Invasive Shrubs are Leafed Out: Timing Management

Invasive shrubs such as honeysuckle (Lonicera spp.), privets (Ligustrum spp.), multiflora rose (Rosa multiflora), Japanese barberry (Berberis thunbergii), and autumn-olive (Eleagnus umbellata) have leafed out in Pennsylvania, and some species are flowering. Oftentimes, these invasive shrubs will leaf out earlier than the more desirable native species (which may just be at breaking bud at this time). This difference in leaf phenology (timing) is useful to improve our ability to identify and control invasive shrubs.

As invasive shrubs are leafing out and starting to flower, we can identify these problematic shrubs more easily and begin our control efforts now. Below are examples of mid-spring shoots of common invasive shrubs found in Pennsylvania.

For more information on plant identification and management, search Forest Management at Penn State Extension.

Shrubs can be challenging to control, either due to their large size or large infestations, or both. Thankfully, there are resources available to work towards controlling shrubs and reclaiming land.

Timing Foliar Herbicide Applications for Invasive Shrubs

Early Option: Treat shrubs around bloom period to stop seed production this year.

Invasive shrubs mainly reproduce through seeds and can produce many thousands of seeds each year. Seeds can be spread by wildlife as they are consumed, causing new introductions. Given the investment needed to control invasive shrubs, it's a good idea to prevent new seeds from being spread. Despite the objective, any management needs to be economical, effective, and efficient, which often means the use of an herbicide. Consult and follow pesticide labels whenever using any pesticide products.

Herbicides are the most effective way to control invasive shrubs, but should be applied with some regard to the timing of application. The earliest opportunity for invasive shrub control is after leaf out or early bloom. When applying foliar herbicides, ensure that:

• Daytime temperatures are above 40°F (influences herbicide movement and activity)
• Plants are actively growing (influences herbicide uptake)
• Leaves are fully expanded (important for complete coverage)

Select an herbicide or tank-mix that allows for maximum control of many weed species in a single application. In pastures and hayfields, consider and follow all grazing, haying, and crop intervals found on the label of your chosen herbicides.

Later Option: Target application between September and October to more effectively kill root systems

As long as plants have not undergone a killing frost, herbicides are effective at controlling invasive shrubs in the Fall. Fall is a good time to control woody shrubs as systemic herbicides will be effectively translocated to the root system as the shrub sends resources below ground to prepare for winter.

For more information about herbicide applications (foliar, basal bark, and stump), refer to the following publications:

Invasive Plant Species Management: Exotic Shrubs (PDF)

Invasive Shrub Control Strategies

Other Management Considerations

Occasional mowing of pastures or brush hogging year-round will suppress, but often not kill invasive shrubs due to their established, persistent root system. If shrubs are cut at ground level, many species can "sucker sprout" where the plant sends up new shoots from the cut stump and forms a new shrub. These sprouted shrubs can take several seasons to reach their previous size.

For greater control, pair mowing with a cut stump application, where a 1:1 mixture of herbicide (glyphosate or triclopyr concentrate) and water is applied to the cut stump surface immediately after cutting the plant. This type of application may be done year-round. This combination prevents any resprouting of the shrubs and provides greater control.

Keep in mind that the management of invasive shrubs may take several treatments over multiple years, particularly in dense infestations. Therefore, a shrub management plan should include both initial applications and subsequent retreatments, or maintenance over time, to keep up with control of these invasive shrubs.

For more information on cut stump and step application, see: Invasive Plant Species Management: Exotic Shrubs (PDF).