Field bindweed (Convolvulus arvensis) is a serious noxious weed pest of California agriculture. It is also referred to as field or wild morning glory, creeping jenny and greenvine.
It is a perennial plant with deep roots extending down to three feet which can grow for months without irrigation. It reproduces by seed or rhizomes. Heavy infestations in wheat can reduce yields by 30 percent to 40 percent.
It is equally a problem in tomatoes, beans and alfalfa. In almost all row crops there are practically no herbicides that are effective to control it post-emergence.
Bindweed is also a problem in young orchard plantings that are in full sunlight where it grows vigorously. Field bindweed can harbor potato virus X and tomato spotted wilt virus.
Recommended control measures include summer fallow with cultivations or herbicide applications, and crop rotations that use selective post-emergence herbicides (2,4D in wheat, corn and Roundup Ready varieties). The use of glyphosate herbicide in the spring prior to planting, or fall following harvest and before the first frost occurs offers a temporary reprieve.
Ineffective bindweed control using glyphosate has led to speculation that issues related to water hardness, pH, or the many different glyphosate brands and formulations on the market may be part of the problem.
Poor water quality such as high pH and water hardness (calcium carbonate content) are known to bind the glyphosate molecule and reduce control. Throughout San Joaquin County, water quality varies, especially in the southern and eastern county areas, where total water hardness can be very high, exceeding 500 ppm from calcium carbonate.
In an effort to better understand glyphosate performance and water quality, we conducted a bindweed study this spring, south of Tracy, using different brands and rates of glyphosate, two surfactants, and two amounts of ammonium sulfate which are recommended by label. Limited in the space we had available, we selected only two brands of glyphosate commonly used, Roundup WeatherMAX 5.5SC and Buccaneer 4SC applied at different rates.
The water source was an agricultural well that typically has very hard water exceeding 450 ppm of calcium carbonate. Cations (Ca, Mg, Fe, Mn) react with glyphosate to form an insoluble salt which does not kill weeds. Studies have shown a 50 percent reduction in weed control with glyphosate using water hardness at 500 ppm.
Ideally, the spray solution pH when using glyphosate should be in the 4.0 - 5.0 range and water hardness as close to 0 as possible or at least below 100 ppm. Water hardness and pH testing was conducted in our lab to measure changes that occur when adding herbicide, surfactant or ammonium sulfate in the water.
The herbicide treatments were applied on June 14 to a thick roadside population of bindweed at varying growth stages ranging from 30 percent without flowers, 50 percent flowering and 20 percent setting seed. Plants were healthy and growing vigorously.
Both brands of glyphosate were effective in lowering the pH and water hardness of the spray solution to the recommended range when the rate of 2.0 lbs/A or higher was used. The addition of acidifiers, surfactants, and ammonium sulfate did help reduce the pH and hardness but not as much as did the herbicide. The acidifier lowered the pH more than it lowered water hardness.
The ammonium sulfate however, lowered the water hardness more than lowering the pH.
Bindweed control averaged more than 90 percent for all treatments after six weeks. There were differences on how fast the burn-down occurred depending on the rate, but no differences from the surfactants and ammonium sulfate.
The 4.0 lb rate has continued to be the best treatment to date and reached 90 percent control within two weeks which was twice as fast as the 2.0 lb rate.
The take-home message is to know your water quality by testing after adding herbicides/surfactants to see if there is any improvement made.
This will help you make the correct decision for your agricultural sprays in the future.
Water quality also affects the efficacy of many insecticides. Water quality test kits are inexpensive for pH and hardness. The kits can be purchased at most stores carrying spa and swimming pool supplies or online. Since this bindweed test is under a non-irrigated condition, plants are not regrowing as they would in an irrigated field but we will continue to observe these plots and monitor long-term results.
Unfortunately, we have not discovered any new silver bullets to easily control field bindweed problems. In the long term, eradication will still require continuous treatments of herbicides or cultivations as plants regrow until the carbohydrate reserves in roots are depleted and unable to propagate any new shoots or roots.