Magazine - Short or Long Term Weed Control

By Jerry Spencer in Consultancy on 4th Jun 2008 5:55

When carrying out short or long term weed control there are a number of desirable characteristics that a good residual non selective (total) herbicide should have:

1. It should not move or leach once applied.
2. It should have a broad spectrum of control
3. It should be safe to use near non target weeds such as shrubs or ornamentals.
4. It should have a low or zero mammalian toxicity.
5. It should be safe to aquatic organisms.
6. It should have a specific period of control and then allow replanting.
7. It should be UV stable.

From a landscaper perspective weed control needs to be robust and of low maintenance as once an area has been treated it is generally highly desirable to not have to return and carry out a further application. In order to achieve this the operator needs to:

1. Select the right product for the job
2. Apply it at the right time and rate for the particular soil type present.
3. Ensure accurate and complete coverage over the entire area.
Other factors such as soil disturbance and rainfall are beyond the control of the operator but should still be taken into consideration.

How long will control last?


Individual pesticides have unique properties, and many variable factors can effect their persistence:

Pesticide half-life: The more stable the pesticide, the longer it takes to break down. This can be measured in terms of its half life, the longer it takes to break down, the higher its persistence. Degradation time is measured in "half-life." Each half-life unit measures the amount of time it takes for one-half the original amount of a pesticide in soil to be deactivated.

The half life is unique to individual products but variable depending on specific environmental and application factors. Most pesticides break down or "degrade" over time as a result of several chemical and micro-biological reactions in soils. Sunlight breaks down some pesticides.

Mobility in soil: All pesticides have unique mobility properties, both vertically and horizontally through the soil structure. Residual herbicides applied to directly to soil are designed to bond to the soil structure.

Solubility in water: Many pesticides are soluble in water out of necessity so that they can be applied with water and be absorbed by the target. The higher the solubility of the pesticide, the higher the risk of leaching. Residual herbicides are generally of lower solubility to aid soil binding but their persistency in the soil can cause other problems.
Chemical properties of herbicides commonly used in field nursery production.

Chemical properties of herbicides commonly used in field nursery production.

Herbicide	Trade names	Half-life (days)	Vapor pressure (mm Hg)	Major degradation pathways
isoxaben	Gallery	50-120	3.9 x 10-7	Primarily microbial
oxyfluorfen	Goal	35	2.0 x 10-6	Photo-degradation
simazine	Princep	60	2.2 x 10-8	Chemical under low pH, Microbial under high pH
napropamide	Devrinol	70	4 x 10-6	Photodegradation, some microbial
metolachlor	Pennant, Dual	15-25	3.1 x 10-5	Primarily microbial, some photodegradation
dichlobenil	Casoron	60	1 x 10-3	Volatilization, microbial degradation
pronamide	Kerb	60	8.5 x 10-5	Chemical and microbial degradation
oxadiazon	Ronstar	60	7.8 x 10-7	Not known
oryzalin	Surflan	20-128	1 x 10-8	Photodegradation, microbial
pendimethalin	Pendulum	44	9.4 x 10-6	Photodegradation, some microbial
prodiamine	Factor Barricade	69-120	2.5 x 10-8	Photodegradation, some microbial
trifluralin	Treflan	45	1.1 x 10-4	Photodegradation, some microbial

Knowing the half life can in fact help determine the rate of application of a specific herbicide and so in turn help with determining a maintenance programme. This is best exemplified with many pre-emergent herbicides where they have an initial application rate followed by a maintenance application.
Below is an example of how you can use this concept. Let us say one pre-emergent has a half life of approximately 44 days, whereas an alternative has a half life of 60 days and the period of weed activity is generally rate related. The minimum activity level is 15 grams and the rate applied is 400g.

Pendimethalin		Oxadiazon
Half life of 44 days		Half life of 60 days
No of days	Product remaining	No of days	Product remaining
At application	400	At application	400
44	200	60	200
88	100	120	100
132	50	180	50
176	25	240	25
220	12.5	300	12.5

This shows that herbicide A has an expected control length of 7 months and herbicide B 10 months. In each case factors such as pH, temperature, rainfall, organic matter and soil type will influence this rate of breakdown.
While having the desirable characteristic of having a predetermined longevity before it rapidly breaks down is ideal, you can see that if you want the product to break down quickly you have to select not only the correrct product but use it at the right rate and use one with a short half life.

Off site movement
This is probably the major cause for concern when using herbicides with long term residuals as there is always potential for movement following heavy rainfall events.
Some products are absorbed through the roots of the plant as it feeds on nutrients while others are absorbed through the growing point or coleoptiles of emerging seedlings.
Where there are existing deep rooted plants a product that will move into the soil profile is desirable, if the target is germinating seedlings or or shallow rooted plants a less soluble product or one that adheres to clay particles would be preferable.

Because of the solubility of products used, applying them correctly is vitally important to get the best results and ensure that they move into the surface as quickly as possible.
1. Apply to moist soil if possible as this ensures that the active is taken in and dispersed evenly through the top 10-15cm of the soil. Once in the surface most products will move sideways unless subjected to mechanical action.
2. Avoid applications in seasons of heavy rain or storms as these may well result in non target damage.
3. Do not apply to waterlogged soils.
4. Do not apply within the rootzone of desirable species; this is usually two times the height of the tree depending on species and other conditions.

Generally applying these in late autumn or early winter will give good control of winter germinating annuals and in late winter early spring for summer growing species.

Head to Jerry's website at: www.etpturf.com.au/

References :

Street J, Gardner D, Deborah Holdren & Pamela Sherratt, Dept. of Horticulture & Crop Science, The Ohio State UniversityTurfgrass Weed Control and Herbicide Update Residential lawn care tips

Macspred Australia Newsletter 2005

http://oregonstate.edu/dept/nursery-weeds/feature_articles/herbicide_carryover/table.pdf

Read more articles in Consultancy, by Jerry Spencer or from June 2008.