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.

Tomato leaves turning yellow
Yellow tomato leaves.

Nitrogen 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. 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.

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.


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.

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!


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.


Blight

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 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 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.

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.


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 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), you can take measures to protect the plants. If you have a garage or shed, you can move 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.


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.

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