenly stop growing. And I learned there was more than one possible cause of these stunted plants!

If you want to grow a big crop of juicy fruits, the first step is growing healthy plants. However, this is often easier to say than do! If you want to know why your tomatoes are stunted, stay with me to learn nine possible reasons why your plants are short and weak.

Cold Temperatures

Every gardener can relate to the itch to get plants in the ground in the spring. The first warm spell arrives, and we haul out our seedlings and watering cans despite the deep-seated knowledge that colder weather will return. While planting during cooler temperatures is fine for spring crops like lettuce and kale, it will set back your tomatoes.

Most people know that frost will turn healthy, green tomato plants into brown, crispy shells of their former selves, but not everyone knows that cool temperatures also have problematic consequences, like stunted growth. Tomato plants like temperatures that remain above 50°F (10°C) during the day and night. Even if temperatures rise into the 70s or 80s (20s Celsius) during the day, nighttime temps in the 40s (~4°C) will stall your tomato plants’ growth.

Therefore, planting your tomatoes earlier in the year doesn’t necessarily result in earlier harvests. To avoid cold-stressed plants, wait until nighttime temperatures are consistently above 50°F (10°C). If you’re planting in a protected structure like a high tunnel, use the temperatures in the structure rather than natural outdoor temperatures as your guide.

Too Small of a Container

Starting tomatoes from seeds offers all kinds of benefits. It allows you to grow unique varieties you can’t find in local nurseries, allows you to grow the seeds you saved from last year’s crop, and can even help you save some money. However, stunted seedlings are a disappointing yet common occurrence.

There are numerous reasons why seedlings become stunted, but small containers are one of the big ones. If your seedlings look healthy and green, then suddenly stop growing or begin turning yellow, they may be rootbound. This phenomenon occurs when the plant’s roots outgrow the pot and begin growing along the container’s edges.

While the ultimate goal is to avoid rootbound plants, you should transplant or up-pot your seedlings as soon as you notice they’re rootbound. The plants will take a few days to settle into their new home, but once they do, the roots will start growing freely. Once this happens, the plant’s stem and leaves will also resume growing.

Keeping an eye out for rootbound plants is especially important if you start your seeds in small trays to save space. While there’s nothing wrong with planting tomato seeds in 128-cell or 72-cell trays, you should bump up the seedlings to larger cells or containers once their roots reach the edges of the cells. I find that tomatoes need to be up-potted every two to four weeks if they’re growing in ideal conditions.

Sudden Changes in Temperature

Even when daytime temperatures reach the 80s and 90s, tomatoes can still experience temperature-related stress. That’s because sudden swings in temperature shock the tomato plants’ systems, resulting in stunted growth. You know how the first 80°F (27°C) degree in May feels much hotter than a similar day in July because our bodies haven’t acclimated to the heat? Well, tomato plants operate similarly.

If you experience a sudden cold snap or early heat wave, your tomatoes may show slowed growth in the week ahead. Once temperatures stabilize, the plants will resume normal growth.

Another type of temperature swing occurs when you move tomatoes from indoors to outdoors. Hardening off is a key step if you want to limit the stress associated with this shift. This process involves slowly increasing the time your indoor-grown tomato seedlings spend outside.

I like to harden off my tomato seedlings over the course of a week by letting the plants spend a few hours outside the first day and a few more the next until they spend an entire 24 hours outdoors. At this point, they’re ready to head into the ground.

If you choose to grab your tomato seedlings from a greenhouse or an indoor set of grow lights and place them directly in the ground, expect the plants to protest. Not only are they dealing with much colder temperatures, but they’re also facing intense sunlight, wind, and a new type of soil. These factors lead to stress and, therefore, stunted plants.

If you don’t have the time to fully harden off your plants, at least let them spend a few days outdoors in pots before planting them in the ground. This will help limit stress and, therefore, stunted growth.

Lack of Nutrients

Tomatoes are heavy feeders, requiring large amounts of nutrients to thrive. They require lots of nitrogen, phosphorus, and potassium to thrive, along with smaller doses of secondary nutrients and micronutrients. While potting mixes and native soils typically contain some of these essential elements, you’ll eventually have to add fertilizer to supply your plants with the necessary nutrients.

If you plant your tomatoes into a raised bed filled with coco coir, perlite, and other nutrient-poor materials, the plants will soon become stunted and show other signs of nutrient deficiencies. These same symptoms can occur when growing seedlings in pots and raising tomato plants in native soil.

Since tomato plants require a wide swath of nutrients to thrive, and many deficiencies result in stunted growth, it can be difficult to determine exactly which nutrient your plant lacks. A soil test can help, as can knowing the signs of specific nutrient deficiencies. Here are the signs of a few common tomato nutrient deficiencies.

If you’re still unsure which nutrients your soil lacks, apply a tomato fertilizer to cover all your bases. You can find many different types of fertilizers designed for tomatoes—there’s granular and liquid, slow-release and quick-acting, high nitrogen and high potassium, and, and, and. The trick is choosing a product you feel comfortable with and applying it following the product instructions. 

Over-applying fertilizer can lead to serious harm, so resist the urge to douse your nutrient-starved plants in an attempt to help them recover. It’s also important to note that many plants are past the point of recovery once they show extreme signs of nutrient deficiency. Therefore, it’s best to fertilize regularly to prevent any deficiencies.

Root Rot

Most people are more interested in juicy tomatoes and healthy green foliage than they are in the plants’ roots. But healthy roots are an essential part of healthy plants! Unhealthy tomato roots often lead to issues with water and nutrient uptake, which contribute to stunted growth.

One of the major issues tomato roots face is root rot. This is a generic term people use to describe numerous fungi that attack plant roots and cause them to become limp, slimy, and/or stunted. Since root rot is more likely to occur in moist conditions, overwatering often precedes this disease.

Treating root rot is easier than preventing it, so make sure to avoid overwatering and avoid planting your tomatoes in areas where previous plants were infected. If your plants develop this disease, decrease the amount you water.

Not Enough Light

Tomato plants require full sun; there’s no way around it. If you plant your tomato plants in an area with dappled light or partial shade, expect to end up with stunted plants. While a few hours of morning or evening shade won’t harm your plants, plant them in an area with at least ten hours of direct light. Since plants rely on light energy to convert water and carbon dioxide into the sugars they use to grow and support vital processes, too little light means the plants can’t grow properly.

The same logic applies to seedlings grown under grow lights or in an indoor area without enough light. Weak grow lights or lights that are too far away from your plants often lead to stunted seedlings. Sometimes, the seedlings become elongated as they stretch towards the light. While this “leggy” growth makes the plants appear tall, their thin stems and small leaves will signal that they’re unhealthy.

If you plan on starting tomato seeds indoors, ensure you have the proper setup to provide them with enough light. You should place most grow lights just a few inches above the tops of the seedlings to make sure they receive enough light energy.

Incorrect Soil pH

Even if the soil contains all the nutrients your tomato plants need to grow and thrive, improper soil pH prevents the plant from accessing these nutrients. That’s because soil pH greatly impacts plant nutrient availability. Plants grown in soil with a pH that’s too high or too low can look similar to plants grown in nutrient-poor soil.

The ideal soil pH for tomatoes is between 6.0 and 7.0. Since a neutral soil pH is 7.0, tomatoes prefer a slightly acidic to neutral pH. While plants won’t experience extreme symptoms if they’re grown in soil a bit outside the ideal range, a pH below 5.5 or above 7.5 will show some signs of nutrient deficiencies. 

Plants will have difficulty accessing iron, calcium, manganese, copper, and zinc if the pH is too high. A pH above 8.0 also limits tomato plants’ ability to access nitrogen. A pH that’s too low means plants will have difficulty accessing molybdenum, phosphorus, calcium, and magnesium.

Since altering soil pH is a slow process, it’s best to test your soil the year before you plan on planting. Adding lime raises the pH, and adding sulfur lowers the soil pH. If you don’t have time to correct the pH, you can mix a potting mix with low pH into a soil with high pH or potting mix with high pH into soil with low pH.

Not Enough Water

Like all plants, tomatoes need water to remain healthy. If they can’t access the water they need, they’ll face numerous issues. Wilted leaves can be an obvious sign that your plants need more water, but there are other subtle signs.

When tomatoes don’t receive enough water, they experience significant declines in photosynthesis. This decreased photosynthesis leads to weaker plants with stunted growth. Maintaining consistent, moderately moist soil will allow for high rates of photosynthesis and healthy plants.

Water also helps with essential plant processes like transpiration (how a plant cools itself), nutrient and sugar movement, and proper turgor pressure inside of cells. In short, if tomato plants don’t have enough water, they can’t remain healthy and grow properly.

The ideal irrigation schedule depends on lots of factors, including temperature, humidity, soil type, and plant size. In general, you should water your tomato plant at least a few times a week if no rain falls. Setting up a drip irrigation system and putting it on a timer can help keep your plants well-watered if you’re traveling or just tend to forget about watering.

Poor Soil

Many people are concerned with soil nutrients, but other soil factors influence tomato plant growth. If these factors are out of whack, the plants can become stunted and unhealthy in other ways. Before you plant your tomatoes in the ground, take a good look at the soil structure and any added inputs.

First, make sure the soil is loose. Compacted soil limits water drainage, which increases the likelihood that roots will develop disease. It also limits aeration, so plant roots can’t complete the necessary gas exchange. And when the soil doesn’t contain enough air space, plant roots have a difficult time growing.

Another thing to look out for is any addition of organic matter. While fully decomposed compost provides your plants with a boost of nutrients and beneficial microbes, there is such a thing as bad compost. If you add carbon-rich compost that hasn’t fully broken down, the compost will rob the soil of nitrogen for as long as a few months. 

I hate to admit it, but I’ve made this mistake myself! The result was plants that looked like they’d been frozen in time. Although the grass grew and tree canopies shaded my yard as spring changed to summer, my plants remained the same size and eventually turned yellow. Once I figured out what was causing the severe stunting, I was able to remedy the problem by adding nitrogen-rich blood meal. However, I recommend avoiding this problem from the start!