When to Treat Invasive Plants: A Seasonal Calendar

For most woody plants and late-season perennials, the best window is late summer to early fall, from about July until the leaves begin to change color. Spring is the window for spring ephemerals such as lesser celandine, and for garlic mustard before it flowers. The plant’s own food movement sets the date, not the calendar month alone.

The one rule behind the calendar

A systemic herbicide moves through the plant’s sap. It works best when the plant is moving food in the direction you want the herbicide to go.

In late summer and fall, deciduous trees and many perennials move carbohydrates down into their roots for winter. The University of Maryland Extension describes this for tree of heaven: applications are most effective from about July until the onset of autumn leaf color, because that is when the tree is moving carbohydrates down into its roots.

Spring ephemerals run the other direction on a shorter schedule. Lesser celandine emerges early, flowers, and is gone by late May in Pennsylvania. Penn State Extension says its underground parts keep storing nutrients for the next spring. A systemic herbicide applied while the leaves are still green moves into those underground parts.

That is why one calendar cannot cover every plant. The table below puts the windows side by side.

The month-by-month calendar

Japanese stiltgrass does not fit the spring-ephemeral or the woody-plant row. It is an annual grass: seed germinates in late winter or early spring, and the mechanical window is late summer before seed set. The Japanese stiltgrass profile gives the month-by-month detail.

The table is a planning aid, not a prescription. Your local frost date, the species, and the product label decide the exact day.

Why the window differs by plant type

Deciduous trees and shrubs. They build reserves in summer and move them down in fall. A late-summer or early-fall application rides that flow to the roots. Penn State Extension gives July 1 to the onset of fall color for tree of heaven, and says applications outside that window only injure aboveground growth.

Burning bush is a woody shrub with a longer foliar window. It leafs out early and holds leaves late, so Penn State Extension gives mid-May to the onset of fall color, while cut-stump and basal bark treatments can be done outside that window. The burning bush profile covers the state sale rules.

Evergreen and semi-evergreen vines. Japanese honeysuckle keeps green leaves well into winter in mild areas. UConn’s invasive plant program says it can be controlled with mid- to late-fall sprays of triclopyr or glyphosate, and that an autumn application also reduces injury to nontarget plants that have gone dormant.

Spring ephemerals. Lesser celandine is green for a short time. Penn State Extension gives late March to early May for foliar treatment, and says the plant is generally gone by late May in Pennsylvania. Miss that window and you have lost the year.

Biennials. Garlic mustard spends its first year as a rosette and its second year bolting, flowering, and setting seed. Penn State Extension says to pull or treat before flowering and before seed shatter. The rosette stage gives a year-long window, and late fall through early spring is a good time to spot-treat because fewer nontarget plants are in the way.

Poison hemlock is also a biennial. Penn State Extension says the most effective approach is to treat first-year rosettes rather than larger mature plants. It also warns that all parts of the plant are toxic, and that the sap is a skin irritant. Do not start this job without reading the safety section of the poison hemlock removal guide.

Weather, rain, and temperature

The label sets the temperature limits and the rain-fast period for the product you buy. Two sourced examples show why the label matters:

• Penn State Extension says to apply lesser celandine herbicide when the air temperature is at least 40°F.
• For garlic mustard, the same source says glyphosate has been effective in the dormant season as long as temperatures are above freezing.

Do not spray when the forecast calls for rain within the label’s rain-fast window. Do not spray when wind will carry the product onto plants you want to keep. Do not spray a blooming plant if the product or label warns about pollinators.

Herbicides: names, not rates

The extension sources on this page name these active ingredients:

• Glyphosate for lesser celandine, garlic mustard, tree of heaven, Japanese knotweed, and Japanese honeysuckle.
• Triclopyr for tree of heaven, Japanese honeysuckle, and shrub honeysuckle.
• 2,4-D for garlic mustard and poison hemlock.
• Diquat, pelargonic acid, and 2,4-D as post-emergent options for poison hemlock.
• Dicamba, imazapyr, and metsulfuron methyl as options for tree of heaven.

This page does not give mixing rates, dilutions, or application rates. Rates differ by product, formulation, site, and state. Read and follow the product label; the label is the law. If the label and this page disagree, the label wins.

When to stop and call someone

Stop and get site-specific advice if:

• The plant is in or near water. Aquatic-labeled products and buffer rules are different.
• The site is pasture or used by livestock. Grazing and haying restrictions are on the label.
• The stand is large, or it crosses a property line.
• The plant is poison hemlock and you cannot avoid contact with the sap.
• You are not sure of the plant’s identity. A wrong treatment can kill a native and leave the invasive.

For disposal after treatment, see how to dispose of invasive plants. For method-by-method instructions, browse the removal guides. If you have not chosen a method yet, start with how to get rid of invasive plants, which routes you by plant type and state rules. For the identification step, start with the identification guides, or go straight to the Japanese knotweed, tree of heaven, and lesser celandine profiles.

The short version

• Woody plants and late-season perennials: July to the onset of fall color.
• Japanese knotweed: after flowering to first frost.
• Spring ephemerals: late March to early May, while the leaves are green.
• Garlic mustard: before it flowers and before seed shatter; rosettes can be treated late fall through early spring.
• Poison hemlock: first-year rosettes, with full protective equipment.
• Every plant, every time: read the label; the label is the law.

For a spring-only view of the same calendar, including what to look for and act on in March, April, May and June, see the spring invasive plants checklist.