Broadleaf Weed Control in Spring
As the turf begins to wake up this spring, turf managers should begin preparations for controlling all the annual broadleaf weeds that have established over the winter months. The first step in the process is to scout the turf and identify the established broadleaf weeds that warrant control. It’s also a good idea to record all the weeds that were present in previous years and, more importantly, take note of any new weeds that may have decided to move in. Early detection is critical in preventing the spread of new weed species, particularly perennial broadleaf weeds, throughout the turfgrass areas.
Broadleaf weeds like white clover and broadleaf plantain are often problematic in turf grown on soils with nutrient deficiencies, improper pH, poor drainage, or excessive compaction. Turf mown at lower than recommended heights often have reduced vigor and recuperative capabilities from disease, traffic, heat, shade, and other environmental stresses. Broadleaf weeds like chickweed, dandelion and oxalis may thrive under various soil types and site conditions but are often present in turf stands under stress.
Post-Emergent Broadleaf Herbicide Applications
There are a ton of different post-emergent herbicides available for weed control in turf; however, they differ greatly in efficacy on the various turf species. Herbicide combinations of two or more active ingredients will generally improve the spectrum of weeds controlled and are often sold as prepackaged formulated mixtures or as combination fertilizer products. Most broadleaf herbicides active ingredients are sold under various trade names that may differ in formulation and concentration, so turf managers should carefully read and follow label directions for specific products.
Turf managers applying post-emergent herbicides in spring must consider the agronomic impact of the environment and the application timing on efficacy for weed control and, more importantly, turf safety. Turfgrasses are generally more sensitive to broadleaf herbicides during initial spring growth. Post-emergent herbicide applications in early spring may inhibit turfgrass growth and delay initial spring green up.
Post-emergent broadleaf herbicides applied during cool weather in early spring often have reduced efficacy, which necessitates follow up applications to fully control the targeted weeds. Efficacy of broadleaf weed control generally improves when temperatures are consistently above 70 degrees. As temperatures increase in spring, herbicides are absorbed and translocated more effectively as weeds resume active growth. Post-emergent herbicide applications in late spring and early summer may have reduced economic value as winter annual broadleaf weeds begin to die off from heat stress.
Herbicide Absorption Influences Efficacy
Post-emergent broadleaf herbicides are primarily absorbed through the foliage of the targeted weeds, but not injure the turfgrass itself. This selective control is achieved due to the turf’s ability to metabolically inactivate the herbicides before the absorbed compound becomes physiologically toxic to the turf plant. Weeds controlled by the herbicides are unable to metabolize the chemical, which is why they die off after application. The key to success lies with the foliar absorption levels of the herbicide necessary to induce the injury to the weed that leads to its death.
Spray adjuvants may be applied with post-emergent herbicides to enhance the foliar absorption and reduce surface tension on the treated leaves of the targeted weeds, ultimately provided better broadleaf weed control. High quality adjuvants are recommended that contain approximately 80 to 90 percent active ingredient. Although adjuvants will likely improve weed control, enhanced activity will also increase the risk of potential turf injury, so turf managers should be cautious when phytotoxicity is a concern.
Herbicide absorption is also influenced by growth, maturity, and the physiological health of the broadleaf weeds. Plants under heat stress in late spring may absorb less herbicide compared to unstressed plants, which will reduce the effectiveness of herbicide application. Additionally, stressed weeds with inhibited growth will translocate less material within the plant that will also lead to reduced weed control, leading to additional herbicide applications to achieve total control of the targeted weeds. Broadleaf herbicides should only be used on unstressed turf with adequate soil moisture. Turf should not be mowed for at least one day before and after treatments.
Granular combination products are generally easier to handle and apply with spreaders than sprayable formulations. However, foliar absorption of these products may be more critical compared to liquid formulations. Sprayable applications are generally more effective and provide better coverage over treated areas than granular herbicides applied with spreaders. Most granular post-emergence herbicides for broadleaf weed control should be applied in early morning under heavy dew so herbicides can stick to leaves.
Winter Annual Broadleaf Weeds
Winter annual broadleaf weeds germinate in fall or winter and grow actively into spring. As temperatures begins to increase, these winter annuals flower, produce seeds and die in summer. Winter annual broadleaf weeds are commonly found on turf that was thinned or weakened from traffic, disease or injury during and fall and winter. Turf grown in shade, compacted soils or with poor drainage are prone to broadleaf weed infestations during fall and spring.
Perennial Broadleaf Weeds in Spring
Perennial broadleaf weeds in spring are more difficult to control than winter annuals due to better heat tolerance and their ability to reproduce by seed or vegetative stems in spring. Post-emergence herbicides may be less effective in spring than fall because of active shoot growth and the plant’s allocation of energy from stems to leaves. Simple perennials may have a fleshy taproot at the base of the plant, while complex perennials have rhizomes and stolons. Herbicides often burn-back shoot growth of perennial weeds but regrowth may occur from vegetative stems which would likely warrant repeat applications to achieve full control of the targeted weeds.
Broadleaf Weed Herbicides for New Turfgrass
Broadleaf weed herbicides should not be applied to newly seeded turf. Seedlings should be given approximately six to eight weeks after it first emerges before even considering herbicide application. All herbicides have the potential to cause some foliar yellowing but responses to treatments depend on turfgrass species, growth, and maturity.