Growing tomatoes can be a little tricky, especially if it’s your first time. One the most common problems that can occur with tomato plants is yellowing leaves. These yellow leaves can be cut off the plant without causing damage if they are removed below the first set of flowers.
This article will explore more about what causes yellow leaves on tomato plants, how you can prevent this problem and when to remove the leaves.
What causes tomato leaves to turn yellow?
Yellow leaves on a tomato plant can be caused by a number of things including:
- Nutrient deficiencies
- Plant diseases
- Herbicide damage
One thing you should remember is that young seedlings are more likely to develop yellow leaves than mature plants. This is especially true with tall and gangly plants that are sold planted in a small pot. Once they run out of room and nutrients the plant can suffer and develop yellow leaves.
When to remove yellow leaves on tomato plants
There are times when it is best to remove the yellow leaves on tomato plants, other times it can be best to leave them on. Check out when you should prune off tomato leaves below.
1. When the leaves have started to turn brown
Yellowing leaves that have started to turn brown on the edges tips or center of the leaf will be damaged beyond repair. These can be pruned off near the base of the stem, snipped cleanly with secateurs or snapped off with your finger tips.
2. If there is fungal or bacteria growth
Any time there is a fungal disease such as sooty mold or if there are bacterial leaf spots the leaf should be cut off. This will stop the leaf from touching other healthy leaves and will stop any disease spreading.
When not to prune off yellow leaves on tomato plants
Here are the top times you should leave yellow leaves on your tomato plant. If the leaf is yellow and the problem is not caused by a communicable fungus or bacteria the leaf can recover.
1. When the problem is caused by nitrogen deficiency
If your tomato leaves have turned yellow due to nitrogen deficiency you do not need to prune them off. Leave them on the plant and feed the plant with a nitrogen based fertilizer. The leaf itself can return to green and continue to grow if the problem is caught early.
2. When the problem is caused by iron or magnesium deficiency
If the plant has yellow leaves while the veins are still green it could be a nitrogen or magnesium deficiency. The leaves can recover from these problems if it is given iron chelates for iron deficiency, Epsom salts for magnesium deficiency or an all purpose fertilizer that contains these nutrients.
Once the nutrient deficiency has been fixed the leaf can recover and return to green.
3. When the plant only has 2-4 mature leaves
Do not cut off yellow leaves if the plant is young and it only has 2-4 mature leaves. Removing the leaves can stunt the plant’s its growth or kill the plant altogether. Plant them out out in full sun, in well-draining soil improved with compost and water it well.
Young plants will often develop yellow leaves due to a nitrogen deficiency. Planting them out in good soil is usually the easiest way to solve the problem.
Top causes of yellow leaves on tomato plants
Here are the top causes of yellowing leaves on tomato plants. Read through each one to see what could be affecting your plants.
1. Cramped pots
If you buy tomato plants from a nursery you might find yellow leaves on some of them. This is because they often plant too many seeds in one pot. The seedlings quickly run out of room and nutrients, particularly nitrogen and their leaves can turn yellow.
These seedlings can be saved and if you plant them in soil improved with compost and aged cow manure and water them well. Feed them with a gentle fertilizer, like pelleted chicken manure and their leaves can green up and the plant can thrive.
The easiest way is to choose healthy seedlings from our local garden center. Avoid seedlings with yellow leaves and yellow or brown spots as this could be a sign of disease.
2. Lack of nitrogen
Tomatoes are heavy feeders, which means they need a lot of nutrients to grow and produce fruit and leaves. Young tomato plants that are not getting enough nitrogen can develop yellow leaves.
Nitrogen deficiencies usually result in the entire leaf being yellow. Too much nitrogen, on the other hand, will cause the plant to grow lots of green leaves and less flowers and fruit.
To avoid young plants developing yellow leaves, plant them in improved soil. I like to add aged cow manure and compost. These are mild fertilizers that contain nitrogen but not in high amounts. If you have particularly poor soil, add a handful of pelleted chicken manure for each plant which will give them longer lasting nitrogen.
Once flowers emerge from the plant, do not add any more nitrogen fertilizer. This will allow the nitrogen to phosphorus ratio to decrease and for the plant to focus on flower and fruit production.
3. Lack of potassium
If your plants are low in potassium, it usually results in the edges of your leaves turning yellow, then brown, then they’ll have a burnt look. Add fertilizer that is higher in potassium like sulfate of potash. This can be sprinkled around the root base and watered in.
4. Lack of magnesium
Magnesium deficiency can cause yellow leaves with green veins on your tomato plants. A magnesium deficiency can be fixed by mixing two tablespoons of Epsom salts with a gallon of water and spraying this on the plants. If the leaves start to turn green within a week, you will know that magnesium was the problem.
5. Iron deficiency
If tomato leaves lose their color, the leaves will look light green, yellow even white. Iron deficiency can be treated with iron chelates watered onto the plant. You could also choose a general tomato or fruit fertilizer that contains iron.
6. Soil pH
In addition to checking the soil for nutrients, you should also check the pH number of the soil. Tomatoes grow best in soil that has a pH level of 6.5.
7. Fungus, bacteria or insects
Bacterial or fungal disease can cause damage to tomato leaves turning them yellow. Soil-born fungus such as Fusarium oxysporum will quickly damage the leaves turning them yellow and then brown.
I always like to remove the leaves affected by any fungus or bacteria as soon as possible. This will help to reduce the spread and reduces my need to spray the plant with fungicides.
How to prevent yellow leaves on tomato plants
Here are my top tips to prevent yellow leaves on tomato plant and help them to grow and thrive.
1. Consistent water
Make sure you water the plants consistently. Instead of adding a little bit of water daily, water deeply about twice a week. Let the water penetrate about a foot into the soil. Deep and infrequent watering is always best for tomato plants.
2. Avoid herbicides
Don’t use herbicides, even if you notice weeds near your tomato plants. Instead, you can weed by hand and even use mulch to make sure the weeds don’t become a problem.
3. Watch for sunburn
Watch out for sunscald, or sunburn. This can be a problem with both newly planted tomatoes and even older plants. Never leave tomato plants in the direct sun but instead make sure that they have some shade. They need warmth from the sun but not direct sun, in other words.
4. Avoid over feeding
When you use fertilizer, always follow the directions. This will prevent fertilizer burn and will make sure the plants get the nutrients they need. If the directions mention “sidedressing,” this means you need to apply the fertilizer 4-6 inches away from the plants instead of directly on the plants.
5. Don’t dig in mulch material
Avoid digging unfinished compost, fall leaves, bark mulch or hay in the soil. Be careful with your organic matter and make sure you don’t use too much of it. Too much sawdust or leaves, for example, can cause the soil to be temporarily depleted of important nitrogen. Soil bacteria will absorb nitrogen from the soil as they break down the mulch.
Yellow leaves on tomato plants can happen due to nutrient deficiency, pest or fungal attack or even a lack of water. Check the soil, leaves and stems to see if there are any pests attacking the plant. Fed it with a good quality organic fertilizer and water it well to help it recover.
I am an accredited practicing dietitian, experienced gardener and a dedicated cook. I love writing and sharing my experience so you can learn from my successes and mistakes.