Johnsongrass (Sorghum halepense) is an invasive and problematic weed across agricultural lands, home gardens, and various landscapes. This aggressive plant competes fiercely with cultivated crops for light, water, and nutrients. Johnsongrass also releases chemicals that can inhibit the growth of nearby plants. Its extensive underground rhizome system and prolific seed production make effective management a considerable challenge.
Types of Herbicides for Johnsongrass Control
Herbicides are categorized based on how they affect plants and when they are applied. Selective herbicides kill specific types of plants, such as grasses, while leaving other plants, like broadleaf crops, unharmed. In contrast, non-selective herbicides kill nearly all vegetation they contact, making them suitable for bare ground applications or spot treatments.
The timing of herbicide application also defines two main categories. Pre-emergent herbicides are applied to the soil before Johnsongrass seeds germinate, forming a chemical barrier that prevents seedling emergence. Post-emergent herbicides are applied directly to actively growing Johnsongrass plants, targeting the foliage and stems.
Herbicides are further classified by their movement within the plant. Systemic herbicides are absorbed by the plant and then translocated throughout its vascular system, reaching roots and rhizomes. This movement is particularly effective for controlling perennial weeds like Johnsongrass, which have extensive underground structures. Contact herbicides, conversely, kill only the plant parts they directly touch, making them less effective for weeds that can regrow from underground rhizomes.
Common Herbicides and Their Actions
Glyphosate is a widely used non-selective, systemic herbicide effective for controlling Johnsongrass. It inhibits the enzyme EPSP synthase, which is necessary for aromatic amino acid production, halting protein synthesis and leading to plant death. Glyphosate is effective for Johnsongrass control in fallow fields, non-cropped areas, or as a spot treatment, as it will affect any green vegetation it contacts.
Sethoxydim is a selective, post-emergent herbicide designed to control grassy weeds, including Johnsongrass, in broadleaf crops. This herbicide is an acetyl-CoA carboxylase (ACCase) inhibitor, interfering with lipid synthesis in grass plants. Without proper lipid production, cell membrane formation is disrupted, leading to growth cessation and plant death. Its selectivity allows use in crops like soybeans, cotton, and peanuts without harming them.
Quizalofop is another selective, post-emergent grass herbicide operating through the same ACCase inhibition mechanism as sethoxydim. It is highly effective against Johnsongrass and commonly applied in broadleaf cropping systems. The herbicide is absorbed through foliage and translocated to the grass plant’s growing points, including rhizomes, to disrupt metabolic processes. Other ACCase inhibitors like fluazifop and clethodim are also used for Johnsongrass control.
Strategic Application for Best Results
Effective herbicide application for Johnsongrass control relies on precise timing to maximize herbicide uptake and translocation. For post-emergent herbicides, application is most effective when Johnsongrass plants are actively growing, 18 to 24 inches tall, before producing seed heads. At this stage, the plant has sufficient foliage to absorb the herbicide and translocate it efficiently to the rhizomes, ensuring better control of the underground system. Pre-emergent herbicides, conversely, must be applied to the soil before Johnsongrass seeds germinate, typically in early spring, to form a protective barrier.
Several application methods can be employed. Broadcast spraying covers an entire area, suitable for widespread infestations in fallow fields. Spot treatment targets individual Johnsongrass patches, minimizing herbicide use. Wick applicators, which wipe herbicide onto taller weeds, are useful where Johnsongrass is significantly taller than desirable vegetation, preventing contact with the crop.
Environmental conditions significantly impact herbicide efficacy. Ideal temperatures for post-emergent applications range from 65 to 85 degrees Fahrenheit, promoting active plant growth and herbicide absorption. Adequate soil moisture before application enhances uptake. High winds should be avoided to prevent drift. Avoid applying herbicides if rain is expected within 4 to 6 hours, as precipitation can wash the product off.
Safety precautions are important during herbicide application. Always read and follow all instructions on the herbicide product label, including application rates, timing, and safety guidelines. Wear appropriate personal protective equipment (PPE), such as gloves, eye protection, and long-sleeved clothing, to minimize exposure. Store herbicides in original containers in a secure, cool, dry place, and dispose of empty containers responsibly to prevent environmental contamination and accidental exposure.
Due to Johnsongrass’s capacity for regrowth from rhizomes and seeds, a single herbicide application is often insufficient for complete control. Multiple applications may be necessary throughout the growing season or in successive seasons to deplete the rhizome energy reserves and control new seedlings. Monitoring treated areas and reapplying as new growth emerges is common practice for sustained suppression.
Integrated Approaches to Johnsongrass Management
Herbicides are effective, but their use against Johnsongrass is enhanced when combined with other management strategies. Cultural practices, like crop rotation, can disrupt Johnsongrass life cycles by introducing varied control methods. Planting competitive cover crops can suppress Johnsongrass growth by shading the soil and competing for resources. Proper fertilization practices can boost crop competitiveness, reducing the weed’s impact.
Mechanical control methods, including tillage and mowing, complement herbicide use. Deep plowing brings rhizomes to the surface for desiccation, while repeated cultivation fragments and exhausts the rhizome system. Mowing prevents Johnsongrass from producing seeds, reducing future infestations, though it doesn’t eliminate rhizomes. These methods often need precise timing to be effective and avoid spreading the weed.
Preventing the spread of Johnsongrass is an important long-term strategy. This involves cleaning farm equipment after working in infested areas to avoid carrying seeds or rhizome fragments to clean fields. Using certified, weed-free crop seeds ensures new infestations are not introduced. Effective Johnsongrass control is an ongoing effort benefiting from a multi-faceted approach, combining chemical, cultural, and mechanical tactics for sustainable management.