Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 1 of 3 (Management and Maintenance Issues)
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In horticultural lingo, yellowing of leaves on vegetable plants such as tomatoes is a “general symptom” or “generalized symptom,” meaning one that can result from many different causes.
The challenge with diagnosing plant problems from a general symptom, or even a few combined general symptoms, can be compared to trying to diagnose what’s wrong with a human who has a headache. Is the headache caused by muscle tension, a cold or infection, alcohol hangover or addiction withdrawal, eye strain, or something more serious like a fractured skull bone? The symptom of having a headache alone is not enough to diagnose, understand, and take steps to address the problem.
In the category of unprovable gardening theories and hot-summer-day surmises, I would guess that almost every gardener who has ever grown more than about 10 tomato plants has had at least a few yellowing leaves.
Even though a general and common symptom, yellowing leaves are almost always an indicator that something has gone wrong. It may be mild, temporary, or unlikely to affect fruiting and harvest; or it may be a serious or even terminal problem. One exception is minimal natural shading, as when a tomato plant grows tall or bushy enough to begin to shade its own lower leaves. See Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 2 of 3 (Abiotic, Environmental, and Soil Problems)
Below are common causes of yellowing tomato leaves that are caused by management or maintenance, with advice for diagnosing and resolving each problem. These are written for California gardeners but also apply to tomato-leaf-yellowing in other areas.
Overwatering: Plants that are regularly overwatered commonly develop yellowing leaves. If you never saw wilting leaves and are watering regularly, you may be overwatering. Too much water in soil reduces oxygen availability to plant roots, stresses plants, may prevent uptake of nutrients, and encourages soil-borne diseases.
Advice: If your soils are constantly wet (as opposed to moist, which usually allows sufficient air to remain in soil), reduce watering.
Underwatering: Tomato plants that are temporarily water-stressed will have wilted leaves that are still green. Tomato plants that are regularly or chronically water-stressed conserve water in roots and stems while allowing leaves to yellow, usually starting with lower and older leaves. The symptoms on tomatoes of overwatering and underwatering are similar. For plants stressed by underwatering, usually plants will wilt at least once before leaves begin to yellow. If water remains insufficient or with repeated wilting, yellow leaves will brown and die. If you have paid attention to your plants but have not seen them wilt and are worried that you aren’t watering enough, you may be overwatering. Wilting can also be caused by root damage or root disease even if watering is sufficient or appropriate. In some conditions, such as in loose, sandy, or other rapidly draining soil during hot weather, tomato plants may become water-stressed even when watered regularly.
Advice: If you watered irregularly, saw wilting, or have reason to believe that watering may have been insufficient before leaves yellowed, increase watering. Tomato plants prefer evenly moist soil with a slight dry-down between waterings.
Poor Drainage and Standing Water: This has the same effect as overwatering. It is possible for compacted soil or underground obstructions such as large stones to prevent drainage and for there to be standing water below the surface of soil while the soil surface can be dry. Soil and roots in containers can block drainage holes and allow standing water to collect.
Advice: Grow tomatoes in well-drained soils or containers. It’s usually difficult to improve poor drainage for established plants. In some cases reducing watering or breaking up compacted soils outside of tomato root systems may help. Conduct an infiltration/drainage test before planting tomatoes, or near established plants but outside the root zones to look for wet or muddy soil. If belowground soil is wet and smells swampy or otherwise unpleasant, it may have become anaerobic and temporarily toxic to plant roots. Investigate to be sure that water runs through containers and comes out drain holes.
Shock or Transplant Shock: Tomatoes are relatively tolerant of transplanting as compared to other vegetables, but they can be shocked by rough handling of roots, loss of roots, or major changes in environmental conditions, especially when transplanted. The lush, healthy nursery seedlings you purchased were likely grown under ideal conditions in greenhouses with temperature, humidity, fertilizers, and other conditions carefully managed by professionals. Moving plants from these conditions to the potentially harsh realities of your garden or yard, including full sun, hot or cold weather, unamended or dead soils, and a different watering schedule can easily cause shock to tomato plants. Loss of roots and root disturbance, which can be caused by everything from nearby cultivation or rototilling to harvesting companion root crops and removing weeds, can also shock tomato plants.
Advice: Handle tomato transplants with care. Harden off plants by putting them out in the area where they will be planted for progressively longer periods each day before transplanting. Avoid root disturbance as much as possible when transplanting and for established plants. Pay attention to changes in environmental conditions, especially extreme changes in temperature, watering, or sunlight, and take steps to protect plants.
Herbicides, Pesticides, or Chemicals: Many herbicides, especially broad-spectrum herbicides, cause leaf yellowing, either from direct contact or through absorption via plant roots, often producing an irregular or mottled pattern of yellowing. It requires only small quantities of some herbicides, chemicals, and substances on plants or in soil to cause yellowing and/or other harm. Herbicides and other chemicals may drift when applied on neighboring or nearby properties.
Advice: Use herbicides and chemicals with caution and under conditions that will minimize or avoid drift or other exposure to your tomato plants. If you believe herbicides or other harmful chemicals are arriving from a nearby property, inquire with neighbors. It may help to use row covers or other physical barrier placed between plants and the direction of drift, or between plants and prevailing winds or breezes.
See customized advice and information for growing tomatoes in your Southern California zipcode
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 2 of 3 (Abiotic, Environmental, and Soil Problems)
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 3 of 3 (Pests and Diseases)
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