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Weedkiller/Herbicide Damage to Plants 


This is a list of herbicides, weedkillers, and the types of injuries that may be caused when they come into contact with sensitive plants: The damage from weedkiller and herbicide application can be rectified to a certain extent, depending on the type of weedkiller used.

2,4-D and MCPA cause unequal rates of growth in the stem petioles (leaf stems) and leaves. The result is a bending and twisting of the plant and cupping of the leaves. There may also be a development of an excessive support root system with gall-like growths occurring on the lower stems and roots.

Trichloroacetic acid (TCA) damage causes browning of leaf tips, stunting of plants and eventual death of the plants. TCA is a short-term agent which can be used for total vegetation control.

Trifluralin damage results in seedlings not emerging.

Dicamba causes an excess of tissues, twisting and bending of petioles and stems; fiddle-heading of potatoes (bunched, curled leaves of new growth, similar to the shape of a head of a violin); leaf yellowing and browning, and; chlorosis (yellowing) and browning in evergreens.

Diquat and Paraquat result in a yellowing and then blackening of tissues. When they drift they cause brown spots to develop on the leaves they contact.

Bromacil will cause leaf margins to turn brown as well as the interveinal areas (between the leaf veins) followed by death of the entire plant. Conifer needles turn yellow, then brown and will later drop. This is a soil sterilant which should not be used in or near the garden or yard since water movement can carry it to the roots of plants in untreated areas and where it will be taken up by the plant.

Atrazine and Simazine results in yellowing, wilting and browning of plants. Damage is first seen on new growth.

Glyphosate affects plants by first causing a wilting, mottling, browning and eventually death. In some cases, leaves lose their green colour and appear white or purple. Leaf and stem deformities may occur similar to that of 2,4-D, dicamba and picloram. Cupping and chlorosis of newly emerged leaves may also occur. Perennial plants sprayed with glyphosate one season may show no apparent effect, but the following spring the newly emerged leaves will be chlorotic. Take precautionary measures when spraying near sucker-producing plants, such as lilac, honeysuckle and poplar. If Roundup (or another formulation of glyphosate) is inadvertently sprayed onto the sucker, this chemical will move into the plant, however the damage will not be noticed until the following season. When the new growth appears, it may lack the green pigment and appear white or purple.

Picloram causes young plants to wilt and die, while older plants will remain erect until death. Plants that are more tolerant may only show chlorosis and crinkling of the leaf edges. Plants which are very sensitive to even a trace of picloram are potatoes, tomatoes, beans, peas, faba beans, sunflowers and sugarbeets. Levels as small as 10 parts per billion of picloram in the soil is enough to cause damage to these susceptible plants.

Corrective measures:

Sometimes mistakes or accidents occur when applying herbicides. Chemical drift may contact sensitive crops or plants, the wrong product may be mistakenly used, etc. In some cases, exposed plants or entire areas may be rescued if timely and appropriate measures are taken.

2,4-D, Banvel (dicamba), paraquat and glyphosate drift: If drift is reaching the garden as well as ornamentals, water heavily with sprinklers. Water will dilute the chemical and wash it to the soil where it has less harmful effects on the plants. The sooner it is washed off the better chance for recovery. Delaying more than three or four hours between exposure and washing off the plants may eliminate the chance for recovery.

Activated Charcoal

Herbicides which are taken up by roots (including total vegetation control chemicals) can be detoxified by using activated charcoal at a rate of 0.5 kg per 9 m2 and incorporating it to a depth of 15 cm. Herbicides which can be effectively adsorbed by activated charcoal are: 2,4-D; 2,4,5-T; atrazine (Aatrex); chloramben (Amiben); amino-triazole (Amitrole T); dicamba (Banvel); dichlobenil (Casoron); chlorpropham (CIPC); dinitramine (Cobex); chlorthal-dimethyl (Dacthal); diphenamid (Enide); EPTC (Eptam); bromacil (Hyvar X); diuron (Karmex); pronamide (Kerb); alachlor (Lasso); linuron (Lorox/Afolan); simazine (Princep); metribuzin (Lexone/Secncor); terbacil (Sinbar); monuron (Televar); picloram (Tordon); and trifluralin (Treflan). Activated charcoal is carried by some horticultural centres.

If tree roots have grown into an area that was treated with a soil sterilant, a trench may be dug near the tree and all roots should be cut off to prevent further uptake of the sterilant. This will be a shock to the tree but will prevent further damage by the sterilant.

Although these corrective measures work, it is far better to prevent accidents with herbicides by using precautionary measures.



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