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Master Gardener: Tomato plants might be overwatered
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Master Gardener: Tomato plants might be overwatered

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My tomato plants are starting to get some yellow leaves. What am I doing wrong? — V.S.

This is a question we have been getting a lot recently and the answer is that you are probably not doing anything wrong.

In normal circumstances, yellow leaves can indicate a nutrient deficiency in iron or perhaps nitrogen, but nothing about our spring has been normal.

Not only have we had cooler temperatures which works against the development of our summer vegetable crops, but we have also had a lot of rain. The Oklahoma Mesonet shows that we have had between 4 and 6 inches of rain depending on where you live in the past 30 days: a lot of this in the past two weeks.

Yes, plants need water, but too much water can have a negative effect on our vegetable gardens. Plant roots draw up water and nutrients into the plant and the water evaporates through the leaves in a process called transpiration. Sometimes during the summer (especially in plants with large leaves) water exits the plant via transpiration faster than the roots can draw water up to replace it. When this happens, it looks like you are under-watering your plants, but the next morning they look fine because they have had a chance to replenish the lost water. This situation can be kind of tricky because it looks like you need to water your plants (and you might) but overwatering also has negative effects.

Plant roots need air to breathe. Good garden soil is about 25% air, but overwatering via either our efforts or Mother Nature’s can push this air out of the soil. When roots get waterlogged, this affects the roots’ ability to draw up nutrients. This can result in yellowing leaves. If this persists, long-term damage can be done to the plant’s root system. In cases where the tomato is planted in a container and suffers from overwatering, the plant can die.

If your tomato plants are in containers, be sure there is adequate drainage. Drainage occurs through holes in the bottom of the container. If you purchased a container, they oftentimes come without drainage holes in the bottom because the manufacturer doesn’t know if you intend to use this container indoors or outdoors. Drainage holes when using the container indoors can get messy.

Fortunately, most of them have knock-out plugs that, with the help of a screwdriver and a small hammer, can be punched out to give you the needed drainage. If your container doesn’t have drainage holes or knockouts, you will need to drill some holes in the bottom to ensure good plant health.

If your tomato plant is in the ground, we kind of just have to wait for this rain to stop, but after we get through this rainy season, your tomato plant should start to look better.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

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