Controlling Weeds With Pre-Emergence

Perennials live for two years or more and are difficult to control because they reproduce both from seed and from vegetative buds on the roots. Pre-emergence herbicides have little affect on shoots arising from these vegetative buds, and no effect on vegetative buds of the perennials that have deep root systems. Pre-emergence herbicides are usually applied in spring for germinating perennial seeds. Examples of deep-rooted perennials are field bindweed (the morning glory weed') and Canada thistle.

Pre-emergence herbicides are the backbone of current turf-grass weed management. Their primary target is annual grasses, but some small-seeded annual broad leaves can be controlled. The landscape contractor applies these herbicides to the target area before weed-seed germination.

To kill winter annuals like Common chickweed (Stellaria media) pre-emergence must be applied in the late summer. Chickweed is found throughout North America except for in the far west. Primarily a weed of turfgrass, lawns and winter small grains, its leaves range from 1/2 to 1 1/4 inches in length, are light green in color and smooth or possibly hairy toward base and on the petioles. Flowers are in small clusters at the ends of stems, are (3-6 mm wide) and consist of five white petals that are deeply lobed, giving the appearance of 10 petals.

Pre-emergence herbicides are activated by 1/4 to 1/2-inch of rainfall or irrigation, after which a ''herbicide barrier'' forms at or just below the soil surface. When the roots of germinating seeds come in contact with the herbicide barrier, their growth is inhibited.

A variety of factors affect the performance of pre-emergence herbicides, including application timing in relation to weed-seed germination, soil type, and environmental conditions, like temperature and rainfall, target-weed species and biotype. For best results, apply pre-emergence herbicides just prior to weed-seed germination.

Don't Apply Too Early

Applying too early may result in reduced control or no control due to leaching and/or normal herbicide degradation. Pre-emergence herbicides must be in place and activated before weed-seed germination begins. For optimum performance, rainfall or irrigation should occur within 24 hours of application to move the herbicides into the upper layer of the soil.

Above and below: There are several pre-emergent herbicides labeled for the control of annual grasses, such as crabgrass, but not all of these products are registered for use in all species of turfgrass. Atrazine, metalochlor, napropamide, oryzalin, pronamide and simazine are registered only for use on warm-season turfgrass. These herbicides can be found under brand names Aatrex, Pennant & Magnum, Sulflan, Kerb, and Princep respectively.

sulflan

Type of Weeds

When using pre-emergence herbicides, it is important to know whether the weeds you want to control are annuals, biennials or perennials. The type of weed it is determines when it germinates, and if the herbicide is applied at the wrong time it will not be effective when the seeds you are targeting are sprouting. (see photographs for examples.)

Do's and Don'ts

1. Use pre-emergent herbicides only on established lawns, because they will affect most germinating seeds, including desirable ones. Although pre-emergence herbicides are used primarily to control annual grasses, they also help control annual broadleaf weeds, minimizing the need for control of those weeds later in the season.
2. In controlling annual grasses, it is important to positively identify the grass before acting, to avoid wasting time and chemicals treating perennial grasses.
3. Apply chemicals in spring when soil reaches 50 degrees F to 55 degrees F. Pre-emergence should be sprayed at the soil level where germination occurs. Once weedy grasses are observed, it is too late for pre-emergence control.
4. Annual grasses are generally difficult to control once they have infested turf. Selective post-emergent control is best accomplished if annual grass seedlings are treated when they are small (i.e., five-leaf stage or younger).

Benefin and benefin-containing products provide effective control of crabgrass. While Benefin and DCPA may breakdown more rapidly in thatchy turfgrass, herbicides containing bensulide, pendimethalin, prodiamine and dithiopyr, can work for up to four months. Benefin can be found under the brand name: Balan, while the other chemicals can be found in brand names: Besumec & Betasan, (bensulide); Pendulum (pendimethalin). A recent preemergent herbicide introduction for use in turfgrass is dimethinamid-p (Tower herbicide). Tower is labeled for preemergence control of goosegrass and certain other grassy weeds and is safe for use on short cut creeping bentgrass.

A summer annual, purslane (Portulaca oleracea) needs to be sprayed in the spring months before it germinates in order to be effectively eliminated. Also known as ''little hogweed,'' it is found throughout North America. Recommended pre-emergents include dithiopyr and pendimethalin. Post-emergents include Dicamba, MCPP, MSMA and 2,4-D.

Soxaben (DowElanco's Gallery) controls a wide range of winter-annual broadleaf weeds, including common chickweed, henbit, bittercress and sowthistle. It also provides pre-emergent control of perennial broadleaf weeds such as dandelion, buckhorn plantain and white clover as they germinate.

Although there is some debate about its effectiveness, corn gluten is said to be effective, while being environmentally friendly. Among the weeds controlled with pre-emergent application of corn gluten are crabgrass, creeping bentgrass, smart weed, dandelions, redroot bigweed, purslane, lambsquarter, foxtail, barnyard grass, and Bermuda grass. Corn gluten meal works by inhibiting root formation in weeds at the time of germination. It doesn't inhibit roots of mature plants or transplants unless it is used at a very high rate of 80 pounds/1000 sq. ft. or more. Weeds germinate and form a shoot, but no root, which prevents growth. A short drying period is required after germination, because too much water may allow the plants to recover and form roots.

Ask For Help

If you are not sure of the identification of a problem weed, take a sample to your County Extension Agent or County Weed Office for identification and appropriate herbicide recommendations.

Top 10 Reasons Your Pre-emergence Failed

1. You failed to read the label – Herbicide labels contain a lot of information about application timing, rates, spray volume, soil conditions, and how to incorporate or activate the herbicide. Effective herbicide management begins with reading the label.
2. You disrupted the chemical barrier – Pre-emergence herbicides form a chemical barrier over the soil/container surface. The barrier is typically 1/2 to 1-inch thick (depending on product, soil type, and method of incorporation). Herbicides do not prevent weeds from germinating; instead they control weeds as they germinate within the chemical barrier.
3. Did not cover a broad weed spectrum – No single pre-emergence herbicide provides control of all weeds. Some herbicides are effective in controlling broadleaf weeds but provide poor control of grasses. These products would include formulations of isoxaben, simazine, or oxyfluorfen.
4. You failed to incorporate the herbicide with irrigation – The herbicide label will provide instruction for how much irrigation to apply after application. First, many people are switching to drip irrigation, with some placing the drip tape underground. Capillary movement of water is insufficient to adequately incorporate the herbicide.
5. You routinely over-watered your plants – Over-watering will reduce herbicide efficacy. Excessive water allows weeds that would otherwise suffer from sub-lethal herbicide toxicity to survive.
6. You used a lower herbicide rate – There is a concentration in the soil at which an herbicide will inhibit weed growth. When lower than recommended rates are used, the herbicide concentration in the chemical barrier starts out too low.
7. You failed to use proper sanitation – Even under the best circumstances, using the most effective herbicides applied at the proper time and in the proper manner, the chemical barrier will not be perfect. There will always be some small areas where the chemical barrier is insufficient for controlling weeds. In addition, the chemical barrier begins to break down almost immediately after application.
8. You top-dressed fertilizers – Topdressing fertilizer is a common practice in container production. It is also a practice that will likely lead to poor weed control.
9. You did not calibrate your equipment – Improper herbicide calibration will result in improper herbicide rates being applied to your plants. When applying herbicides with a sprayer, be sure the sprayer is properly calibrated.
10. The soil that already had weeds – Herbicides registered for use in nursery plants will not kill existing weeds. Many of the herbicides used in nursery plants work by inhibiting root growth. But even small weeds generally have a deep, well-developed taproot, which when greater than one inch deep will not be affected by herbicide activity. Sites must be weed-free before applying pre-emergence herbicides.