'Tis The Season - Winter Invasive Plant Control

Just because most of the leaves have fallen doesn't mean your vegetation management activities have to stop. Depending on the types of vegetation present, there are a number of invasive species management activities that are useful in the winter season.

First, What Targets?

Invasive plants come in many forms, and that form determines the best approach to manage them, including the timing of the control operation. Let's summarize the types of vegetation, and determine which are susceptible to winter work. One approach is to categorize by "Plant Growth Forms". This approach considers plant life cycle and growth habit, accounting for whether the species is woody or herbaceous, and whether the plant grows as a single "clump" or expands by way of spreading roots, rhizomes, or stolons. Plant growth form determines what control practices are useful, and when. Table 1 summarizes Plant Growth Forms and the part of the plant where effective control is focused. Based on this model of Plant Growth Forms, plant types we can treat in winter are winter annuals, first- and second-year biennials, non-suckering woody, and woody vines (Table 2).

Plant Growth Form (PGF) combines a plant's life cycle and growth habit into eight distinct management categories. Annuals live one season and die after setting seed. Summer annuals germinate in the spring and summer, and winter annual germinate in the fall and overwinter. Biennials live two growing seasons then die after seeding. Herbaceous perennials live more than two years, and the annual growth dies back to the ground each season. Non-suckering woody plants do not generate new shoots from their roots, while suckering woody species form multi-stemmed colonies by producing new shoots from their root system. Woody vines are suckering species, but climb on other plants. Winter annuals, biennials, non-suckering woody, and woody vines can be usefully impacted with winter operations.

Table 2. The Plant Growth Forms (PGF) susceptible to winter season work are summarized by operations (OP) and timing by month. The operational year is divided by months, and shading indicates the operation is useful. FOL=foliar herbicide application, MECH=stem cutting, STEM=stem-applied herbicide

Treatment Options by Plant Growth Form

There are chemical and mechanical options available in the winter season. Herbaceous plants that are not dormant can be foliar-treated with herbicide as long as the plants are above freezing when treated. The full array of woody stem treatments is available in the winter – stump treatment, basal bark, and hack-and-squirt. Mechanical treatments (mowing or cutting) are useful against non-suckering woody plants and woody vines.

Winter Annuals

Winter annuals are annual species that germinate late in the growing season, overwinter, then flower early the next season. The most common winter annuals in PA are members of the mustard family, such as yellow rocket (Barbarea vulgaris), which is common in ag fields and open areas, and hairy bittercress (Cardamine hirsuta, Figure 1), which is becoming more common in woodland edges. Other common winter annuals include the mints henbit (Lamium amplexicaule) and purple deadnettle (Lamium purpureum); common chickweed (Stellaria media); downy brome (Bromus tectorum); and some natives in the aster family such as marestail (Conyza canadensis) and daisy fleabane (Erigeron strigosus).

Any green, non-dormant plant can be treated with a foliar application. Research conducted at Ohio State (Frey et al., 2007) on garlic mustard (Alliaria petiolata) demonstrated that glyphosate was effective when the air and soil surface temperatures were above freezing. Also, by working in above-freezing conditions, you avoid contending with icing in sprayers. Be sure to use surfactant, and ideally schedule the application for when there is a comparatively warm spell.

Figure 1. Hairy bittercress (Cardamine hirsuta) is an increasingly common winter annual weed. The left panel shows an individual plant, and the right panel shows a patch in the field. Winter annuals germinate late in the growing season and overwinter. Winter annuals and biennials can be foliar-treated during dormant season when air and soil surface temperatures are above freezing.

Biennials

In the Plant Growth Form scheme, biennials are separated into distinct classes for first- and second-year plants. For this discussion, they are the same plants as biennials overwinter once. As with winter annuals, the winter operation you can direct against biennials is a foliar application in above-freezing conditions. The most common biennial in woodland settings is garlic mustard (Figure 2). Other common targets in woodlands and natural areas are poison hemlock (Conium maculatum, Figure 3), common teasel (Dipsacum fullonum), and exotic thistles.

Figure 2. Garlic mustard (Alliaria petiolata) is a common woodlands biennial. The overwintering rosettes can be treated with a foliar application of glyphosate.

 
Figure 3. Poison hemlock (Conium maculatum) is an exotic biennial that lives up to its name - it is lethally toxic to humans and livestock. It is common in riparian settings, and is a viable target for dormant season foliar herbicide applications with glyphosate.

Non-suckering Woody Plants

Useful winter operations on woody plants are limited to non-suckering types because applying herbicide or cutting stems of suckering species does not impact the root system and results in more stems over a wider footprint. Examples of target tree species would be Norway maple (Acer platanoides), European alder (Alnus glutinosa), and callery pear (Pyrus calleryana). There are numerous shrub species in the non-suckering category, such as shrub honeysuckle(s) (Lonicera spp.), autumn olive (Elaeagnus umbellata), multiflora rose (Rosa multiflora), and Japanese barberry (Berberis thunbergii). Many of these species are detailed on the Penn State Extension website.

A situation where a mechanical-only operation has value is when a stand of exotic shrubs is leveled with a forestry mower (mastication) to facilitate selective management of the resprouts (Figure 4). Resprouting following mastication is often reduced compared to saw-cutting, possibly due to the physical damage/shattering of the root collar area. Resprouts of non-suckering species can be foliar-treated late the next growing season.

Figure 4. For well-established exotic woody plant infestations, a useful first step is to level (or masticate) the vegetation with a forestry mower, chewing the stems to ground level, then selectively managing the resprouts the following growing seasons to release desirable species and suppress target exotic species.

You can stem-treat non-suckering woody species throughout the dormant season, using the cut-stump (Figure 5) or hack-and-squirt method, with glyphosate or triclopyr herbicide; or basal bark applications (Figure 6) with oil-soluble triclopyr products. This is more anecdotal than data-driven, but basal bark treatments seem to be more effective later in the dormant season, as November/December treatments on larger stems have provided less effect than expected in the past. Considering this physiologically, a November-applied treatment to a dormant stem just sits there for several months before there is metabolic activity to affect. As soon as they are applied, herbicides begin to break down.

Figure 5. Stems of non-suckering woody species can be cut and stump-treated throughout the year, including the dormant season. The most commonly used herbicides are glyphosate or triclopyr.

Figure 6. The basal bark method applies an oil-based mixture of triclopyr herbicide to entire circumference of the the lower 12- to 18-inches of target woody stems. In the dormant season, the herbicide diffuses through the bark and kills the cambium tissue, chemically girdling the stem.

Woody Vines

Problematic woody vine species include Oriental bittersweet (Celastrus orbiculatus), porcelainberry (Ampelopsis brevipedunculata), wisteria species (Wisteria spp.), Japanese honeysuckle (Lonicera japonica), and often native grape species (Vitis spp.) are overly abundant in woodlots. Winter is a great time to deal with woody vine infestations by cutting. This does not affect the root system of the plant, but it does eliminate the aerial portion. Vines are problematic on two fronts – their suckering behavior, and that most of their foliage is out of reach of sprays and intermixed with desired canopies. Cutting vines to the ground eliminates the aerial growth and forces the roots to resprout. This creates an opportunity to foliar-treat the regrowth in the summer when the foliage is at ground level.

You can cut vines however it works best for you. This will depend on the number and the size, and your predilection for power tools. A good pair of hand pruners or loppers and a pruning saw (Figure 6) will cover the full range of vine diameters. Powered options are many, ranging from small battery-powered saws to lightweight chainsaws. A common approach when cutting vines is to use the "window cut" which is a two-cut system that cuts at ground level and 3- to 5-ft high (Figure 7). It does double the work, but it vastly reduces the chance of missing vines when you get to a dense infestation, and it eliminates having a readily available "trellis" for the newly emerging resprouts the following spring. You can cut vines any time of year. When you cut them, leave them in the tree. Don't pull the vines from the tree. It only increases the chance you will damage the tree and and also pull large, woody debris onto your head. If you are planning to foliar-treat the regrowth, plan the operation to allow six to eight weeks of regrowth before spraying. This will allow the new vine canopy to develop enough that it will translocate applied herbicide to the root system. If you treat too soon after cutting, the herbicide will defoliate the new growth, but not translocate to the roots.

Figure 7. A wide range of tools can be used to cut woody vines, from hand pruners to chainsaws. The pictured vine could readily be cut with a good set of loppers or a folding pruning saw. The choice is a matter of the number and size of the vines, and preferences of the vine cutter.

Figure 8. The "window cut" approach to clearing vines is extra work, but greatly reduces the chance of missing vines. The left panel is "before". and the right panel is the same site after the Oriental bittersweet vines were cut at ground level and 3 feet above ground.

Summary

The dormant season provides ample opportunities to manage invasive and competing vegetation. It is a great opportunity to enjoy the outdoors while improving the habitat value of your property. Also, in terms of growth, it's a slow time. The work you did on your last outing is still visible the next time you work. Nature is not still in the winter, but it does provide what feels like a pause and a months-long window to make progress on your vegetation management objectives.

References

Frey, M.N., C.P. Herms, and J. Cardina. 2007. Cold Weather Application of Glyphosate for Garlic Mustard (Alliaria petiolata) Control. Weed Technology, 21:656-660.