Tomatoes are easily the favorite vegetable grown in South Carolina home gardens. However, when their potential diseases, insect pests and physiological disorders are considered, it’s understandable that growing tomatoes can present some challenges. While some of these problems cannot be prevented, glyphosate damage can usually be avoided completely by taking some precautions.
Glyphosate is a broad-spectrum, non-selective, systemic herbicide that is the active ingredient in a number of consumer products, such as:
- Ace Concentrate Weed & Grass Killer,
- Bonide Kleen-up Grass & Weed Killer,
- Eliminator Weed & Grass Killer Super Concentrate,
- Gordon’s Groundwork Concentrate 50% Super Weed & Grass Killer,
- Hi-Yield Super Concentrate Killzall Weed & Grass Killer,
- Martin’s Eraser Systemic Weed & Grass Killer,
- Monterey Remuda Full Strength 41% Glyphosate,
- Roundup Weed & Grass Killer Super Concentrate (50.2%),
- Southern States Grass & Weed Killer Concentrate,
- Tiger Brand Quick Kill Grass & Weed Killer,
- Ultra Kill Weed & Grass Killer Concentrate,
- Zep Enforcer Weed Defeat III.
Most of these concentrated products contain 41% glyphosate, are available in various sizes, and are for use by home gardeners. Read the product label and mix according to the label directions. These products have “caution” as the safety signal word on the labels.
Once glyphosate enters a plant, it moves preferentially to actively growing areas. Glyphosate kills plants by interfering with a plant chemical that is necessary for the production of amino acids (building blocks of protein) required for new growth.
Damage by glyphosate usually results from spray drift, either from an application by the home gardener or by a neighbor, or from glyphosate residue in a multi-use pesticide sprayer. The amount of damage to tomatoes varies depending on several factors including the amount of exposure, growing conditions, cultivar affected, and stage of growth.
In addition to the diagnostic initial symptoms seen in Photo 1, other symptoms include cupped, crinkled and small leaflets with or without mottling similar to that from virus infections. Glyphosate-injured tomato fruits are often smaller and irregularly shaped. Depending on the amount of damage sustained, plants may recover from glyphosate injury or over time, damaged areas may turn brown and die (Photo 2). With lethal doses, necrosis (death) typically starts at the top of the plant and moves downward.
Glyphosate Use Precautions
As mentioned previously, glyphosate damage to tomatoes is completely avoidable so long as certain precautions are taken. For weed control in vegetable gardens, allow at least 3 days between application of glyphosate and planting of tomatoes. Hooded or shielded spray applications between rows of tomatoes are not recommended. Herbicides (and all other pesticides) should never be sprayed when even slightly breezy conditions exist. As much as possible, avoid using glyphosate near vegetable gardens.
If an application of glyphosate is necessary for weed control around other vegetables in the garden, utilize a cardboard shield to prevent drift to non-target plants. When applying glyphosate near or in the garden, adjust the spray nozzle to form large droplets rather than a mist, so as to reduce drift. When using a pump sprayer, pump it up to only about half normal pressure. In addition, keep the spray nozzle close to target plants when applying the herbicide. Finally, sprayers used for glyphosate should not be used for applying other pesticides, such as fungicides and insecticides.
If tomatoes are started in a hobby greenhouse, do not use glyphosate to control weeds on the greenhouse floor, as small amounts of spray drift can severely injure tomatoes and other crops in the greenhouse. Only spray glyphosate in an empty greenhouse.
Alternative Conventional Herbicide for Use Near Tomato Plants
Sethoxydim: Some products containing the active ingredient, sethoxydim, can be used for post-emergence grass weed control around tomato and many other vegetable crops in the home vegetable garden. Most of these sethoxydim products also require the addition of 2 tablespoons of a spray adjuvant (called spreader stickers or crop oils) for best weed control. The available products include:
- Bonide Grass Beater Over-the-Top Grass Killer Concentrate (13%); no crop oil or spreader sticker required,
- Ferti-lome Over-the-Top II (18%); requires use of Hi-Yield Spreader Sticker,
- Hi-Yield Grass Killer Postemergence Grass Herbicide (18%); requires use of Hi-Yield Spreader Sticker,
- Monterey Grass Getter (18%); requires use of Monterey Crop Oil Concentrate,
- Poast Herbicide (18%); requires use of crop oil concentrate.
All sethoxydim products listed have “warning” as the safety signal word on the label. As with all pesticides, read and follow all label instructions and precautions. Use these products with caution due to potential leaf injury if the temperature is greater than 90 °F with the relative humidity greater than 60%. Do not use sethoxydim around corn, which is a grass crop.
Alternative Natural-based, Burn-down Herbicides
For those who would prefer not to use glyphosate for weed control in vegetable gardens, landscape beds, or areas to be kept free of weeds, several non-selective, burn-down herbicides are available that are based on more natural products. This does not mean that they are safer for the individual doing the spraying – caution is always advised. Even natural products may irritate or burn the skin, or injure the eyes, especially in the concentrated form. Read the product label for safe use and protective clothing (such as coveralls). It is advisable to wear rubber boots to prevent contact when walking through areas being sprayed, as well as wearing protective goggles. Wear a pair of rubber or top quality dish washing gloves to help protect your hands and forearms from exposure, especially when mixing and adjusting the sprayer nozzle. Also keep in mind that sprayer wands often leak.
Please note that burn-down herbicides do not translocate into the root system, which means that for perennial and tougher to kill weeds, the weeds may regrow from the roots and require additional sprays for control. These products control actively growing, emerged, green vegetation. However, by being persistent with the spraying of any weed regrowth, even the toughest of weeds can be controlled. Do not allow sprays to contact desirable plants.
Herbicidal Soap: Herbicidal soap (ammoniated soap of fatty acids) can be used for post-emergence general weed control. Shield tomato plants to prevent contact with spray. Examples include:
- Weed-Aside Herbicidal Soap from Gardens Alive,
- Monterey Herbicidal Soap from Planet Natural.
Similar products containing herbicidal soaps are labeled for use in the vegetable garden, but only prior to planting the crop. Examples include:
- Bayer Natria Grass & Weed Killer RTU
- Schultz Garden Safe Brand Weed & Grass Killer RTU.
All products listed have “caution” as the signal word on the label, except Monterey Herbicidal Soap, which has “warning.” RTU means pre-mixed and ready to use.
Pelargonic Acid: Pelargonic acid is a naturally-occurring fatty acid found in many plants. Herbicides containing pelargonic acid are labeled for post-emergence, non-selective, weed control. As such, non-target plants, such as tomatoes, must be shielded to prevent spray contact and potential injury. Examples of herbicides containing pelargonic acid include:
- Scythe Herbicide (57% pelargonic acid)
- BioSafe Weed Control Concentrate (40% ammoniated nonanoate) OMRI
- Mirimichi Green Pro Weed Control Concentrate (40% ammoniated nonanoate) OMRI
- Mirimichi Green Pro Weed & Grass Control RTU (premixed) (5% ammoniated nonanoate) OMRI
Note: Pelargonic acid is a fatty acid which occurs naturally as esters in the oil of pelargonium. It is often called nonanoic acid. The ammonium salt of nonanoic acid, ammoniated nonanoate, is an herbicide. These products have “warning” as the safety signal word on the label.
Plant Essential Oil-based Herbicides: Examples include:
- SafeGro Weed Zap (contains 45% cinnamon oil & 45% clove oil) (OMRI)
This cinnamon and clove oil product has “caution” as the safety signal word.
Orange Oil (d-limonene) -based Herbicides: These include:
- Avenger Weed killer Concentrate (70% d-limonene) Concentrate; and RTU (OMRI)
This orange oil product has “caution” as the safety signal word.
Acetic Acid-based Herbicides: Examples include:
- Summerset Brand All Down Concentrate (23% acetic acid & 14% citric acid); also RTU (8% acetic acid & 6% citric acid)
These acetic acid products have “danger” as the safety signal word. Acetic acid can cause eye damage, so also wear eye protection (goggles).
For information on the best conditions for growing a healthy tomato plant, see HGIC 1323, Tomato.
For non-chemical weed control options in the garden, see HGIC 1253, Controlling Weeds by Cultivating & Mulching.