Good weed management includes correct identification, appropriate cultural practices to minimise problems, avoidance of weed introduction and, finally, chemical use if required. Turfgrass professionals throughout the world have encompassed the reduction in availability of chemical control agents for both weeds and other problems in turfgrass management and, in my opinion, the number and range of available compounds for weed control will continue to decline in future years, forcing turfgrass managers to seriously consider other options in the fight against weeds.
Cultural management of weeds in turfgrass includes the use of mowing, fertilisation, cultivation, irrigation, and selection to impact upon weed populations. It is easy to forget that promoting a vigorous, healthy sward is critically important in controlling weed numbers. Having a good management programme in place, including correct nutrition, irrigation and aeration, will not only increase the health of the sward but will also promote the sward's cover and limit the possibilities of weed growth.
Good nutrition helps to maintain turfgrass density and vigour. By increasing the turf stands cover, light will be prevented from reaching the soil surface which will, in turn, delay the germination of seeds in the spring due to the cooling effect of the turf canopy on the soil surface. Inadequate fertilisation, in particular nitrogen, leads to an open sward that is more susceptible to weed infestation and may also weaken the sward, reducing disease and stress tolerance levels. However, excessive nitrogen applications can lead to succulent soft growth which, in time, can cause grass death and allows weeds to grow. It is also important to remember that excessive quantities of phosphorus in the soil, particularly near the surface, can encourage development of germinating weed seeds.
The goal of every turfgrass manager is to have strong healthy grass and this is also achieved through using the correct mowing height. Turfgrass mowed too short will become open, often with a poorly developed rooting system, which will encourage weed and disease invasion. Mowing recommendations include frequent mowing at the tallest height specified for the specific turf and particular situation. This practice, as well as helping the plant to carry out its physiological process, will also reduce available light levels to the soil surface. Proper mowing height is critical to maintaining turf density. In general, mowing below the optimum height will increase invasion of weedy grasses.
Over-watered turf often grows very rapidly, allowing turfgrass plants with shallow and weak root systems to develop, subsequently allowing weeds and diseases to readily invade. On the other hand, under-irrigation causes open turf stands, and grasses can go dormant, allowing weeds to flourish. Once weeds are established, they can often grow well in drought environments since many have exceptionally deep roots.
Activities such as aerification, vertical mowing and slitting are methods of reducing thatch and soil compaction. Compaction reduces the amount of pore spaces in the soil, which may favour the growth of certain weeds such as knotweed since it can tolerate lower levels of oxygen in the soil. Shade can also favour weed growth, since it reduces light quantity and quality leading to turfgrass stands with reduced density. Tree pruning and in extreme situations tree removal may be required.
Biological weed control
Recently, the concept of using biological control agents as a weed control strategy has gained much publicity, particularly with the increasing concern in relation to possible environmental damage and human health risks stemming from synthetic herbicide use. Methods using natural weed antagonists to control weed growth have been used and these agents are commonly referred to as bioherbicides.
Biological weed control consists of using natural pathogens to selectively injure target weeds. However, the pathogen must not injure desirable plants. A particular problem with the use of such control has been the ability of the antagonist to persist in the environment that they are introduced. Biological control is made up of two strategies:
• Classic control whereby an organism is selected that can establish and sustain in the particular environment.
• Inundative control which consists of repeat application of biological control agents.
These inundative biocontrol agents are applied at high concentrations for immediate effect. However, they do not sustain long-term weed damage and are often sensitive to environmental conditions. An example of inundation control using micro-organisms includes dandelion control using Phoma herbarum.
Microbial inoculants containing selected blends of bacteria and fungi have gained much attention as possible tools to control both weeds and diseases in turfgrass. It has been shown that deleterious rhizobacteria can colonise the root surfaces of weeds and suppress growth while subsequently increasing growth of desirable plants. Xanthomonas campestris pv. Poa annua has been identified as an organism to control annual meadowgrass in turf sward's.
The organism works by preventing water movement through the xylem of the grass, however it does not negatively impact upon most other desirable grasses. Gange found that the addition of a fungal inoculant containing four species of arbuscular mycorrhiza, had a positive impact on Agrostis stolonifera total biomass and a negative impact on Poa annua total biomass. Research at University College Dublin by Butler and Hunter, found that application of a commercially available microbial inoculant containing both bacteria and fungi significantly reduced Poa annua germination in a bentgrass sward. In many instances Poa annua invades golf greens within a few years of construction and this research may be a useful strategy in controlling annual meadow grass established on golf greens. Pyricularia setariae has been found to be a biological control for green foxtail.
Having said all this, I realise that many readers will be thinking "where can I get these biological control agents?"
Up to now, the majority of these agents were commercially unavailable and this meant that talk about using them was useless. However, recently numerous microbial inoculants products have come onto the market. Many of these inoculants contain blends of micro-organisms, many of which are not specifically designed to control weeds. In my opinion, it is only a matter of time before mass production of specifically produced biological weed control agents are commonly available throughout the world. Many biological control agents are extremely new and their ability to control their host weed has not yet been determined.
One problem that many in the industry have with microbial type products is the lack of consistency of the inoculant products. This was highlighted by Professor Alan Gange during a recent meeting when he noted that some of the micro-organisms contained within microbial inoculant products may not be alive when they are applied to a turfgrass sward and this needs to be addressed in order to maximise the potential benefits of these products.
Another issue that may be a problem with microbial inoculants containing various species of mycorrhizal fungi is that some fungi contained within may be antagonistic to other fungi and reduce any benefit of applying the fungal product, suggesting that, in some cases, less fungal species within an inoculant may be more beneficial.
In conclusion, I would like to state that it is essential that every effort is made to ensure that cultural practices are used to every advantage to control weed invasion. Research carried out at Michigan State University found that herbicide use, coupled with improved cultural practices are required to maintain maximum turfgrass cover long term. In my experience, biological control agents are a very useful method in helping to control weed mass in turfgrass. However, in modern turfgrass management, many people expect mass eradication of weeds using biological control methods and this is not possible. Biological control will not eradicate noxious weeds and residual levels of the weed populations must be expected. Instead, these agents help to reduce weed numbers to more acceptable levels. However, presently in many situations this is not acceptable to many people, although as herbicide regulations continue to become stricter we will have no choice but to seek the use of biological and cultural control methods in weed management.
About the author: Dr. Tim Butler is a Sportsturf Science Agronomist and Independent Expert on all aspects of Golf Course, Sports Pitch and Race Track construction, renovation, drainage, agronomy and environmental management. Contact Tim at firstname.lastname@example.org