W.M. STALL
Horticultural Sciences Department
University of Florida, Gainesville, FL 32611-0690
PO Box 110690

All vegetable crops are designated as "minor crops" by EPA. The small comparative acreage of vegetable crops along with the relative high value per acre is not conducive for manufacturers to invest the large sums of money needed to develop herbicides for specific small acreage crops. Even with government-University cooperation in establishing tolerances on vegetables through the IR-4 program, the number of herbicides registered for use on vegetables ranges from few to none.

The loss of methyl bromide use on the vegetables grown on mulched beds threatens to cause a problem with weed management, especially with nutsedges within that production system. Methyl bromide was the closest thing we had to a "silver bullet" for weed control in the plant bed. At the present time, and in the future, at least short term, there is no one product that can replace all the control spectrum and have the crop safety that was found with the methyl bromide use. Also, unfortunately, we do not have the herbicides available to add to the fumigant alternatives for methyl bromide for total weed control. Therefore, non-chemical management options will have to be incorporated into the production system for vegetables grown in Florida.

Weed Management Methods

Weed management typically includes combinations of two or more of the following means of control: mechanical, cultural, or chemical.

Mechanical control includes field preparation by plowing or disking, cultivation, mowing, hoeing and hand pulling of weeds. Seedbed preparation, by plowing or disking, exposes many weed seeds to variations in light, temperature and moisture. For some weeds, this process breaks weed seed dormancy, making possible early season control with herbicides or additional cultivation. Plowing can also bury weed seeds causing them to loose viability and die before being returned to the soil surface.

Where fields are known to contain a large number of hard-to-control weeds, an off season fallow treatment can reduce the weed seed bank and the subsequent number of weeds emerging in the crop if done properly. Fallowing, whether all mechanical or a combination of mechanical and chemical, can also reduce other pest pressures that may survive on weeds growing in the field. Mechanical fallowing simply is allowing weeds to germinate and emerge and then disking them under before they have a chance to propagate. When this is done several successive times, the seed bank of many weeds in the soil is significantly reduced. Successive diskings is the key since some weeds in the soil such as nightshades will not all germinate due to allelopathic compounds given off by the emerging seedlings. These compounds keep the remaining nightshade seeds dormant. These dormant seeds will not germinate until the growing weeds are killed. Other weed seeds must be either scarified or be near the soil surface to germinate. Several diskings can cause these seeds to germinate and then the plants can be killed. With perennial grasses and asexual propagated plants such as nutsedges, mechanical fallowing must be supplemented with chemicals. It took disking every two weeks for two years to eliminate nutsedges in a scientific study. In another study, only 76% of the nutsedge in an area was controlled when they were hoed every week for 14 weeks. When nutsedges and perennial grasses are treated with a labeled systemic, nonelective herbicide and then disked after the herbicide has translocated (usually two weeks) significant numbers of weed can be controlled. It is recommended that this process be repeated at least one more time for adequate control and more times for fields with high weed populations.

Cultivation and hoeing are acceptable methods of weed control and in several vegetable crops, the only in-season method of weed control. A study with watermelon has shown that hoeing twice at the proper timing is as effective as using PRE and POST herbicides. Care must be taken not to cultivate too deeply in many crops where the roots are shallow. Deep cultivation can prune the crops roots and reduce yield.

In most cases, early season weed control is most important. In studies with watermelon and tomatoes, pigweeds must be controlled during the first four weeks of the crops growth. Sedges and grasses should be controlled from 3-6 weeks during the crops growth to eliminate yield loss. When done properly, it does not matter whether the control method is chemical or mechanical.

Cultural control methods include mulching, crop rotation, cover cropping, flooding and other. Polyethylene mulch warms the soil, reduces fertilizer leaching, help in water conservation and weed control. Black or other non-clear mulches suppress many grass and broadleaf weeds. Nutsedges, however, will germinate and pierce the mulch. Nutsedges cannot penetrate paper mulches, but paper mulches are not in general use due to other factors. Clear mulches will allow the soil to warm faster and higher than opaque or black mulches. Weeds, however, will grow under the clear mulches. Clear mulches may be used, however, for soil solarization during the summer months. Germinating weeds will be killed by the high heat under the mulch when used in this manner.

Crop rotation is another cultural method that if used correctly can enhance weed control. Knowledge of the weeds present in the field and growing a crop that has labeled herbicides that will control those weeds or using a crop whose growth habit will smother or shade out the weeds or whose growth allows cultivation to control the weeds are long term planning processes that can reduce weed competition. In planning rotations, the use of herbicides with long residues should be avoided.

Cover crops can also be used to reduce weed buildup in fields. Cover crops can shade out certain weeds, compete with weeds and cause them not to flower or seed, or in some cases be allelopathic to weeds and kill them. Herbicides may be applied with some cover crops. Again, be aware of the plant-back restrictions.

Another weed management method is the use of a stale-seed-bed. This method is designed to control the early emerging weeds that are not or cannot be controlled with the herbicide or cultural control method used. The seed bed is formed, fertilizer incorporated and/or mulch layed if it is used. Before the crop seed is planted or emerged or before transplanting, the emerged weeds are sprayed with a non-selective contact or systemic herbicide. Since the soil will not be disturbed except for planting or transplanting, the emerged weeds are killed and new weed seeds are not brought to emergence depth. This method is used successfully in controlling nutsedges with glyphosate that have emerged through polyethylene mulch.

Another method of weed management that is often overlooked is the destruction of the crop after harvest along with the weeds that have emerged during the later part of the season. The crop destruct reduces insects and diseases that can move to adjacent fields and the weed destruct keeps new weed seeds from being added to the soil seed-bank. It also reduces the increases in asexual propagated plants such as nutsedges. Yellow nutsedge can put out 70 new tubers (nuts) every two months. Keeping the weeds from propagating will reduce the weed problems encountered during the next cropping.