Other Practices to Control Weeds

Flame Weeding

Flame weeding is used in Europe, but has not been widely used in the United States for the last forty to fifty years. While there are many types of flame weeders (see below), all models burn liquid propane to create temperatures high enough to kill exposed plant parts, insects, and, if hot enough, weed seeds. Burned plants do not immediately collapse and may appear undamaged, but if flaming was effective, they will die within a few days. If gentle pressure on the leaf results in an immediate water-soaked appearance in the pinched area, the leaf has been damaged enough to collapse within a few days.

Two examples of the use of flame weeding in vegetables are described in Grower Examples 2 and 3:

Grower Example 2: Dan Tawczynski uses a liquid propane flamer made from a fertilizer sidedresser modified by the addition of high-pressure hoses, valves, levers, and spray nozzles from an old weed sprayer. He directs the flames so they extend 4 to 5 feet back from the no zzles. He can travel at 4 to 5 mph on a calm day, making wide turns at the end of the field. The flames reach a temperature of up to 6,300 degrees F. He flames potato plants just as they emerge from the ground. Flaming kills emerged weeds, surface wee d seeds, and Colorado potato beetles. Above-ground potato foliage is also killed but new shoots regrow within a few days.

Grower Example 3: The three Yokiel brothers use a cultivator-flamer with burners staggered on either side of the row and set to burn through the row. Four-inch shovels run ahead of the burners, cultivating between rows. Flame temperatures reach 3,600 degrees F. The Yokiels report that their flamer is most effective when weeds are less than 2 inches high and when the field is dry and bare of mulch or stubble. Corn fields are flamed when the corn is no more than 1½ to 2 inches tall so that the growing point is still below the soil surface. When the corn is 8 to 10 inches tall the field is again flamed, aiming burners under corn leaves.

Laser Weeding

Although still experimental, and potentially dangerous if used improperly, weed control with carbon dioxide lasers has potential as a means of reducing seed production by weeds already in the field without damaging the crop.

Biological Control

There is considerable research underway on using diseases or insects to control weeds, but successful biological control of weeds has been achieved in only about 30 percent of the cases. At this time there are only a few examples of successful imple mentation of biological controls for weeds. The North Carolina Department of Agriculture Biological Control Program is releasing weevils that eat the seed of musk thistle. Scientists in Georgia have found that Puccinia rust can be used to control nutsedge and this rust will be marketed in the United States as 'Dr. Biosedge.' Since rust spores are highly sensitive to impurities in surface water and surfactants, the rust must be applied using deep well water and special surfactants.

It has been suggested that biological and chemical weed control programs can be integrated to quickly reduce weed populations to the economic threshold levels or below while at the same time improving long term control. However, the interactions betwee n herbicides and biocontrol insects are complex. Most insects are not affected by herbicides applied at field rates, but toxicity to some insects has been documented, especially when chemicals are applied during oviposition and early larval development.

Tips on Controlling Yellow Nutsedge

Yellow nutsedge, a shade-intolerant perennial, produces tubers or 'nutlets' that can remain dormant for years before sprouting. In the early summer, the plant grows rapidly, spreading by rhizomes and roots. In mid-to-late summer when the days get shorter, the plants produce nutlets on the ends of the rhizomes. The plants die in the fall and the tubers separate from each other. These tubers sprout the following year and can grow through plastic mulch and even through potato tubers.

The key to controlling nutsedge is to prevent late-summer tuber formation. This can be done by following the spring vegetable crop with a midsummer fallow period in which the field is disked weekly or biweekly. It is especially important to disk regularly after the first of August, the main period of tuber formation. Planting of fall crops should be post-poned as long as possible and the regime must be continued for several years.

In fields with severe nutsedge problems which are not susceptible to winter erosion, disking when a hard freeze is expected may bring more tubers to the surface where they will be killed. In cold winters, an occasional timely disking can reduce the number of viable tubers by 70 to 80 percent.

Scouting-based Weed Management in Cucumber

Like other IPM practices, weed management depends on scouting at the appropriate time. For cucumbers, scouting for weeds takes place a year or season prior to planting. Control action depends on the number of weed plants in a 100 square foot (10 ×10 foot) block. Table 7.3 describes weed control strategies in North Carolina.

Natural Herbicides. Just as the natural compound Bt (Bacillus thuringiensis toxin) is effective on a range of insects, a naturally occurring plant poison or "phytotoxin" AAL-Toxin, has been isolated from the pathogenic fungi, Alternaria a lternata f. sp. lycopersici. On susceptible tomato lines, this fungi causes stem canker. AAL-Toxin has been shown to have a wide range of phytotoxicity.Within families, however, susceptibilities vary. Highly susceptible species include such important weeds as black nightshade, prickly sida, jimsonweed, hemp sesbania, sicklepod, northern joint vetch and spurred anoda. All monocots and tomato cultivars resistant to stem canker are also resistant. Lettuce, squash and okra were only injured at relatively high concentrations. This compound has been patented as a herbicide, but not yet developed commercially. Nevertheless, it illustrates the potential for developing herbicides based on natural phytotoxins.