Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 2 of 3 (Abiotic, Environmental, and Soil Problems)
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Below are common causes of yellowing tomato leaves that are caused by abiotic (non-biological), environmental, and soil problems, with advice for diagnosing and resolving each problem. These are written for California gardeners but also apply to tomato-leaf-yellowing in other areas.
Shading and Natural Shading: Tomatoes and many other herbaceous plants regularly abandon leaves and grow new ones for various reasons including to maximize exposure to sunlight and feed themselves efficiently. Lower leaves or any leaves that don’t receive enough sunlight may turn yellow, brown, and die. A bushy or densely growing tomato plant in particular may shade its own lower leaves. Increased or complete shading of tomato plants will cause larger proportions of leaves to turn yellow, and eventually may result in spindly, leggy plants that grow as tall as possible in an attempt to reach sunlight. Common causes of shading in gardens include taller or vigorously growing nearby plants (including other tomato plants), trellising or fencing, and seasonal changes in sunlight that shift shading patterns from plants or structures as the sun rises higher or lower in the sky.
Advice: A small portion of lower leaves yellowing on tomato plants is usually not cause for concern when the plant otherwise appears healthy and you can confirm that yellowing leaves are mostly or entirely shaded. Plan carefully for seasonal changes in sunlight when first planting tomatoes. Space plants sufficiently for minimal shading when plants are mature.
Sunburn: Dark-green leaves on tomato plants usually indicate insufficient sunlight, too much nitrogen fertilizer, or another source of excessive nitrogen; while pale or light-green leaves on a plant in full sun often indicate too much sunlight. Sunburn often occurs on tomatoes in full sun during hot weather, or when sun/shade conditions change, such as after seedlings are transplanted or when a container plant is moved into full sun from partial shade. Plants can also be sunburned due to normal seasonal changes in sunlight (the sun moving higher or lower in the sky seasonally may drastically change sunlight conditions). Sunburn often starts with yellowing on leaves and if extreme proceeds to white or brown dead areas, and can also cause lesions, yellowing, or white areas on tomato fruits. It usually occurs in a relatively irregular, grouped or semi-spotting pattern on each individual leaf, and will occur only on leaves that are overexposed to sunlight. Leaves lower down or partially shaded on a plant won’t show sunburn.
Advice: Plan carefully for sun conditions before planting tomatoes. If sun conditions in your yard post a challenge, consider growing tomatoes in containers on wheeled carts or wheeled plant stands and move plants to shade during hot weather. Provide sufficient water and shade during hot afternoons. See GardenZeus Tips for Shading Vegetables During Hot Weather
Frost: Some mild-winter California areas (Such as GardenZeus CA Zone 12: Southern Inland, More Extreme; and GardenZeus CA Zone 13: Southern Inland, Less Extreme) may be subject to overnight frosts from November or December through February or March. Symptoms of frost damage to tomatoes include severely wilted leaves that turn brown or black, especially at the tops and outside edges of plants, and varying degrees of yellowing and browning to other leaves depending upon severity of frost damage. Tomato plants with short or intermittent exposure to frost, or with mild or moderate frost damage only, usually recover and can still yield well.
Advice: Monitor weather reports and cover tomato plants overnight with sheets of cloth or plastic, cardboard, or similar when frost is expected or temperature may drop into the 30s.
Chlorosis: Chlorosis caused by iron deficiency is one of the most common problems that occurs with cultivated plants in California. Symptoms of chlorosis in plants appear as interveinal yellowing on leaves, with new leaves being pale-green or yellow. Iron deficiency in plants often occurs when there is sufficient iron in soils, but the iron is chemically unavailable to plants because of soil alkalinity.
Advice: Apply chelated iron fertilizers, acidify soil, and/or encourage a living soil that will naturally manage soil pH. See related articles:
Down and Dirty 5: What is the Mother of All Southern California Landscape and Garden Problems?
Down and Dirty 6: No Tomato Harvest When You Did Everything Right
Down and Dirty 7: Mudpies and Fizz: Easy Home Tests for Soil pH
Beware of overly simple or blanket gardening advice! Nitrogen deficiency may be so commonly suggested or assumed as the reason for yellowing leaves on tomato plants that this advice may lead to too much nitrogen fertilizer being applied and plants being harmed by salt burn. Symptoms of nitrogen deficiency are among the easiest to recognize on tomatoes and other herbaceous plants, and are the inverse or opposite of iron chlorosis, with new leaves being darker green and older leaves being paler green, yellow, or interveinally yellow. Nitrogen is a mobile nutrient, meaning that plants can move it from older leaves to newer leaves, and tomato plants readily do so to grow new leaves. Only in extreme cases of nitrogen deficiency will the majority of leaves on a tomato plant turn yellow, and even in these cases the newest leaves will be greener.
Advice: Fertilize with nitrogen. GardenZeus recommends diluted urea or a cup of chicken manure diluted in 4 gallons of water (half cup if fresh manure) and mixed thoroughly, applied as a soil drench about every 2 to 4 weeks during the growing season or until hot weather arrives. Adding too much nitrogen may result in rapid growth and lush, high-carbohydrate leaves that attract insect infestation; and slowed or reduced yields.. Note: brown leaf tips and edges often indicate salt burn from too much nitrogen or other fertilizer, and can be caused by other sources of nitrogen or soil salts such as pet urine or wild-animal urine.
Other nutrient deficiencies such as of magnesium, potassium, sulfur, and calcium may cause leaf discolorations, yellowing, brown/necrotic leaves or leaf areas, and similar symptoms. Tomato-leaf yellowing from nutrient deficiencies occur commonly when tomatoes or other heavy feeders are planted repeatedly in the same beds or areas, when soils are infertile or exhausted, or when soil pH is alkaline (or highly acidic, which is rare in California urban soils). Exact nutrient deficiency may be difficult or impossible to determine with a visual inspection, even by experts or professionals.
Advice: If your soil is well amended with compost and organic matter, it’s possible but unlikely that you have serious nutrient deficiencies. Test your garden soil regularly, every year or two, and amend or otherwise address problems and deficiencies. Learn how to encourage and maintain a thriving microbial ecosystem in garden soil. Avoid repeated plantings of tomatoes or other heavy feeders in the same beds or areas.
See customized advice and information for growing tomatoes in your Southern California zipcode
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 1 of 3 (Management and Maintenance Issues)
Yellow Leaves on Tomato Plants: A GardenZeus Guide, Part 3 of 3 (Pests and Diseases)
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