Weeds Pages
Dodder
Dodder is a parasitic plant; its seedlings must attach to a suitable host to survive. In addition to tomatoes, other known hosts of dodder include safflower, sugarbeets, alfalfa, asparagus, honeydew melon, onions, carrots, nightshades, and numerous other broadleaf weeds. Rotations are generally not effective in eliminating dodder because it has a wide host range and its seeds can remain viable for years. However, rotation to nonhost crops such as cotton, corn, cereals, and garlic can help reduce seed numbers. The standard way to control dodder has been to destroy the host tomato plants as soon as dodder is observed. If dodder is flowering, remove the host plants from the field and burn them to kill the seed. Pendimethalin (Prowl H20) preemergence applications, which are registered in transplanted tomatoes only, have been shown to reduce dodder germination and emergence by 80%. Several tomato varieties, including Heinz 9492, 9553, 9888, 9992, 9997, 1100, CXD 233, and PX 665 have shown good resistance to dodder infestation, reducing seed production.
Most dodder germinates between March 1 and May 20, so late planting can help reduce problems with dodder.
Field Bindweed
A deep-rooted perennial, field bindweed is difficult to control once it becomes established. Herbicides used in tomatoes are not effective against established plants; however, trifluralin (Treflan) as a layby treatment will control seedling field bindweed. Seedlings can be easily identified from established bindweed plants by the presence of cotyledons. Established plants require cultivation for control; in drip irrigated fields bindweed is becoming more problematic. By eliminating bindweed competition in tomatoes for 6 to 8 weeks after tomato emergence, full yields can be obtained. Field bindweed is best controlled after a cereal crop, where actively growing bindweed can be treated in fall with glyphosate (Roundup Weathermax, Glyphomax, Touchdown, etc).
In heavily infested fields a reclamation blade can be used to provide sufficient control to grow a crop of tomatoes relatively free of field bindweed. Use the reclamation blade following the harvest of a crop that dries or uses up the soil moisture to a depth of 18 to 20 inches. This method has been most effective following a cereal crop, safflower, or sugarbeets. Perform the blading during the summer months so that the severed rhizomes will dehydrate or desiccate. It takes about 10 to 12 months for the rhizome to regrow sufficiently from the 18- to 20-inch depth to adversely affect tomato plants grown in the field.
With the end of harvest, where there are high field bindweed populations, an herbicide control program in late fall can be effective if the bindweed has sufficient soil moisture to remain vigorous and flowering. Note plant-back restrictions of certain materials.
Nightshade
Related to tomatoes and potatoes, the nightshade family includes black nightshade, hairy nightshade, cutleaf nightshade, groundcherry, and several others. These annual weeds are resistant to many herbicides commonly used in tomatoes. Soil fumigation with metam sodium (Vapam) is an effective pre-plant treatment. Rimsulfuron (Matrix) provides control of most of the nightshade species when applied after seeding or transplanting as a preemergence treatment and incorporated with water from sprinklers or rain within 5 to 7 days of treatment. Rimsulfuron also provides postemergence control if the weeds are at the cotyledon stage. Nightshade seedlings can be partially controlled by a postemergence directed spray of metribuzin at the 5- to 6-true-leaf stage of direct-seeded or established tomato transplants. Rotating to crops where available herbicides control nightshade (e.g., Roundup Ready corn or cotton) helps to reduce seed levels in the soil.
Many weeds in the nightshade family, such as hairy, black, and cutleaf nightshade as well as groundcherries, germinate in the top 1 to 2 inches of soil. In fields heavily infested with seeds of these weeds, deep plowing with a well-adjusted moldboard plow (or Kverneland plow) can significantly reduce seed emergence. It is essential to completely invert the soil to bury the seeds below the germinating zone. In most cases, a standard or conventional moldboard plow does not invert the soil profile adequately to bury weed seeds uniformly.
Nutsedge
Nutsedge is a perennial that reproduces primarily through abundantly produced tubers. Tubers remain viable in the soil for several years until conditions are favorable for growth. Each tuber contains four to seven buds, each capable of producing a plant. Generally, only one bud will germinate on any tuber, but if it is destroyed by cultivation or herbicide treatment, a new plant will grow from one of the other buds. Partial suppression of yellow nutsedge can be achieved with an application of s-metolachlor (Dual Magnum) at layby. Halosulfuron (Sandea) is effective in suppressing nutsedge after it emerges. Preplant use of metam sodium will also give some help with yellow nutsedge control in the tomato seedline. Cultivation can be used to help suppress nutsedge, but must be done by the 5- or 6-leaf stage or new tubers will have formed. Rotating to different crops, such as corn or beans, where available herbicides suppress nutsedge helps to avoid buildup of this weed.
Yellow and purple nutsedge rarely emerge from dormant tubers that are buried deeper than 4 to 6 inches. Therefore, during late fall, specialty plows (Wilcox or Kverneland) can be used to invert the soil to at least 12 inches deep to minimize the emergence of nutsedge in the tomato field. It is essential that the specialty plow completely invert the soil for this method of control to be effective.