Tomatoes are one of my favorite vegetables to grow. I love how they grow and how their leaves smell, and I love to cook with tomatoes too. Most summers, I grow a couple of dozen tomato plants and it’s still not enough. Of all the crops I grow each year, my household goes through tomatoes the quickest. Sauces, ketchup, soups: you name it, we tomato it. So there’s nothing worse than experiencing an issue with your tomato plants, like leaf curl and tomato leaves turning yellow or white. In the case of tomato leaves turning yellow, there’s good news: most of the reasons this happens can be fixed, leading to healthy, productive tomato plants.
If you’re reading this, chances are, you’re already experiencing a problem with yellowing tomato leaves. I feel for you! It’s happened to me before. I’d like to share what I know about this problem. In this article, we’ll explore some of the most common reasons I’ve found that tomato leaves turn yellow and how we can fix and undo the damage to your tomato plants.
The most common reasons for tomato leaves turning yellow
In my experience, there are 8 more common reasons for tomato leaves turning yellow, including overly shaded seedlings, improper hardening, nutrient deficiency, fertilizer burn, disease, and improper watering. The tough thing about gardening is that your plants can’t tell you what’s wrong, so think of this list as a diagnostic checklist. As we run down the list, we can rule out least likely reasons for yellow tomato leaves and land on the most likely culprit.
Poor seedling care is a big cause of tomato leaves turning yellow
If you’ve brought home some new tomato plants or grown them from seed and are discovering that your young tomatoes have yellow leaves, don’t fret. Sometimes when tomatoes are grown indoors or in a greenhouse setting, they get crowded by the leaves of other plants, which reduces the sunlight reaching each plant’s leaves. Yellowing is most common on lower leaves, as they receive the least amount of sunlight. This is pretty common among new tomato plants and isn’t usually a big deal.
Seedling crowding can be an issue as well. Many nurseries and gardeners will start a few tomato seeds in a single pod of potting soil and then later thin them out or separate them. Before the thinning occurs, the hungry tomato starts can quickly deplete the potting soil of its nutrients. We’ll talk a little bit more about nutrient deficiencies being a cause for tomato leaves turning yellow, but the issue of nutrient-deficient tomato starts will be resolved once you plant them outdoors and provide a bit more nutrients.
Insufficient ‘hardening’ of your tomato starts
The easiest mistake beginners make when growing tomatoes, and one I’ve made myself, is not properly hardening off your tomato plants before planting them outdoors. When tomatoes are started indoors or in a greenhouse, they experience a pretty consistent climate. Your indoor temperatures don’t fluctuate much, there’s no wind, and light is easily moderated. If we go from that controlled environment to the outdoors without a hardening off period, your tomatoes can be shocked by bigger temperature changes, wind, and sudden, full sunlight.
If you’ve purchased tomato plants and plugged them directly into the ground outside without this hardening off period, it could be a cause for your tomato leaves turning yellow. Damage due to not hardening off your tomatoes is hard to fix. They can sometimes recover but are usually set back by a few weeks. I don’t recommend digging your plants back up, that shock could be the end of them. Instead, you can use the old milk carton trick, where you cut the bottom off of a milk carton and place it over your tomato plants to act as a small greenhouse while they adapt to the outdoor climate.
Nutrient deficiency in the soil leads to tomato leaves turning yellow
Nutrient deficiencies are another reason for tomato leaves turning yellow. There are three common nutrient deficiencies that cause this.
When your tomato plants have insufficient magnesium, their leaves will turn yellow, but the veins of the leaves will remain green. If you suspect this is the problem, we have a much more comprehensive guide to fixing magnesium deficiency in your soil. I’d recommend giving that a read and following the steps on that guide.
If your plants are deficient in nitrogen, you won’t see the green veins like with magnesium deficiency. The entire leaf of a tomato plant will turn yellow if it’s not getting enough nitrogen. Adding a bit more nitrogen to your soil may help eliminate this problem, but don’t do too much. Nitrogen is great for healthy foliage, but too much can cause your tomatoes to grow excessive foliage at the expense of flowering.
Finally, a potassium deficiency causes yellowing tomato leaves, but they tend to yellow more at the edges of the leaf as opposed to the whole leaf. Eventually, the leaf will start to turn brown, almost like it’s been burned. Adding fertilizer with an adequate amount of potassium will help eliminate this problem and contribute to larger fruit.
Nutrient deficiency and nitrogen burn can be a one-two knockout for tomato plants if you aren’t careful. If you had yellow tomato leaves and thought it was a nutrient deficiency, adding too much nitrogen can cause the problem to get worse. Excessive nitrogen can cause what’s called nitrogen burn, which is where the leaves of your tomato plant begin to look burned, turning yellow, brown, and then dying off. If you’ve fertilized your tomatoes with a heavy nitrogen fertilizer and are experiencing nitrogen burn, don’t fret, but do act quickly.
You can begin to undo the damage of nitrogen burn by heavily soaking the soil around your tomato plants and flushing some of the nitrogen out of the soil. We’re talking a heavy watering though: 5-10 inches of water is what’s needed to flush out the excess nutrients. If your flush was successful, you should see healthier, new foliage begin to emerge at the top of your plants.
Some diseases can cause the yellowing of tomato leaves as well. Early blight, which is a soil fungus, will first turn leaves yellow, eventually leading to brown spots. Septoria leaf spot is another fungus that has similar leaf yellowing symptoms. Both diseases, if left untreated, can kill your tomato plants.
Fusarium wilt, Verticillium wilt, and Bacterial wilt are diseases that negatively impact the plant’s roots, making it more difficult to uptake water and nutrients to the leaves, stems, flowers, and fruit.
The difficult part of dealing with tomato diseases is that they often look the same, making a positive diagnosis difficult. If it’s one of the wilts, your plant unfortunately will not survive and needs to be discarded immediately to prevent spread. For the fungal diseases, simply remove the impacted leaves and apply a fungicide.
Making sure you water your tomato plants the right way is key to growing healthy tomato plants. The wrong way to water tomatoes is by providing a little bit of water every day. This trains your tomatoes to grow shallow roots, which can lead to plants that fall over and are more susceptible to dry periods. What you want to do is water deeply every week or two. This trains your tomato plants to grow nice, deep, resilient roots.
Underwatering can be a reason for tomato leaves turning yellow. Watering too little means your plants aren’t getting enough water to uptake nutrients and grow.