Why Are My Tomato Leaves Turning Yellow?

If you have grown tomatoes before, you have likely run into some problems. In this article, I’ll discuss one of the most common issues, tomato leaves turning yellow.

Thankfully, yellowing tomato leaves is usually not a major concern. There are several possible causes for yellow leaves on your tomatoes, and I’ll go through each of them here.

In addition, I will share some advice on how to deal with each issue to fix your yellow leaf problem as soon as possible! Let’s get started.

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Tomato leaves turning yellow
Yellow tomato leaves.

1. Nutrient Deficiency

The most common cause of tomato leaves turning yellow is a nutrient deficiency. Though there are several possible deficiencies to blame, nitrogen is by far the most likely.

Nitrogen is the most essential nutrient for all plant life, and it is responsible for healthy foliage growth and a deep green color. It is also the most likely nutrient to become deficient in your soil.

Without enough, tomato leaves will become pale green and eventually yellow. If the deficiency becomes severe enough, the leaves may die and fall from the plant.

How to Know it is Nitrogen

There is a simple trick to determine if your tomato leaves are yellow from a nitrogen deficiency. If a lack of nitrogen is to blame, the yellowing will begin at the bottom of the tomato plant and spread upwards over time.

Yellow tomato leaves on bottom of plant.
Yellow tomato leaves on bottom of plant.

The reason for this is that nitrogen is a mobile nutrient, meaning that plants can move the nutrient around the plant as needed. When nitrogen is running low, the tomato plant will take the remaining supply from older leaves at the bottom of the plant and move it up towards new foliage.

If you see that the lower leaves are yellow and the upper, younger leaves are not, your plant likely requires nitrogen. However, another mobile nutrient can cause yellowing, though it is less commonly the cause.

Magnesium vs. Nitrogen Deficiencies

Magnesium will cause yellow tomato leaves, starting at the bottom, but the pattern of the yellowing will differ from nitrogen. There is a simple method to distinguish a magnesium deficiency from nitrogen.

  • Magnesium deficiencies will result in the leaves turning yellow between leaf veins. The veins will remain green.
  • Nitrogen deficient leaves will turn yellow entirely, veins and all.

If you suspect a magnesium deficiency rather than nitrogen, try amending the soil with all-natural epsom salt, or a diluted cal-mag spray. Learn more about magnesium’s role in plant life here.

How To Fix a Nitrogen Deficiency

The first and most important step is to feed your tomato plants. Tomatoes are fast-growing plants that require lots of nutrient to develop properly. If nitrogen is lacking, the plant will eventually become stunted and will not grow more branches or leaves.

To feed a nitrogen-lacking tomato plant, use one of our recommended fertilizers as soon as possible. In the future, we would recommend amending your soil before transplanting tomatoes into it. When the soil is fertile and alive with beneficial bacteria and nutrients, the plants will do best.

Recommended Fertilizers:

Note: Yellow tomato leaves will not turn back to a green color. However, adding nitrogen should stop the spreading of yellow leaves and normal growth should resume.

If adding fertilizer doesn’t seem to be the right solution for you, there are several other possible causes for your plant’s yellowing leaves.


2. Overwatering

When tomato plants receive too much water, a variety of issues will appear. One of them can be yellowing leaves. Without adequate drainage, the root systems of your plants essentially begin to drown.

Watering tomato plant at base
Watering tomato plant at base.

I recommend doing your best to evenly water your tomatoes throughout the season. This means not allowing the plants to become overly dry, but never soaking the plants.

Sometimes, overwatering is out of your control. An abnormally heavy rainfall may put stress on your tomatoes.

Do whatever you can to allow for better drainage. If you are growing your tomatoes in pots, make sure that the water can drain out the bottom (remove the bottom saucer). If planting in the ground, consider planting on a mound to allow water to flow away from the root system.

Most importantly, learn when your tomatoes actually need water. Root systems can run very deep for in-ground tomatoes, meaning that water can be drawn from far underground before watering becomes necessary. If it doesn’t rain much near you, try to stick to a schedule (one light watering per week, maybe more if it is especially hot).

Overwatering is a common mistake for growers of all types of plants. Yellow leaves are one result from it, but splitting tomatoes is another!


3. Poor Airflow

When airflow is limited, tomatoes become very vulnerable to issues. The plants will have difficulty transpiring moisture from their leaves, inviting disease and blight.

Yellow leaves may be a result of this poor aeration. I have noticed that plants that have not been bottom-pruned will often have spotty yellow leaves around mid-summer.

The solution is simple: prune unnecessary branches. By keeping the bottom 12-18″ of branches trimmed, your tomatoes will breathe easy. Your plants will also look much more well-maintained instead of becoming bushy and messy.

Bottom-pruned tomato plant.

In addition to bottom-pruning, I recommend periodically trimming higher leaf branches. As long as the plant has enough remaining leaves to photosynthesize, the fruits will still develop properly.

It can feel cruel, pruning away perfectly good foliage, but don’t worry. Your tomatoes can handle pruning, and in the end will be healthier for it.

Read all about pruning tomatoes properly here.


4. Poor Drainage

Soil drainage is incredibly important for tomato plants. If the soil is compacted or is heavy in clay, it may not drain well, leading to uneven water availability.

If you plant directly in the ground, you should perform a soil drainage test to see where you stand.

Drainage test:

  • Dig a small hole in your garden about 18-20 inches deep and fill it with water, allowing to drain overnight.
  • The next day, fill it again and measure the depth each hour until it is mostly empty.
  • If your soil drains <1 inch of water per hour, you have poor drainage.

Better drainage will help all of your vegetable plants, not just tomatoes. If your soil drains too slowly, try some of these methods to improve drainage.

  • Avoid walking on garden soil (especially when it is wet). Walking on your garden soil will compress the soil, collapsing air pockets and leading to poor drainage. Design your garden with walkways and always avoid walking where crops will be planted.
  • Add compost to soil each spring. Compost helps improve aeration by adding fluffy, nutrient-rich material to the surface. As it breaks down, it improves drainage (and many other things!).
  • Keep plants in the soil. Raw soil will erode and dry out, making it hydrophobic and difficult to use. Proper plant spacing helps make sure none of your soil is exposed to direct sunlight. Plant cover crops in the off season, and make sure tomatoes are properly spaced to avoid this problem.

5. Blight (and Other Disease)

If you’ve grown tomatoes before, you are likely aware of blight. This disease thrives in moist conditions, so the above two solutions will help fight against it.

Essentially, when your tomato leaves and branches are crowded, there is less room for water to evaporate. This creates the perfect environment for blight to set in and take hold.

Yellow tomato leaves are less common from blight, more often appearing as black spots on leaves and branches. However, yellowing is often seen in addition to the black spots.

Black and yellow spots from tomato blight on leaves and branches.
Tomato blight on leaves and branches.

Again, the solution is to provide better aeration. Do this by bottom pruning and regularly trimming excess foliage throughout the season.

Also, be sure to provide adequate space between each tomato plant in your garden. The ideal spacing for tomato plants is around 2-3 feet from each other, depending on the size of the variety.


6. Cold Weather

Towards the end of the season, as temperatures drop, tomato plants will begin to stress. If you are expecting particularly cold weather, some leaves may turn yellow as they die off.

Cold weather will cause tomato leaves to turn yellow more randomly around your plants. This ‘thinning out’ pattern is different from a nutrient deficiency, which is more linear.

If you are expecting cold weather (below 50°F/10°C), you can temporarily protect the plants. If you have a garage or shed, move any potted plants indoors overnight. Otherwise, you can use garden cloth to cover and protect the plants temporarily.

As the ground cools, the roots will suffer as well. To prevent damage, add mulch around the base of your tomatoes to insulate the ground and keep it warm.

These methods will help you elongate your growing season by a few weeks. This is a great way to encourage the plant to ripen up its final fruits before the winter sets in.


7. Seedlings Turning Yellow

When a tomato seedling sprouts, the first set of leaves are called ‘cotyledons.’ These are not true leaves, and are meant to get the plant off to a good start.

Transplanting tomato seedlings into soil
Young tomato plant with yellowing cotyledons.

After a week or two, true leaves begin to form, and the seedling leaves are no longer required for growth. As a result, the cotyledons usually turn yellow and fall off.

This is completely natural and is nothing to worry about! For new growers, This can cause some worry about the health of the young plants.

But rest assured, the seed leaves turning yellow on young tomato plants is to be expected. Don’t be mistaken into thinking it is a nutrient deficiency.


8. Sun Scald

If you start your tomatoes from seed, moving the plants outdoors should be done slowly. Sun scald is when your plant’s leaves become burned from direct sun exposure.

Sun scald usually appears more white than yellow, but in extreme cases, the leaves can die and fall off. Thankfully, you can avoid sun scald by hardening off your tomatoes properly.

Sun scald on tomato leaves
Sunburned tomato leaves with curled edges.

Young plants are highly susceptible to getting sunburnt. As the weather is warming up, slowly transition the plants outdoors, increasing the amount of direct sunlight they receive each day.

After a few weeks, the plants will be ready to handle full sun conditions without burning the leaves. Plus, newer foliage that forms will be much more robust and able to handle the sunlight very well.

Sun scald isn’t too much of an issue, as tomatoes are fast growers. Burn foliage will soon be replaced with newer, stronger leaves that will resist sun scald.


9. Pests

Another possible cause of tomato leaves yellowing is a pest infestation. There are a wide variety of insects that feed on tomato plants, any many of them can lead to yellowing and dying leaves.

Aphid on tomato leaf
Aphid on tomato leaf.

It is important to be watchful for pest problems in early spring. This is when many species begin to return, including aphids, whiteflies, spider mites and more.

If you notice curled leaves, yellow spots, or other abnormalities on your new foliage, you may have a pest problem.

Solutions:

  • Plant a variety of other plants and flowers. Use beneficial companion plants to encourage good bugs in your garden. There is no better way of controlling a pest problem than by attracting the pest’s natural predators!
  • Use insecticidal soap. If you can’t plant other varieties or are growing indoors, you may need to resort to killing off the pests. Insecticidal soap is the safest way to kill many types of pests without damaging your plants. Be sure to dilute it and cover the leaves top and bottom.

I hope this article helps you figure out why your tomato leaves are turning yellow. Thankfully, yellowing tomato leaves is not a death sentence! With these tools and techniques, your plants will be back to thriving in no time.

Calvin author picture

Hi! I’m Calvin, and I love growing my own tomatoes and other veggies at home. It seems my garden grows in size every year! Learn more about me here.

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