Knowing how to keep grass free of broadleaf weeds is a key aspect of lawn care.
Broadleaf weeds are perhaps the main problem in home lawns. Apart from them being unsightly, the nature of their growth habit often means that sunlight to the grass is reduced and turf is smothered. In parts of Australia where warm season grasses are the common lawn turf, the problem is exacerbated as the broadleaf weeds tend to actively grow in the cooler months, the very time warm season grass growth slows or stops, rendering them helpless to compete against the smothering growth of broadleaf weeds. Weeds also tend to compete for soil moisture and nutrients, compromising the resources available to the turf plant.
Broadleaf weeds can be grouped into perennial weeds (grow season after season) and annual weeds (emerge, grow, flower, seed and die in one season). The nature of their life-cycles dictates the preferred control strategies to remove the weed.
Perennial weeds are often a challenge to control as they will have tubers, taproots or rhizomes for energy storage which, coupled with their general stress resistance, often requires several herbicide applications for effective control. Specialised seeds and dormancy periods also make control more challenging.
Examples: creeping oxalis (Oxalis corniculata, pictured below), white clover (Trifolium repens, main picture above) and cat’s ear (Hypochoeris radicata).
Annual weeds do at least offer the opportunity for a proactive turf manager to apply pre-emergent herbicides, preventing the weeds appearing in the first place. Dealing with annuals post emergence will require regular spraying as the plants will continue to germinate and emerge over an extended period.
Examples: cape weed (Arctotheca calendula), bindii (Soliva sessilis) and chickweed (Stellaria media).
Plantains are a common and slightly unusual group of weeds, in that they contain both annual and perennial species.
There are a number of key factors in broadleaf weed control, outlined below.
Choice of product
Selecting the correct product is key, with a wide range of products available (both targeted and broad spectrum products). Correct weed identification and knowledge of the type of turf grasses present (to avoid phytotoxicity issues or damage to the turf) is vital for success.
Targeting weeds at the seedling stage is ideal, though this can often be difficult as germination of many species is staggered over the growing season. As such, multiple applications are often required to gain effective control over a weed population with post-emergent herbicides.
A combined approach of pre- and post-emergent herbicides is generally the most effective for annuals, though timing preventative applications for perennials is more difficult when existing weed populations are present.
Water quality can impact herbicide performance.t cannot be assumed that town water is suitable – sometimes it can be quite hard with a pH of up to nine. Reading the label is important in order to understand whether water characteristics are critical for a particular product. Furthermore, to reduce the potential impact of water quality on the herbicide, avoid leaving pre-mixed herbicide solutions in a tank for more than 3-4 hours.
Application and spray droplet size
Herbicides can be applied as spot treatment applications or broadcast applications. The choice of application will depend on the target weed, turf grass present and product used. For example, pre-emergent herbicides are used as broadcast treatments as you obviously don’t know where the weeds will emerge, but treatment of persistent perennial weeds may often involve spot applications with a ‘stronger’ or weed-specific application.
The shape of the spray cone is important when considering spot versus broadcast applications, but droplet size is also important for performance. As a rule of thumb, smaller droplet sizes should be used for smaller leafed weeds and larger droplets for larger broadleaf weeds. However, waxy or hairy leaves can also impact the behaviour of spray droplets and in such circumstances the inclusion of a surfactant or wetting agent would be beneficial.
Spray droplet size is also important when considering spray drift and the possibility of causing damage to non-target plants. Small droplet sizes are more prone to spray drift and volatilisation in windy and warm weather. Consideration of environmental conditions before spraying is very important before undertaking herbicide treatments.
Apart from the need to consider weather conditions in respect to potential non-target e ects, the impact of sunlight (UV), rain and irrigation can impact herbicide performance. All of this will depend on the physical characteristics of the herbicide and its mode of action.
For example, rain or irrigation for a foliar absorbed herbicide is obviously detrimental, resulting in the removal of the herbicide from the leaf surfaces. However, if root uptake is required, the product will need to be watered in post application. Application timing can be optimised to negate some of these effects, for example applying only if rain is not forecast or applying late in the day to minimise UV degradation. Adjuvants can be applied to boost performance, such as products to improve rain-fastness, which maximises both the amount of product and time available for foliar absorption.